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| <h1>Google C++ Style Guide</h1> |
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| <div class="main_body"> |
| |
| <h2 id="Background" class="ignoreLink">Background</h2> |
| |
| <p>C++ is one of the main development languages used by |
| many of Google's open-source projects. As every C++ |
| programmer knows, the language has many powerful features, but |
| this power brings with it complexity, which in turn can make |
| code more bug-prone and harder to read and maintain.</p> |
| |
| <p>The goal of this guide is to manage this complexity by |
| describing in detail the dos and don'ts of writing C++ code |
| . These rules exist to |
| keep the code base manageable while still allowing |
| coders to use C++ language features productively.</p> |
| |
| <p><em>Style</em>, also known as readability, is what we call |
| the conventions that govern our C++ code. The term Style is a |
| bit of a misnomer, since these conventions cover far more than |
| just source file formatting.</p> |
| |
| <p> |
| Most open-source projects developed by |
| Google conform to the requirements in this guide. |
| </p> |
| |
| |
| |
| <p>Note that this guide is not a C++ tutorial: we assume that |
| the reader is familiar with the language. </p> |
| |
| <h3 id="Goals">Goals of the Style Guide</h3> |
| |
| <p>Why do we have this document?</p> |
| |
| <p>There are a few core goals that we believe this guide should |
| serve. These are the fundamental <b>why</b>s that |
| underlie all of the individual rules. By bringing these ideas to |
| the fore, we hope to ground discussions and make it clearer to our |
| broader community why the rules are in place and why particular |
| decisions have been made. If you understand what goals each rule is |
| serving, it should be clearer to everyone when a rule may be waived |
| (some can be), and what sort of argument or alternative would be |
| necessary to change a rule in the guide.</p> |
| |
| <p>The goals of the style guide as we currently see them are as follows:</p> |
| <dl> |
| <dt>Style rules should pull their weight</dt> |
| <dd>The benefit of a style rule |
| must be large enough to justify asking all of our engineers to |
| remember it. The benefit is measured relative to the codebase we would |
| get without the rule, so a rule against a very harmful practice may |
| still have a small benefit if people are unlikely to do it |
| anyway. This principle mostly explains the rules we don’t have, rather |
| than the rules we do: for example, <code>goto</code> contravenes many |
| of the following principles, but is already vanishingly rare, so the Style |
| Guide doesn’t discuss it.</dd> |
| |
| <dt>Optimize for the reader, not the writer</dt> |
| <dd>Our codebase (and most individual components submitted to it) is |
| expected to continue for quite some time. As a result, more time will |
| be spent reading most of our code than writing it. We explicitly |
| choose to optimize for the experience of our average software engineer |
| reading, maintaining, and debugging code in our codebase rather than |
| ease when writing said code. "Leave a trace for the reader" is a |
| particularly common sub-point of this principle: When something |
| surprising or unusual is happening in a snippet of code (for example, |
| transfer of pointer ownership), leaving textual hints for the reader |
| at the point of use is valuable (<code>std::unique_ptr</code> |
| demonstrates the ownership transfer unambiguously at the call |
| site). </dd> |
| |
| <dt>Be consistent with existing code</dt> |
| <dd>Using one style consistently through our codebase lets us focus on |
| other (more important) issues. Consistency also allows for |
| automation: tools that format your code or adjust |
| your <code>#include</code>s only work properly when your code is |
| consistent with the expectations of the tooling. In many cases, rules |
| that are attributed to "Be Consistent" boil down to "Just pick one and |
| stop worrying about it"; the potential value of allowing flexibility |
| on these points is outweighed by the cost of having people argue over |
| them. </dd> |
| |
| <dt>Be consistent with the broader C++ community when appropriate</dt> |
| <dd>Consistency with the way other organizations use C++ has value for |
| the same reasons as consistency within our code base. If a feature in |
| the C++ standard solves a problem, or if some idiom is widely known |
| and accepted, that's an argument for using it. However, sometimes |
| standard features and idioms are flawed, or were just designed without |
| our codebase's needs in mind. In those cases (as described below) it's |
| appropriate to constrain or ban standard features. In some cases we |
| prefer a homegrown or third-party library over a library defined in |
| the C++ Standard, either out of perceived superiority or insufficient |
| value to transition the codebase to the standard interface.</dd> |
| |
| <dt>Avoid surprising or dangerous constructs</dt> |
| <dd>C++ has features that are more surprising or dangerous than one |
| might think at a glance. Some style guide restrictions are in place to |
| prevent falling into these pitfalls. There is a high bar for style |
| guide waivers on such restrictions, because waiving such rules often |
| directly risks compromising program correctness. |
| </dd> |
| |
| <dt>Avoid constructs that our average C++ programmer would find tricky |
| or hard to maintain</dt> |
| <dd>C++ has features that may not be generally appropriate because of |
| the complexity they introduce to the code. In widely used |
| code, it may be more acceptable to use |
| trickier language constructs, because any benefits of more complex |
| implementation are multiplied widely by usage, and the cost in understanding |
| the complexity does not need to be paid again when working with new |
| portions of the codebase. When in doubt, waivers to rules of this type |
| can be sought by asking |
| your project leads. This is specifically |
| important for our codebase because code ownership and team membership |
| changes over time: even if everyone that works with some piece of code |
| currently understands it, such understanding is not guaranteed to hold a |
| few years from now.</dd> |
| |
| <dt>Be mindful of our scale</dt> |
| <dd>With a codebase of 100+ million lines and thousands of engineers, |
| some mistakes and simplifications for one engineer can become costly |
| for many. For instance it's particularly important to |
| avoid polluting the global namespace: name collisions across a |
| codebase of hundreds of millions of lines are difficult to work with |
| and hard to avoid if everyone puts things into the global |
| namespace.</dd> |
| |
| <dt>Concede to optimization when necessary</dt> |
| <dd>Performance optimizations can sometimes be necessary and |
| appropriate, even when they conflict with the other principles of this |
| document.</dd> |
| </dl> |
| |
| <p>The intent of this document is to provide maximal guidance with |
| reasonable restriction. As always, common sense and good taste should |
| prevail. By this we specifically refer to the established conventions |
| of the entire Google C++ community, not just your personal preferences |
| or those of your team. Be skeptical about and reluctant to use |
| clever or unusual constructs: the absence of a prohibition is not the |
| same as a license to proceed. Use your judgment, and if you are |
| unsure, please don't hesitate to ask your project leads to get additional |
| input.</p> |
| |
| |
| |
| <h2 id="C++_Version">C++ Version</h2> |
| |
| <p>Currently, code should target C++17, i.e., should not use C++2x |
| features. The C++ version targeted by this guide will advance |
| (aggressively) over time.</p> |
| |
| |
| |
| <p>Do not use |
| <a href="#Nonstandard_Extensions">non-standard extensions</a>.</p> |
| |
| <div>Consider portability to other environments |
| before using features from C++14 and C++17 in your project. |
| </div> |
| |
| <h2 id="Header_Files">Header Files</h2> |
| |
| <p>In general, every <code>.cc</code> file should have an |
| associated <code>.h</code> file. There are some common |
| exceptions, such as unittests and |
| small <code>.cc</code> files containing just a |
| <code>main()</code> function.</p> |
| |
| <p>Correct use of header files can make a huge difference to |
| the readability, size and performance of your code.</p> |
| |
| <p>The following rules will guide you through the various |
| pitfalls of using header files.</p> |
| |
| <a id="The_-inl.h_Files"></a> |
| <h3 id="Self_contained_Headers">Self-contained Headers</h3> |
| |
| <p>Header files should be self-contained (compile on their own) and |
| end in <code>.h</code>. Non-header files that are meant for inclusion |
| should end in <code>.inc</code> and be used sparingly.</p> |
| |
| <p>All header files should be self-contained. Users and refactoring |
| tools should not have to adhere to special conditions to include the |
| header. Specifically, a header should |
| have <a href="#The__define_Guard">header guards</a> and include all |
| other headers it needs.</p> |
| |
| <p>Prefer placing the definitions for template and inline functions in |
| the same file as their declarations. The definitions of these |
| constructs must be included into every <code>.cc</code> file that uses |
| them, or the program may fail to link in some build configurations. If |
| declarations and definitions are in different files, including the |
| former should transitively include the latter. Do not move these |
| definitions to separately included header files (<code>-inl.h</code>); |
| this practice was common in the past, but is no longer allowed.</p> |
| |
| <p>As an exception, a template that is explicitly instantiated for |
| all relevant sets of template arguments, or that is a private |
| implementation detail of a class, is allowed to be defined in the one |
| and only <code>.cc</code> file that instantiates the template.</p> |
| |
| <p>There are rare cases where a file designed to be included is not |
| self-contained. These are typically intended to be included at unusual |
| locations, such as the middle of another file. They might not |
| use <a href="#The__define_Guard">header guards</a>, and might not include |
| their prerequisites. Name such files with the <code>.inc</code> |
| extension. Use sparingly, and prefer self-contained headers when |
| possible.</p> |
| |
| <h3 id="The__define_Guard">The #define Guard</h3> |
| |
| <p>All header files should have <code>#define</code> guards to |
| prevent multiple inclusion. The format of the symbol name |
| should be |
| |
| <code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>.</p> |
| |
| |
| |
| <div> |
| <p>To guarantee uniqueness, they should |
| be based on the full path in a project's source tree. For |
| example, the file <code>foo/src/bar/baz.h</code> in |
| project <code>foo</code> should have the following |
| guard:</p> |
| </div> |
| |
| <pre>#ifndef FOO_BAR_BAZ_H_ |
| #define FOO_BAR_BAZ_H_ |
| |
| ... |
| |
| #endif // FOO_BAR_BAZ_H_ |
| </pre> |
| |
| |
| |
| <h3 id="Forward_Declarations">Forward Declarations</h3> |
| |
| <p>Avoid using forward declarations where possible. |
| Instead, <code>#include</code> the headers you need.</p> |
| |
| <p class="definition"></p> |
| <p>A "forward declaration" is a declaration of a class, |
| function, or template without an associated definition.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Forward declarations can save compile time, as |
| <code>#include</code>s force the compiler to open |
| more files and process more input.</li> |
| |
| <li>Forward declarations can save on unnecessary |
| recompilation. <code>#include</code>s can force |
| your code to be recompiled more often, due to unrelated |
| changes in the header.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Forward declarations can hide a dependency, allowing |
| user code to skip necessary recompilation when headers |
| change.</li> |
| |
| <li>A forward declaration may be broken by subsequent |
| changes to the library. Forward declarations of functions |
| and templates can prevent the header owners from making |
| otherwise-compatible changes to their APIs, such as |
| widening a parameter type, adding a template parameter |
| with a default value, or migrating to a new namespace.</li> |
| |
| <li>Forward declaring symbols from namespace |
| <code>std::</code> yields undefined behavior.</li> |
| |
| <li>It can be difficult to determine whether a forward |
| declaration or a full <code>#include</code> is needed. |
| Replacing an <code>#include</code> with a forward |
| declaration can silently change the meaning of |
| code: |
| <pre> // b.h: |
| struct B {}; |
| struct D : B {}; |
| |
| // good_user.cc: |
| #include "b.h" |
| void f(B*); |
| void f(void*); |
| void test(D* x) { f(x); } // calls f(B*) |
| </pre> |
| If the <code>#include</code> was replaced with forward |
| decls for <code>B</code> and <code>D</code>, |
| <code>test()</code> would call <code>f(void*)</code>. |
| </li> |
| |
| <li>Forward declaring multiple symbols from a header |
| can be more verbose than simply |
| <code>#include</code>ing the header.</li> |
| |
| <li>Structuring code to enable forward declarations |
| (e.g. using pointer members instead of object members) |
| can make the code slower and more complex.</li> |
| |
| |
| </ul> |
| |
| <p class="decision"></p> |
| <ul> |
| <li>Try to avoid forward declarations of entities |
| defined in another project.</li> |
| |
| <li>When using a function declared in a header file, |
| always <code>#include</code> that header.</li> |
| |
| <li>When using a class template, prefer to |
| <code>#include</code> its header file.</li> |
| </ul> |
| |
| <p>Please see <a href="#Names_and_Order_of_Includes">Names and Order |
| of Includes</a> for rules about when to #include a header.</p> |
| |
| <h3 id="Inline_Functions">Inline Functions</h3> |
| |
| <p>Define functions inline only when they are small, say, 10 |
| lines or fewer.</p> |
| |
| <p class="definition"></p> |
| <p>You can declare functions in a way that allows the compiler to expand |
| them inline rather than calling them through the usual |
| function call mechanism.</p> |
| |
| <p class="pros"></p> |
| <p>Inlining a function can generate more efficient object |
| code, as long as the inlined function is small. Feel free |
| to inline accessors and mutators, and other short, |
| performance-critical functions.</p> |
| |
| <p class="cons"></p> |
| <p>Overuse of inlining can actually make programs slower. |
| Depending on a function's size, inlining it can cause the |
| code size to increase or decrease. Inlining a very small |
| accessor function will usually decrease code size while |
| inlining a very large function can dramatically increase |
| code size. On modern processors smaller code usually runs |
| faster due to better use of the instruction cache.</p> |
| |
| <p class="decision"></p> |
| <p>A decent rule of thumb is to not inline a function if |
| it is more than 10 lines long. Beware of destructors, |
| which are often longer than they appear because of |
| implicit member- and base-destructor calls!</p> |
| |
| <p>Another useful rule of thumb: it's typically not cost |
| effective to inline functions with loops or switch |
| statements (unless, in the common case, the loop or |
| switch statement is never executed).</p> |
| |
| <p>It is important to know that functions are not always |
| inlined even if they are declared as such; for example, |
| virtual and recursive functions are not normally inlined. |
| Usually recursive functions should not be inline. The |
| main reason for making a virtual function inline is to |
| place its definition in the class, either for convenience |
| or to document its behavior, e.g., for accessors and |
| mutators.</p> |
| |
| <h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3> |
| |
| <p>Include headers in the following order: Related header, C system headers, |
| C++ standard library headers, |
| other libraries' headers, your project's |
| headers.</p> |
| |
| <p> |
| All of a project's header files should be |
| listed as descendants of the project's source |
| directory without use of UNIX directory aliases |
| <code>.</code> (the current directory) or <code>..</code> |
| (the parent directory). For example, |
| |
| <code>google-awesome-project/src/base/logging.h</code> |
| should be included as:</p> |
| |
| <pre>#include "base/logging.h" |
| </pre> |
| |
| <p>In <code><var>dir/foo</var>.cc</code> or |
| <code><var>dir/foo_test</var>.cc</code>, whose main |
| purpose is to implement or test the stuff in |
| <code><var>dir2/foo2</var>.h</code>, order your includes |
| as follows:</p> |
| |
| <ol> |
| <li><code><var>dir2/foo2</var>.h</code>.</li> |
| |
| <li>A blank line</li> |
| |
| <li>C system headers (more precisely: headers in angle brackets with the |
| <code>.h</code> extension), e.g. <code><unistd.h></code>, |
| <code><stdlib.h></code>.</li> |
| |
| <li>A blank line</li> |
| |
| <li>C++ standard library headers (without file extension), e.g. |
| <code><algorithm></code>, <code><cstddef></code>.</li> |
| |
| <li>A blank line</li> |
| |
| <div> |
| <li>Other libraries' <code>.h</code> files.</li> |
| </div> |
| |
| <li> |
| Your project's <code>.h</code> |
| files.</li> |
| </ol> |
| |
| <p>Separate each non-empty group with one blank line.</p> |
| |
| <p>With the preferred ordering, if the related header |
| <code><var>dir2/foo2</var>.h</code> omits any necessary |
| includes, the build of <code><var>dir/foo</var>.cc</code> |
| or <code><var>dir/foo</var>_test.cc</code> will break. |
| Thus, this rule ensures that build breaks show up first |
| for the people working on these files, not for innocent |
| people in other packages.</p> |
| |
| <p><code><var>dir/foo</var>.cc</code> and |
| <code><var>dir2/foo2</var>.h</code> are usually in the same |
| directory (e.g. <code>base/basictypes_test.cc</code> and |
| <code>base/basictypes.h</code>), but may sometimes be in different |
| directories too.</p> |
| |
| |
| |
| <p>Note that the C headers such as <code>stddef.h</code> |
| are essentially interchangeable with their C++ counterparts |
| (<code>cstddef</code>). |
| Either style is acceptable, but prefer consistency with existing code.</p> |
| |
| <p>Within each section the includes should be ordered |
| alphabetically. Note that older code might not conform to |
| this rule and should be fixed when convenient.</p> |
| |
| <p>You should include all the headers that define the symbols you rely |
| upon, except in the unusual case of <a href="#Forward_Declarations">forward |
| declaration</a>. If you rely on symbols from <code>bar.h</code>, |
| don't count on the fact that you included <code>foo.h</code> which |
| (currently) includes <code>bar.h</code>: include <code>bar.h</code> |
| yourself, unless <code>foo.h</code> explicitly demonstrates its intent |
| to provide you the symbols of <code>bar.h</code>.</p> |
| |
| <p>For example, the includes in |
| |
| <code>google-awesome-project/src/foo/internal/fooserver.cc</code> |
| might look like this:</p> |
| |
| <pre>#include "foo/server/fooserver.h" |
| |
| #include <sys/types.h> |
| #include <unistd.h> |
| |
| #include <string> |
| #include <vector> |
| |
| #include "base/basictypes.h" |
| #include "base/commandlineflags.h" |
| #include "foo/server/bar.h" |
| </pre> |
| |
| <p><b>Exception:</b></p> |
| |
| <p>Sometimes, system-specific code needs |
| conditional includes. Such code can put conditional |
| includes after other includes. Of course, keep your |
| system-specific code small and localized. Example:</p> |
| |
| <pre>#include "foo/public/fooserver.h" |
| |
| #include "base/port.h" // For LANG_CXX11. |
| |
| #ifdef LANG_CXX11 |
| #include <initializer_list> |
| #endif // LANG_CXX11 |
| </pre> |
| |
| <h2 id="Scoping">Scoping</h2> |
| |
| <h3 id="Namespaces">Namespaces</h3> |
| |
| <p>With few exceptions, place code in a namespace. Namespaces |
| should have unique names based on the project name, and possibly |
| its path. Do not use <i>using-directives</i> (e.g. |
| <code>using namespace foo</code>). Do not use |
| inline namespaces. For unnamed namespaces, see |
| <a href="#Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and |
| Static Variables</a>. |
| |
| </p><p class="definition"></p> |
| <p>Namespaces subdivide the global scope |
| into distinct, named scopes, and so are useful for preventing |
| name collisions in the global scope.</p> |
| |
| <p class="pros"></p> |
| |
| <p>Namespaces provide a method for preventing name conflicts |
| in large programs while allowing most code to use reasonably |
| short names.</p> |
| |
| <p>For example, if two different projects have a class |
| <code>Foo</code> in the global scope, these symbols may |
| collide at compile time or at runtime. If each project |
| places their code in a namespace, <code>project1::Foo</code> |
| and <code>project2::Foo</code> are now distinct symbols that |
| do not collide, and code within each project's namespace |
| can continue to refer to <code>Foo</code> without the prefix.</p> |
| |
| <p>Inline namespaces automatically place their names in |
| the enclosing scope. Consider the following snippet, for |
| example:</p> |
| |
| <pre class="neutralcode">namespace outer { |
| inline namespace inner { |
| void foo(); |
| } // namespace inner |
| } // namespace outer |
| </pre> |
| |
| <p>The expressions <code>outer::inner::foo()</code> and |
| <code>outer::foo()</code> are interchangeable. Inline |
| namespaces are primarily intended for ABI compatibility |
| across versions.</p> |
| |
| <p class="cons"></p> |
| |
| <p>Namespaces can be confusing, because they complicate |
| the mechanics of figuring out what definition a name refers |
| to.</p> |
| |
| <p>Inline namespaces, in particular, can be confusing |
| because names aren't actually restricted to the namespace |
| where they are declared. They are only useful as part of |
| some larger versioning policy.</p> |
| |
| <p>In some contexts, it's necessary to repeatedly refer to |
| symbols by their fully-qualified names. For deeply-nested |
| namespaces, this can add a lot of clutter.</p> |
| |
| <p class="decision"></p> |
| |
| <p>Namespaces should be used as follows:</p> |
| |
| <ul> |
| <li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>. |
| </li><li>Terminate namespaces with comments as shown in the given examples. |
| </li><li> |
| |
| <p>Namespaces wrap the entire source file after |
| includes, |
| <a href="https://gflags.github.io/gflags/"> |
| gflags</a> definitions/declarations |
| and forward declarations of classes from other namespaces.</p> |
| |
| <pre>// In the .h file |
| namespace mynamespace { |
| |
| // All declarations are within the namespace scope. |
| // Notice the lack of indentation. |
| class MyClass { |
| public: |
| ... |
| void Foo(); |
| }; |
| |
| } // namespace mynamespace |
| </pre> |
| |
| <pre>// In the .cc file |
| namespace mynamespace { |
| |
| // Definition of functions is within scope of the namespace. |
| void MyClass::Foo() { |
| ... |
| } |
| |
| } // namespace mynamespace |
| </pre> |
| |
| <p>More complex <code>.cc</code> files might have additional details, |
| like flags or using-declarations.</p> |
| |
| <pre>#include "a.h" |
| |
| ABSL_FLAG(bool, someflag, false, "dummy flag"); |
| |
| namespace mynamespace { |
| |
| using ::foo::Bar; |
| |
| ...code for mynamespace... // Code goes against the left margin. |
| |
| } // namespace mynamespace |
| </pre> |
| </li> |
| |
| <li>To place generated protocol |
| message code in a namespace, use the |
| <code>package</code> specifier in the |
| <code>.proto</code> file. See |
| |
| |
| <a href="https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#package"> |
| Protocol Buffer Packages</a> |
| for details.</li> |
| |
| <li>Do not declare anything in namespace |
| <code>std</code>, including forward declarations of |
| standard library classes. Declaring entities in |
| namespace <code>std</code> is undefined behavior, i.e., |
| not portable. To declare entities from the standard |
| library, include the appropriate header file.</li> |
| |
| <li><p>You may not use a <i>using-directive</i> |
| to make all names from a namespace available.</p> |
| |
| <pre class="badcode">// Forbidden -- This pollutes the namespace. |
| using namespace foo; |
| </pre> |
| </li> |
| |
| <li><p>Do not use <i>Namespace aliases</i> at namespace scope |
| in header files except in explicitly marked |
| internal-only namespaces, because anything imported into a namespace |
| in a header file becomes part of the public |
| API exported by that file.</p> |
| |
| <pre>// Shorten access to some commonly used names in .cc files. |
| namespace baz = ::foo::bar::baz; |
| </pre> |
| |
| <pre>// Shorten access to some commonly used names (in a .h file). |
| namespace librarian { |
| namespace impl { // Internal, not part of the API. |
| namespace sidetable = ::pipeline_diagnostics::sidetable; |
| } // namespace impl |
| |
| inline void my_inline_function() { |
| // namespace alias local to a function (or method). |
| namespace baz = ::foo::bar::baz; |
| ... |
| } |
| } // namespace librarian |
| </pre> |
| |
| </li><li>Do not use inline namespaces.</li> |
| </ul> |
| |
| <h3 id="Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and Static |
| Variables</h3> |
| |
| <p>When definitions in a <code>.cc</code> file do not need to be |
| referenced outside that file, place them in an unnamed |
| namespace or declare them <code>static</code>. Do not use either |
| of these constructs in <code>.h</code> files. |
| |
| </p><p class="definition"></p> |
| <p>All declarations can be given internal linkage by placing them in unnamed |
| namespaces. Functions and variables can also be given internal linkage by |
| declaring them <code>static</code>. This means that anything you're declaring |
| can't be accessed from another file. If a different file declares something with |
| the same name, then the two entities are completely independent.</p> |
| |
| <p class="decision"></p> |
| |
| <p>Use of internal linkage in <code>.cc</code> files is encouraged |
| for all code that does not need to be referenced elsewhere. |
| Do not use internal linkage in <code>.h</code> files.</p> |
| |
| <p>Format unnamed namespaces like named namespaces. In the |
| terminating comment, leave the namespace name empty:</p> |
| |
| <pre>namespace { |
| ... |
| } // namespace |
| </pre> |
| |
| <h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3> |
| |
| <p>Prefer placing nonmember functions in a namespace; use completely global |
| functions rarely. Do not use a class simply to group static functions. Static |
| methods of a class should generally be closely related to instances of the |
| class or the class's static data.</p> |
| |
| |
| <p class="pros"></p> |
| <p>Nonmember and static member functions can be useful in |
| some situations. Putting nonmember functions in a |
| namespace avoids polluting the global namespace.</p> |
| |
| <p class="cons"></p> |
| <p>Nonmember and static member functions may make more sense |
| as members of a new class, especially if they access |
| external resources or have significant dependencies.</p> |
| |
| <p class="decision"></p> |
| <p>Sometimes it is useful to define a |
| function not bound to a class instance. Such a function |
| can be either a static member or a nonmember function. |
| Nonmember functions should not depend on external |
| variables, and should nearly always exist in a namespace. |
| Do not create classes only to group static member functions; |
| this is no different than just giving the function names a |
| common prefix, and such grouping is usually unnecessary anyway.</p> |
| |
| <p>If you define a nonmember function and it is only |
| needed in its <code>.cc</code> file, use |
| <a href="#Unnamed_Namespaces_and_Static_Variables">internal linkage</a> to limit |
| its scope.</p> |
| |
| <h3 id="Local_Variables">Local Variables</h3> |
| |
| <p>Place a function's variables in the narrowest scope |
| possible, and initialize variables in the declaration.</p> |
| |
| <p>C++ allows you to declare variables anywhere in a |
| function. We encourage you to declare them in as local a |
| scope as possible, and as close to the first use as |
| possible. This makes it easier for the reader to find the |
| declaration and see what type the variable is and what it |
| was initialized to. In particular, initialization should |
| be used instead of declaration and assignment, e.g.:</p> |
| |
| <pre class="badcode">int i; |
| i = f(); // Bad -- initialization separate from declaration. |
| </pre> |
| |
| <pre>int j = g(); // Good -- declaration has initialization. |
| </pre> |
| |
| <pre class="badcode">std::vector<int> v; |
| v.push_back(1); // Prefer initializing using brace initialization. |
| v.push_back(2); |
| </pre> |
| |
| <pre>std::vector<int> v = {1, 2}; // Good -- v starts initialized. |
| </pre> |
| |
| <p>Variables needed for <code>if</code>, <code>while</code> |
| and <code>for</code> statements should normally be declared |
| within those statements, so that such variables are confined |
| to those scopes. E.g.:</p> |
| |
| <pre>while (const char* p = strchr(str, '/')) str = p + 1; |
| </pre> |
| |
| <p>There is one caveat: if the variable is an object, its |
| constructor is invoked every time it enters scope and is |
| created, and its destructor is invoked every time it goes |
| out of scope.</p> |
| |
| <pre class="badcode">// Inefficient implementation: |
| for (int i = 0; i < 1000000; ++i) { |
| Foo f; // My ctor and dtor get called 1000000 times each. |
| f.DoSomething(i); |
| } |
| </pre> |
| |
| <p>It may be more efficient to declare such a variable |
| used in a loop outside that loop:</p> |
| |
| <pre>Foo f; // My ctor and dtor get called once each. |
| for (int i = 0; i < 1000000; ++i) { |
| f.DoSomething(i); |
| } |
| </pre> |
| |
| <h3 id="Static_and_Global_Variables">Static and Global Variables</h3> |
| |
| <p>Objects with |
| <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration"> |
| static storage duration</a> are forbidden unless they are |
| <a href="http://en.cppreference.com/w/cpp/types/is_destructible">trivially |
| destructible</a>. Informally this means that the destructor does not do |
| anything, even taking member and base destructors into account. More formally it |
| means that the type has no user-defined or virtual destructor and that all bases |
| and non-static members are trivially destructible. |
| Static function-local variables may use dynamic initialization. |
| Use of dynamic initialization for static class member variables or variables at |
| namespace scope is discouraged, but allowed in limited circumstances; see below |
| for details.</p> |
| |
| <p>As a rule of thumb: a global variable satisfies these requirements if its |
| declaration, considered in isolation, could be <code>constexpr</code>.</p> |
| |
| <p class="definition"></p> |
| <p>Every object has a <dfn>storage duration</dfn>, which correlates with its |
| lifetime. Objects with static storage duration live from the point of their |
| initialization until the end of the program. Such objects appear as variables at |
| namespace scope ("global variables"), as static data members of classes, or as |
| function-local variables that are declared with the <code>static</code> |
| specifier. Function-local static variables are initialized when control first |
| passes through their declaration; all other objects with static storage duration |
| are initialized as part of program start-up. All objects with static storage |
| duration are destroyed at program exit (which happens before unjoined threads |
| are terminated).</p> |
| |
| <p>Initialization may be <dfn>dynamic</dfn>, which means that something |
| non-trivial happens during initialization. (For example, consider a constructor |
| that allocates memory, or a variable that is initialized with the current |
| process ID.) The other kind of initialization is <dfn>static</dfn> |
| initialization. The two aren't quite opposites, though: static |
| initialization <em>always</em> happens to objects with static storage duration |
| (initializing the object either to a given constant or to a representation |
| consisting of all bytes set to zero), whereas dynamic initialization happens |
| after that, if required.</p> |
| |
| <p class="pros"></p> |
| <p>Global and static variables are very useful for a large number of |
| applications: named constants, auxiliary data structures internal to some |
| translation unit, command-line flags, logging, registration mechanisms, |
| background infrastructure, etc.</p> |
| |
| <p class="cons"></p> |
| <p>Global and static variables that use dynamic initialization or have |
| non-trivial destructors create complexity that can easily lead to hard-to-find |
| bugs. Dynamic initialization is not ordered across translation units, and |
| neither is destruction (except that destruction |
| happens in reverse order of initialization). When one initialization refers to |
| another variable with static storage duration, it is possible that this causes |
| an object to be accessed before its lifetime has begun (or after its lifetime |
| has ended). Moreover, when a program starts threads that are not joined at exit, |
| those threads may attempt to access objects after their lifetime has ended if |
| their destructor has already run.</p> |
| |
| <p class="decision"></p> |
| <h4>Decision on destruction</h4> |
| |
| <p>When destructors are trivial, their execution is not subject to ordering at |
| all (they are effectively not "run"); otherwise we are exposed to the risk of |
| accessing objects after the end of their lifetime. Therefore, we only allow |
| objects with static storage duration if they are trivially destructible. |
| Fundamental types (like pointers and <code>int</code>) are trivially |
| destructible, as are arrays of trivially destructible types. Note that |
| variables marked with <code>constexpr</code> are trivially destructible.</p> |
| <pre>const int kNum = 10; // allowed |
| |
| struct X { int n; }; |
| const X kX[] = {{1}, {2}, {3}}; // allowed |
| |
| void foo() { |
| static const char* const kMessages[] = {"hello", "world"}; // allowed |
| } |
| |
| // allowed: constexpr guarantees trivial destructor |
| constexpr std::array<int, 3> kArray = {{1, 2, 3}};</pre> |
| <pre class="badcode">// bad: non-trivial destructor |
| const std::string kFoo = "foo"; |
| |
| // bad for the same reason, even though kBar is a reference (the |
| // rule also applies to lifetime-extended temporary objects) |
| const std::string& kBar = StrCat("a", "b", "c"); |
| |
| void bar() { |
| // bad: non-trivial destructor |
| static std::map<int, int> kData = {{1, 0}, {2, 0}, {3, 0}}; |
| }</pre> |
| |
| <p>Note that references are not objects, and thus they are not subject to the |
| constraints on destructibility. The constraint on dynamic initialization still |
| applies, though. In particular, a function-local static reference of the form |
| <code>static T& t = *new T;</code> is allowed.</p> |
| |
| <h4>Decision on initialization</h4> |
| |
| <p>Initialization is a more complex topic. This is because we must not only |
| consider whether class constructors execute, but we must also consider the |
| evaluation of the initializer:</p> |
| <pre class="neutralcode">int n = 5; // fine |
| int m = f(); // ? (depends on f) |
| Foo x; // ? (depends on Foo::Foo) |
| Bar y = g(); // ? (depends on g and on Bar::Bar) |
| </pre> |
| |
| <p>All but the first statement expose us to indeterminate initialization |
| ordering.</p> |
| |
| <p>The concept we are looking for is called <em>constant initialization</em> in |
| the formal language of the C++ standard. It means that the initializing |
| expression is a constant expression, and if the object is initialized by a |
| constructor call, then the constructor must be specified as |
| <code>constexpr</code>, too:</p> |
| <pre>struct Foo { constexpr Foo(int) {} }; |
| |
| int n = 5; // fine, 5 is a constant expression |
| Foo x(2); // fine, 2 is a constant expression and the chosen constructor is constexpr |
| Foo a[] = { Foo(1), Foo(2), Foo(3) }; // fine</pre> |
| |
| <p>Constant initialization is always allowed. Constant initialization of |
| static storage duration variables should be marked with <code>constexpr</code> |
| or where possible the |
| |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/03c1513538584f4a04d666be5eb469e3979febba/absl/base/attributes.h#L540"> |
| <code>ABSL_CONST_INIT</code></a> |
| attribute. Any non-local static storage |
| duration variable that is not so marked should be presumed to have |
| dynamic initialization, and reviewed very carefully.</p> |
| |
| <p>By contrast, the following initializations are problematic:</p> |
| |
| <pre class="badcode">// Some declarations used below. |
| time_t time(time_t*); // not constexpr! |
| int f(); // not constexpr! |
| struct Bar { Bar() {} }; |
| |
| // Problematic initializations. |
| time_t m = time(nullptr); // initializing expression not a constant expression |
| Foo y(f()); // ditto |
| Bar b; // chosen constructor Bar::Bar() not constexpr</pre> |
| |
| <p>Dynamic initialization of nonlocal variables is discouraged, and in general |
| it is forbidden. However, we do permit it if no aspect of the program depends |
| on the sequencing of this initialization with respect to all other |
| initializations. Under those restrictions, the ordering of the initialization |
| does not make an observable difference. For example:</p> |
| <pre>int p = getpid(); // allowed, as long as no other static variable |
| // uses p in its own initialization</pre> |
| |
| <p>Dynamic initialization of static local variables is allowed (and common).</p> |
| |
| |
| |
| <h4>Common patterns</h4> |
| |
| <ul> |
| <li>Global strings: if you require a global or static string constant, |
| consider using a simple character array, or a char pointer to the first |
| element of a string literal. String literals have static storage duration |
| already and are usually sufficient.</li> |
| <li>Maps, sets, and other dynamic containers: if you require a static, fixed |
| collection, such as a set to search against or a lookup table, you cannot |
| use the dynamic containers from the standard library as a static variable, |
| since they have non-trivial destructors. Instead, consider a simple array of |
| trivial types, e.g. an array of arrays of ints (for a "map from int to |
| int"), or an array of pairs (e.g. pairs of <code>int</code> and <code>const |
| char*</code>). For small collections, linear search is entirely sufficient |
| (and efficient, due to memory locality); consider using the facilities from |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/algorithm/container.h">absl/algorithm/container.h</a> |
| |
| |
| for the standard operations. If necessary, keep the collection in sorted |
| order and use a binary search algorithm. If you do really prefer a dynamic |
| container from the standard library, consider using a function-local static |
| pointer, as described below.</li> |
| <li>Smart pointers (<code>unique_ptr</code>, <code>shared_ptr</code>): smart |
| pointers execute cleanup during destruction and are therefore forbidden. |
| Consider whether your use case fits into one of the other patterns described |
| in this section. One simple solution is to use a plain pointer to a |
| dynamically allocated object and never delete it (see last item).</li> |
| <li>Static variables of custom types: if you require static, constant data of |
| a type that you need to define yourself, give the type a trivial destructor |
| and a <code>constexpr</code> constructor.</li> |
| <li>If all else fails, you can create an object dynamically and never delete |
| it by using a function-local static pointer or reference (e.g. <code>static |
| const auto& impl = *new T(args...);</code>).</li> |
| </ul> |
| |
| <h3 id="thread_local">thread_local Variables</h3> |
| |
| <p><code>thread_local</code> variables that aren't declared inside a function |
| must be initialized with a true compile-time constant, |
| and this must be enforced by using the |
| |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/base/attributes.h"> |
| <code>ABSL_CONST_INIT</code></a> |
| attribute. Prefer |
| <code>thread_local</code> over other ways of defining thread-local data.</p> |
| |
| <p class="definition"></p> |
| <p>Starting with C++11, variables can be declared with the |
| <code>thread_local</code> specifier:</p> |
| <pre>thread_local Foo foo = ...; |
| </pre> |
| <p>Such a variable is actually a collection of objects, so that when different |
| threads access it, they are actually accessing different objects. |
| <code>thread_local</code> variables are much like |
| <a href="#Static_and_Global_Variables">static storage duration variables</a> |
| in many respects. For instance, they can be declared at namespace scope, |
| inside functions, or as static class members, but not as ordinary class |
| members.</p> |
| |
| <p><code>thread_local</code> variable instances are initialized much like |
| static variables, except that they must be initialized separately for each |
| thread, rather than once at program startup. This means that |
| <code>thread_local</code> variables declared within a function are safe, but |
| other <code>thread_local</code> variables are subject to the same |
| initialization-order issues as static variables (and more besides).</p> |
| |
| <p><code>thread_local</code> variable instances are destroyed when their thread |
| terminates, so they do not have the destruction-order issues of static |
| variables.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Thread-local data is inherently safe from races (because only one thread |
| can ordinarily access it), which makes <code>thread_local</code> useful for |
| concurrent programming.</li> |
| <li><code>thread_local</code> is the only standard-supported way of creating |
| thread-local data.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Accessing a <code>thread_local</code> variable may trigger execution of |
| an unpredictable and uncontrollable amount of other code.</li> |
| <li><code>thread_local</code> variables are effectively global variables, |
| and have all the drawbacks of global variables other than lack of |
| thread-safety.</li> |
| <li>The memory consumed by a <code>thread_local</code> variable scales with |
| the number of running threads (in the worst case), which can be quite large |
| in a program.</li> |
| <li>An ordinary class member cannot be <code>thread_local</code>.</li> |
| <li><code>thread_local</code> may not be as efficient as certain compiler |
| intrinsics.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p><code>thread_local</code> variables inside a function have no safety |
| concerns, so they can be used without restriction. Note that you can use |
| a function-scope <code>thread_local</code> to simulate a class- or |
| namespace-scope <code>thread_local</code> by defining a function or |
| static method that exposes it:</p> |
| |
| <pre>Foo& MyThreadLocalFoo() { |
| thread_local Foo result = ComplicatedInitialization(); |
| return result; |
| } |
| </pre> |
| |
| <p><code>thread_local</code> variables at class or namespace scope must be |
| initialized with a true compile-time constant (i.e. they must have no |
| dynamic initialization). To enforce this, <code>thread_local</code> variables |
| at class or namespace scope must be annotated with |
| |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/base/attributes.h"> |
| <code>ABSL_CONST_INIT</code></a> |
| (or <code>constexpr</code>, but that should be rare):</p> |
| |
| <pre>ABSL_CONST_INIT thread_local Foo foo = ...; |
| </pre> |
| |
| <p><code>thread_local</code> should be preferred over other mechanisms for |
| defining thread-local data.</p> |
| |
| <h2 id="Classes">Classes</h2> |
| |
| <p>Classes are the fundamental unit of code in C++. Naturally, |
| we use them extensively. This section lists the main dos and |
| don'ts you should follow when writing a class.</p> |
| |
| <h3 id="Doing_Work_in_Constructors">Doing Work in Constructors</h3> |
| |
| <p>Avoid virtual method calls in constructors, and avoid |
| initialization that can fail if you can't signal an error.</p> |
| |
| <p class="definition"></p> |
| <p>It is possible to perform arbitrary initialization in the body |
| of the constructor.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>No need to worry about whether the class has been initialized or |
| not.</li> |
| |
| <li>Objects that are fully initialized by constructor call can |
| be <code>const</code> and may also be easier to use with standard containers |
| or algorithms.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>If the work calls virtual functions, these calls |
| will not get dispatched to the subclass |
| implementations. Future modification to your class can |
| quietly introduce this problem even if your class is |
| not currently subclassed, causing much confusion.</li> |
| |
| <li>There is no easy way for constructors to signal errors, short of |
| crashing the program (not always appropriate) or using exceptions |
| (which are <a href="#Exceptions">forbidden</a>).</li> |
| |
| <li>If the work fails, we now have an object whose initialization |
| code failed, so it may be an unusual state requiring a <code>bool |
| IsValid()</code> state checking mechanism (or similar) which is easy |
| to forget to call.</li> |
| |
| <li>You cannot take the address of a constructor, so whatever work |
| is done in the constructor cannot easily be handed off to, for |
| example, another thread.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Constructors should never call virtual functions. If appropriate |
| for your code , |
| terminating the program may be an appropriate error handling |
| response. Otherwise, consider a factory function |
| or <code>Init()</code> method as described in |
| <a href="https://abseil.io/tips/42">TotW #42</a> |
| . |
| Avoid <code>Init()</code> methods on objects with |
| no other states that affect which public methods may be called |
| (semi-constructed objects of this form are particularly hard to work |
| with correctly).</p> |
| |
| <a id="Explicit_Constructors"></a> |
| <h3 id="Implicit_Conversions">Implicit Conversions</h3> |
| |
| <p>Do not define implicit conversions. Use the <code>explicit</code> |
| keyword for conversion operators and single-argument |
| constructors.</p> |
| |
| <p class="definition"></p> |
| <p>Implicit conversions allow an |
| object of one type (called the <dfn>source type</dfn>) to |
| be used where a different type (called the <dfn>destination |
| type</dfn>) is expected, such as when passing an |
| <code>int</code> argument to a function that takes a |
| <code>double</code> parameter.</p> |
| |
| <p>In addition to the implicit conversions defined by the language, |
| users can define their own, by adding appropriate members to the |
| class definition of the source or destination type. An implicit |
| conversion in the source type is defined by a type conversion operator |
| named after the destination type (e.g. <code>operator |
| bool()</code>). An implicit conversion in the destination |
| type is defined by a constructor that can take the source type as |
| its only argument (or only argument with no default value).</p> |
| |
| <p>The <code>explicit</code> keyword can be applied to a constructor |
| or (since C++11) a conversion operator, to ensure that it can only be |
| used when the destination type is explicit at the point of use, |
| e.g. with a cast. This applies not only to implicit conversions, but to |
| C++11's list initialization syntax:</p> |
| <pre>class Foo { |
| explicit Foo(int x, double y); |
| ... |
| }; |
| |
| void Func(Foo f); |
| </pre> |
| <pre class="badcode">Func({42, 3.14}); // Error |
| </pre> |
| This kind of code isn't technically an implicit conversion, but the |
| language treats it as one as far as <code>explicit</code> is concerned. |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Implicit conversions can make a type more usable and |
| expressive by eliminating the need to explicitly name a type |
| when it's obvious.</li> |
| <li>Implicit conversions can be a simpler alternative to |
| overloading, such as when a single |
| function with a <code>string_view</code> parameter takes the |
| place of separate overloads for <code>std::string</code> and |
| <code>const char*</code>.</li> |
| <li>List initialization syntax is a concise and expressive |
| way of initializing objects.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Implicit conversions can hide type-mismatch bugs, where the |
| destination type does not match the user's expectation, or |
| the user is unaware that any conversion will take place.</li> |
| |
| <li>Implicit conversions can make code harder to read, particularly |
| in the presence of overloading, by making it less obvious what |
| code is actually getting called.</li> |
| |
| <li>Constructors that take a single argument may accidentally |
| be usable as implicit type conversions, even if they are not |
| intended to do so.</li> |
| |
| <li>When a single-argument constructor is not marked |
| <code>explicit</code>, there's no reliable way to tell whether |
| it's intended to define an implicit conversion, or the author |
| simply forgot to mark it.</li> |
| |
| <li>It's not always clear which type should provide the conversion, |
| and if they both do, the code becomes ambiguous.</li> |
| |
| <li>List initialization can suffer from the same problems if |
| the destination type is implicit, particularly if the |
| list has only a single element.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Type conversion operators, and constructors that are |
| callable with a single argument, must be marked |
| <code>explicit</code> in the class definition. As an |
| exception, copy and move constructors should not be |
| <code>explicit</code>, since they do not perform type |
| conversion. Implicit conversions can sometimes be necessary and |
| appropriate for types that are designed to transparently wrap other |
| types. In that case, contact |
| your project leads to request |
| a waiver of this rule.</p> |
| |
| <p>Constructors that cannot be called with a single argument |
| may omit <code>explicit</code>. Constructors that |
| take a single <code>std::initializer_list</code> parameter should |
| also omit <code>explicit</code>, in order to support copy-initialization |
| (e.g. <code>MyType m = {1, 2};</code>).</p> |
| |
| <h3 id="Copyable_Movable_Types">Copyable and Movable Types</h3> |
| <a id="Copy_Constructors"></a> |
| |
| <p>A class's public API must make clear whether the class is copyable, |
| move-only, or neither copyable nor movable. Support copying and/or |
| moving if these operations are clear and meaningful for your type.</p> |
| |
| <p class="definition"></p> |
| <p>A movable type is one that can be initialized and assigned |
| from temporaries.</p> |
| |
| <p>A copyable type is one that can be initialized or assigned from |
| any other object of the same type (so is also movable by definition), with the |
| stipulation that the value of the source does not change. |
| <code>std::unique_ptr<int></code> is an example of a movable but not |
| copyable type (since the value of the source |
| <code>std::unique_ptr<int></code> must be modified during assignment to |
| the destination). <code>int</code> and <code>std::string</code> are examples of |
| movable types that are also copyable. (For <code>int</code>, the move and copy |
| operations are the same; for <code>std::string</code>, there exists a move operation |
| that is less expensive than a copy.)</p> |
| |
| <p>For user-defined types, the copy behavior is defined by the copy |
| constructor and the copy-assignment operator. Move behavior is defined by the |
| move constructor and the move-assignment operator, if they exist, or by the |
| copy constructor and the copy-assignment operator otherwise.</p> |
| |
| <p>The copy/move constructors can be implicitly invoked by the compiler |
| in some situations, e.g. when passing objects by value.</p> |
| |
| <p class="pros"></p> |
| <p>Objects of copyable and movable types can be passed and returned by value, |
| which makes APIs simpler, safer, and more general. Unlike when passing objects |
| by pointer or reference, there's no risk of confusion over ownership, |
| lifetime, mutability, and similar issues, and no need to specify them in the |
| contract. It also prevents non-local interactions between the client and the |
| implementation, which makes them easier to understand, maintain, and optimize by |
| the compiler. Further, such objects can be used with generic APIs that |
| require pass-by-value, such as most containers, and they allow for additional |
| flexibility in e.g., type composition.</p> |
| |
| <p>Copy/move constructors and assignment operators are usually |
| easier to define correctly than alternatives |
| like <code>Clone()</code>, <code>CopyFrom()</code> or <code>Swap()</code>, |
| because they can be generated by the compiler, either implicitly or |
| with <code>= default</code>. They are concise, and ensure |
| that all data members are copied. Copy and move |
| constructors are also generally more efficient, because they don't |
| require heap allocation or separate initialization and assignment |
| steps, and they're eligible for optimizations such as |
| |
| <a href="http://en.cppreference.com/w/cpp/language/copy_elision"> |
| copy elision</a>.</p> |
| |
| <p>Move operations allow the implicit and efficient transfer of |
| resources out of rvalue objects. This allows a plainer coding style |
| in some cases.</p> |
| |
| <p class="cons"></p> |
| <p>Some types do not need to be copyable, and providing copy |
| operations for such types can be confusing, nonsensical, or outright |
| incorrect. Types representing singleton objects (<code>Registerer</code>), |
| objects tied to a specific scope (<code>Cleanup</code>), or closely coupled to |
| object identity (<code>Mutex</code>) cannot be copied meaningfully. |
| Copy operations for base class types that are to be used |
| polymorphically are hazardous, because use of them can lead to |
| <a href="https://en.wikipedia.org/wiki/Object_slicing">object slicing</a>. |
| Defaulted or carelessly-implemented copy operations can be incorrect, and the |
| resulting bugs can be confusing and difficult to diagnose.</p> |
| |
| <p>Copy constructors are invoked implicitly, which makes the |
| invocation easy to miss. This may cause confusion for programmers used to |
| languages where pass-by-reference is conventional or mandatory. It may also |
| encourage excessive copying, which can cause performance problems.</p> |
| |
| <p class="decision"></p> |
| |
| <p>Every class's public interface must make clear which copy and move |
| operations the class supports. This should usually take the form of explicitly |
| declaring and/or deleting the appropriate operations in the <code>public</code> |
| section of the declaration.</p> |
| |
| <p>Specifically, a copyable class should explicitly declare the copy |
| operations, a move-only class should explicitly declare the move operations, |
| and a non-copyable/movable class should explicitly delete the copy operations. |
| Explicitly declaring or deleting all four copy/move operations is permitted, |
| but not required. If you provide a copy or move assignment operator, you |
| must also provide the corresponding constructor.</p> |
| |
| <pre>class Copyable { |
| public: |
| Copyable(const Copyable& other) = default; |
| Copyable& operator=(const Copyable& other) = default; |
| |
| // The implicit move operations are suppressed by the declarations above. |
| }; |
| |
| class MoveOnly { |
| public: |
| MoveOnly(MoveOnly&& other); |
| MoveOnly& operator=(MoveOnly&& other); |
| |
| // The copy operations are implicitly deleted, but you can |
| // spell that out explicitly if you want: |
| MoveOnly(const MoveOnly&) = delete; |
| MoveOnly& operator=(const MoveOnly&) = delete; |
| }; |
| |
| class NotCopyableOrMovable { |
| public: |
| // Not copyable or movable |
| NotCopyableOrMovable(const NotCopyableOrMovable&) = delete; |
| NotCopyableOrMovable& operator=(const NotCopyableOrMovable&) |
| = delete; |
| |
| // The move operations are implicitly disabled, but you can |
| // spell that out explicitly if you want: |
| NotCopyableOrMovable(NotCopyableOrMovable&&) = delete; |
| NotCopyableOrMovable& operator=(NotCopyableOrMovable&&) |
| = delete; |
| }; |
| </pre> |
| |
| <p>These declarations/deletions can be omitted only if they are obvious: |
| </p><ul> |
| <li>If the class has no <code>private</code> section, like a |
| <a href="#Structs_vs._Classes">struct</a> or an interface-only base class, |
| then the copyability/movability can be determined by the |
| copyability/movability of any public data members. |
| </li><li>If a base class clearly isn't copyable or movable, derived classes |
| naturally won't be either. An interface-only base class that leaves these |
| operations implicit is not sufficient to make concrete subclasses clear. |
| </li><li>Note that if you explicitly declare or delete either the constructor or |
| assignment operation for copy, the other copy operation is not obvious and |
| must be declared or deleted. Likewise for move operations. |
| </li></ul> |
| |
| <p>A type should not be copyable/movable if the meaning of |
| copying/moving is unclear to a casual user, or if it incurs unexpected |
| costs. Move operations for copyable types are strictly a performance |
| optimization and are a potential source of bugs and complexity, so |
| avoid defining them unless they are significantly more efficient than |
| the corresponding copy operations. If your type provides copy operations, it is |
| recommended that you design your class so that the default implementation of |
| those operations is correct. Remember to review the correctness of any |
| defaulted operations as you would any other code.</p> |
| |
| <p>Due to the risk of slicing, prefer to avoid providing a public assignment |
| operator or copy/move constructor for a class that's |
| intended to be derived from (and prefer to avoid deriving from a class |
| with such members). If your base class needs to be |
| copyable, provide a public virtual <code>Clone()</code> |
| method, and a protected copy constructor that derived classes |
| can use to implement it.</p> |
| |
| |
| |
| <h3 id="Structs_vs._Classes">Structs vs. Classes</h3> |
| |
| <p>Use a <code>struct</code> only for passive objects that |
| carry data; everything else is a <code>class</code>.</p> |
| |
| <p>The <code>struct</code> and <code>class</code> |
| keywords behave almost identically in C++. We add our own |
| semantic meanings to each keyword, so you should use the |
| appropriate keyword for the data-type you're |
| defining.</p> |
| |
| <p><code>structs</code> should be used for passive objects that carry |
| data, and may have associated constants, but lack any functionality |
| other than access/setting the data members. All fields must be public, |
| and accessed directly rather than through getter/setter methods. The |
| struct must not have invariants that imply relationships between |
| different fields, since direct user access to those fields may break |
| those invariants. Methods should not provide behavior but should only |
| be used to set up the data members, e.g., constructor, destructor, |
| <code>Initialize()</code>, <code>Reset()</code>.</p> |
| |
| <p>If more functionality or invariants are required, a |
| <code>class</code> is more appropriate. If in doubt, make |
| it a <code>class</code>.</p> |
| |
| <p>For consistency with STL, you can use |
| <code>struct</code> instead of <code>class</code> for |
| stateless types, such as traits, |
| <a href="#Template_metaprogramming">template metafunctions</a>, |
| and some functors.</p> |
| |
| <p>Note that member variables in structs and classes have |
| <a href="#Variable_Names">different naming rules</a>.</p> |
| |
| <h3 id="Structs_vs._Tuples">Structs vs. Pairs and Tuples</h3> |
| |
| <p>Prefer to use a <code>struct</code> instead of a pair or a |
| tuple whenever the elements can have meaningful names.</p> |
| |
| <p> |
| While using pairs and tuples can avoid the need to define a custom type, |
| potentially saving work when <em>writing</em> code, a meaningful field |
| name will almost always be much clearer when <em>reading</em> code than |
| <code>.first</code>, <code>.second</code>, or <code>std::get<X></code>. |
| While C++14's introduction of <code>std::get<Type></code> to access a |
| tuple element by type rather than index (when the type is unique) can |
| sometimes partially mitigate this, a field name is usually substantially |
| clearer and more informative than a type. |
| </p> |
| |
| <p> |
| Pairs and tuples may be appropriate in generic code where there are not |
| specific meanings for the elements of the pair or tuple. Their use may |
| also be required in order to interoperate with existing code or APIs. |
| </p> |
| |
| <a id="Multiple_Inheritance"></a> |
| <h3 id="Inheritance">Inheritance</h3> |
| |
| <p>Composition is often more appropriate than inheritance. |
| When using inheritance, make it <code>public</code>.</p> |
| |
| <p class="definition"></p> |
| <p> When a sub-class |
| inherits from a base class, it includes the definitions |
| of all the data and operations that the base class |
| defines. "Interface inheritance" is inheritance from a |
| pure abstract base class (one with no state or defined |
| methods); all other inheritance is "implementation |
| inheritance".</p> |
| |
| <p class="pros"></p> |
| <p>Implementation inheritance reduces code size by re-using |
| the base class code as it specializes an existing type. |
| Because inheritance is a compile-time declaration, you |
| and the compiler can understand the operation and detect |
| errors. Interface inheritance can be used to |
| programmatically enforce that a class expose a particular |
| API. Again, the compiler can detect errors, in this case, |
| when a class does not define a necessary method of the |
| API.</p> |
| |
| <p class="cons"></p> |
| <p>For implementation inheritance, because the code |
| implementing a sub-class is spread between the base and |
| the sub-class, it can be more difficult to understand an |
| implementation. The sub-class cannot override functions |
| that are not virtual, so the sub-class cannot change |
| implementation.</p> |
| |
| <p>Multiple inheritance is especially problematic, because |
| it often imposes a higher performance overhead (in fact, |
| the performance drop from single inheritance to multiple |
| inheritance can often be greater than the performance |
| drop from ordinary to virtual dispatch), and because |
| it risks leading to "diamond" inheritance patterns, |
| which are prone to ambiguity, confusion, and outright bugs.</p> |
| |
| <p class="decision"></p> |
| |
| <p>All inheritance should be <code>public</code>. If you |
| want to do private inheritance, you should be including |
| an instance of the base class as a member instead.</p> |
| |
| <p>Do not overuse implementation inheritance. Composition |
| is often more appropriate. Try to restrict use of |
| inheritance to the "is-a" case: <code>Bar</code> |
| subclasses <code>Foo</code> if it can reasonably be said |
| that <code>Bar</code> "is a kind of" |
| <code>Foo</code>.</p> |
| |
| <p>Limit the use of <code>protected</code> to those |
| member functions that might need to be accessed from |
| subclasses. Note that <a href="#Access_Control">data |
| members should be private</a>.</p> |
| |
| <p>Explicitly annotate overrides of virtual functions or virtual |
| destructors with exactly one of an <code>override</code> or (less |
| frequently) <code>final</code> specifier. Do not |
| use <code>virtual</code> when declaring an override. |
| Rationale: A function or destructor marked |
| <code>override</code> or <code>final</code> that is |
| not an override of a base class virtual function will |
| not compile, and this helps catch common errors. The |
| specifiers serve as documentation; if no specifier is |
| present, the reader has to check all ancestors of the |
| class in question to determine if the function or |
| destructor is virtual or not.</p> |
| |
| <p>Multiple inheritance is permitted, but multiple <em>implementation</em> |
| inheritance is strongly discouraged.</p> |
| |
| <h3 id="Operator_Overloading">Operator Overloading</h3> |
| |
| <p>Overload operators judiciously. Do not use user-defined literals.</p> |
| |
| <p class="definition"></p> |
| <p>C++ permits user code to |
| <a href="http://en.cppreference.com/w/cpp/language/operators">declare |
| overloaded versions of the built-in operators</a> using the |
| <code>operator</code> keyword, so long as one of the parameters |
| is a user-defined type. The <code>operator</code> keyword also |
| permits user code to define new kinds of literals using |
| <code>operator""</code>, and to define type-conversion functions |
| such as <code>operator bool()</code>.</p> |
| |
| <p class="pros"></p> |
| <p>Operator overloading can make code more concise and |
| intuitive by enabling user-defined types to behave the same |
| as built-in types. Overloaded operators are the idiomatic names |
| for certain operations (e.g. <code>==</code>, <code><</code>, |
| <code>=</code>, and <code><<</code>), and adhering to |
| those conventions can make user-defined types more readable |
| and enable them to interoperate with libraries that expect |
| those names.</p> |
| |
| <p>User-defined literals are a very concise notation for |
| creating objects of user-defined types.</p> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Providing a correct, consistent, and unsurprising |
| set of operator overloads requires some care, and failure |
| to do so can lead to confusion and bugs.</li> |
| |
| <li>Overuse of operators can lead to obfuscated code, |
| particularly if the overloaded operator's semantics |
| don't follow convention.</li> |
| |
| <li>The hazards of function overloading apply just as |
| much to operator overloading, if not more so.</li> |
| |
| <li>Operator overloads can fool our intuition into |
| thinking that expensive operations are cheap, built-in |
| operations.</li> |
| |
| <li>Finding the call sites for overloaded operators may |
| require a search tool that's aware of C++ syntax, rather |
| than e.g. grep.</li> |
| |
| <li>If you get the argument type of an overloaded operator |
| wrong, you may get a different overload rather than a |
| compiler error. For example, <code>foo < bar</code> |
| may do one thing, while <code>&foo < &bar</code> |
| does something totally different.</li> |
| |
| <li>Certain operator overloads are inherently hazardous. |
| Overloading unary <code>&</code> can cause the same |
| code to have different meanings depending on whether |
| the overload declaration is visible. Overloads of |
| <code>&&</code>, <code>||</code>, and <code>,</code> |
| (comma) cannot match the evaluation-order semantics of the |
| built-in operators.</li> |
| |
| <li>Operators are often defined outside the class, |
| so there's a risk of different files introducing |
| different definitions of the same operator. If both |
| definitions are linked into the same binary, this results |
| in undefined behavior, which can manifest as subtle |
| run-time bugs.</li> |
| |
| <li>User-defined literals (UDLs) allow the creation of new |
| syntactic forms that are unfamiliar even to experienced C++ |
| programmers, such as <code>"Hello World"sv</code> as a |
| shorthand for <code>std::string_view("Hello World")</code>. |
| Existing notations are clearer, though less terse.</li> |
| |
| <li>Because they can't be namespace-qualified, uses of UDLs also require |
| use of either using-directives (which <a href="#Namespaces">we ban</a>) or |
| using-declarations (which <a href="#Aliases">we ban in header files</a> except |
| when the imported names are part of the interface exposed by the header |
| file in question). Given that header files would have to avoid UDL |
| suffixes, we prefer to avoid having conventions for literals differ |
| between header files and source files. |
| </li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Define overloaded operators only if their meaning is |
| obvious, unsurprising, and consistent with the corresponding |
| built-in operators. For example, use <code>|</code> as a |
| bitwise- or logical-or, not as a shell-style pipe.</p> |
| |
| <p>Define operators only on your own types. More precisely, |
| define them in the same headers, .cc files, and namespaces |
| as the types they operate on. That way, the operators are available |
| wherever the type is, minimizing the risk of multiple |
| definitions. If possible, avoid defining operators as templates, |
| because they must satisfy this rule for any possible template |
| arguments. If you define an operator, also define |
| any related operators that make sense, and make sure they |
| are defined consistently. For example, if you overload |
| <code><</code>, overload all the comparison operators, |
| and make sure <code><</code> and <code>></code> never |
| return true for the same arguments.</p> |
| |
| <p>Prefer to define non-modifying binary operators as |
| non-member functions. If a binary operator is defined as a |
| class member, implicit conversions will apply to the |
| right-hand argument, but not the left-hand one. It will |
| confuse your users if <code>a < b</code> compiles but |
| <code>b < a</code> doesn't.</p> |
| |
| <p>Don't go out of your way to avoid defining operator |
| overloads. For example, prefer to define <code>==</code>, |
| <code>=</code>, and <code><<</code>, rather than |
| <code>Equals()</code>, <code>CopyFrom()</code>, and |
| <code>PrintTo()</code>. Conversely, don't define |
| operator overloads just because other libraries expect |
| them. For example, if your type doesn't have a natural |
| ordering, but you want to store it in a <code>std::set</code>, |
| use a custom comparator rather than overloading |
| <code><</code>.</p> |
| |
| <p>Do not overload <code>&&</code>, <code>||</code>, |
| <code>,</code> (comma), or unary <code>&</code>. Do not overload |
| <code>operator""</code>, i.e. do not introduce user-defined |
| literals. Do not use any such literals provided by others |
| (including the standard library).</p> |
| |
| <p>Type conversion operators are covered in the section on |
| <a href="#Implicit_Conversions">implicit conversions</a>. |
| The <code>=</code> operator is covered in the section on |
| <a href="#Copy_Constructors">copy constructors</a>. Overloading |
| <code><<</code> for use with streams is covered in the |
| section on <a href="#Streams">streams</a>. See also the rules on |
| <a href="#Function_Overloading">function overloading</a>, which |
| apply to operator overloading as well.</p> |
| |
| <h3 id="Access_Control">Access Control</h3> |
| |
| <p>Make classes' data members <code>private</code>, unless they are |
| <a href="#Constant_Names">constants</a>. This simplifies reasoning about invariants, at the cost |
| of some easy boilerplate in the form of accessors (usually <code>const</code>) if necessary.</p> |
| |
| <p>For technical |
| reasons, we allow data members of a test fixture class in a .cc file to |
| be <code>protected</code> when using |
| |
| |
| <a href="https://github.com/google/googletest">Google |
| Test</a>).</p> |
| |
| <h3 id="Declaration_Order">Declaration Order</h3> |
| |
| <p>Group similar declarations together, placing public parts |
| earlier.</p> |
| |
| <p>A class definition should usually start with a |
| <code>public:</code> section, followed by |
| <code>protected:</code>, then <code>private:</code>. Omit |
| sections that would be empty.</p> |
| |
| <p>Within each section, generally prefer grouping similar |
| kinds of declarations together, and generally prefer the |
| following order: types (including <code>typedef</code>, |
| <code>using</code>, and nested structs and classes), |
| constants, factory functions, constructors, assignment |
| operators, destructor, all other methods, data members.</p> |
| |
| <p>Do not put large method definitions inline in the |
| class definition. Usually, only trivial or |
| performance-critical, and very short, methods may be |
| defined inline. See <a href="#Inline_Functions">Inline |
| Functions</a> for more details.</p> |
| |
| <h2 id="Functions">Functions</h2> |
| |
| <a id="Function_Parameter_Ordering"></a> |
| <h3 id="Output_Parameters">Output Parameters</h3> |
| |
| <p>The output of a C++ function is naturally provided via |
| a return value and sometimes via output parameters.</p> |
| |
| <p>Prefer using return values over output parameters: they |
| improve readability, and often provide the same or better |
| performance. If output-only parameters are used, |
| they should appear after input parameters.</p> |
| |
| <p>Parameters are either input to the function, output from the |
| function, or both. Input parameters are usually values or |
| <code>const</code> references, while output and input/output |
| parameters will be pointers to non-<code>const</code>.</p> |
| |
| <p>When ordering function parameters, put all input-only |
| parameters before any output parameters. In particular, |
| do not add new parameters to the end of the function just |
| because they are new; place new input-only parameters before |
| the output parameters.</p> |
| |
| <p>This is not a hard-and-fast rule. Parameters that are |
| both input and output (often classes/structs) muddy the |
| waters, and, as always, consistency with related |
| functions may require you to bend the rule.</p> |
| |
| <h3 id="Write_Short_Functions">Write Short Functions</h3> |
| |
| <p>Prefer small and focused functions.</p> |
| |
| <p>We recognize that long functions are sometimes |
| appropriate, so no hard limit is placed on functions |
| length. If a function exceeds about 40 lines, think about |
| whether it can be broken up without harming the structure |
| of the program.</p> |
| |
| <p>Even if your long function works perfectly now, |
| someone modifying it in a few months may add new |
| behavior. This could result in bugs that are hard to |
| find. Keeping your functions short and simple makes it |
| easier for other people to read and modify your code. |
| Small functions are also easier to test.</p> |
| |
| <p>You could find long and complicated functions when |
| working with |
| some code. Do not be |
| intimidated by modifying existing code: if working with |
| such a function proves to be difficult, you find that |
| errors are hard to debug, or you want to use a piece of |
| it in several different contexts, consider breaking up |
| the function into smaller and more manageable pieces.</p> |
| |
| <h3 id="Reference_Arguments">Reference Arguments</h3> |
| |
| <p>All parameters passed by lvalue reference must be labeled |
| <code>const</code>.</p> |
| |
| <p class="definition"></p> |
| <p>In C, if a |
| function needs to modify a variable, the parameter must |
| use a pointer, eg <code>int foo(int *pval)</code>. In |
| C++, the function can alternatively declare a reference |
| parameter: <code>int foo(int &val)</code>.</p> |
| |
| <p class="pros"></p> |
| <p>Defining a parameter as reference avoids ugly code like |
| <code>(*pval)++</code>. Necessary for some applications |
| like copy constructors. Makes it clear, unlike with |
| pointers, that a null pointer is not a possible |
| value.</p> |
| |
| <p class="cons"></p> |
| <p>References can be confusing, as they have value syntax |
| but pointer semantics.</p> |
| |
| <p class="decision"></p> |
| <p>Within function parameter lists all references must be |
| <code>const</code>:</p> |
| |
| <pre>void Foo(const std::string &in, std::string *out); |
| </pre> |
| |
| <p>In fact it is a very strong convention in Google code |
| that input arguments are values or <code>const</code> |
| references while output arguments are pointers. Input |
| parameters may be <code>const</code> pointers, but we |
| never allow non-<code>const</code> reference parameters |
| except when required by convention, e.g., |
| <code>swap()</code>.</p> |
| |
| <p>However, there are some instances where using |
| <code>const T*</code> is preferable to <code>const |
| T&</code> for input parameters. For example:</p> |
| |
| <ul> |
| <li>You want to pass in a null pointer.</li> |
| |
| <li>The function saves a pointer or reference to the |
| input.</li> |
| </ul> |
| |
| <p> Remember that most of the time input |
| parameters are going to be specified as <code>const |
| T&</code>. Using <code>const T*</code> instead |
| communicates to the reader that the input is somehow |
| treated differently. So if you choose <code>const |
| T*</code> rather than <code>const T&</code>, do so |
| for a concrete reason; otherwise it will likely confuse |
| readers by making them look for an explanation that |
| doesn't exist.</p> |
| |
| <h3 id="Function_Overloading">Function Overloading</h3> |
| |
| <p>Use overloaded functions (including constructors) only if a |
| reader looking at a call site can get a good idea of what |
| is happening without having to first figure out exactly |
| which overload is being called.</p> |
| |
| <p class="definition"></p> |
| <p>You may write a function that takes a <code>const |
| std::string&</code> and overload it with another that |
| takes <code>const char*</code>. However, in this case consider |
| std::string_view |
| instead.</p> |
| |
| <pre>class MyClass { |
| public: |
| void Analyze(const std::string &text); |
| void Analyze(const char *text, size_t textlen); |
| }; |
| </pre> |
| |
| <p class="pros"></p> |
| <p>Overloading can make code more intuitive by allowing an |
| identically-named function to take different arguments. |
| It may be necessary for templatized code, and it can be |
| convenient for Visitors.</p> |
| <p>Overloading based on const or ref qualification may make utility |
| code more usable, more efficient, or both. |
| (See <a href="http://abseil.io/tips/148">TotW 148</a> for more.) |
| </p> |
| |
| <p class="cons"></p> |
| <p>If a function is overloaded by the argument types alone, |
| a reader may have to understand C++'s complex matching |
| rules in order to tell what's going on. Also many people |
| are confused by the semantics of inheritance if a derived |
| class overrides only some of the variants of a |
| function.</p> |
| |
| <p class="decision"></p> |
| <p>You may overload a function when there are no semantic differences |
| between variants. These overloads may vary in types, qualifiers, or |
| argument count. However, a reader of such a call must not need to know |
| which member of the overload set is chosen, only that <b>something</b> |
| from the set is being called. If you can document all entries in the |
| overload set with a single comment in the header, that is a good sign |
| that it is a well-designed overload set.</p> |
| |
| <h3 id="Default_Arguments">Default Arguments</h3> |
| |
| <p>Default arguments are allowed on non-virtual functions |
| when the default is guaranteed to always have the same |
| value. Follow the same restrictions as for <a href="#Function_Overloading">function overloading</a>, and |
| prefer overloaded functions if the readability gained with |
| default arguments doesn't outweigh the downsides below.</p> |
| |
| <p class="pros"></p> |
| <p>Often you have a function that uses default values, but |
| occasionally you want to override the defaults. Default |
| parameters allow an easy way to do this without having to |
| define many functions for the rare exceptions. Compared |
| to overloading the function, default arguments have a |
| cleaner syntax, with less boilerplate and a clearer |
| distinction between 'required' and 'optional' |
| arguments.</p> |
| |
| <p class="cons"></p> |
| <p>Defaulted arguments are another way to achieve the |
| semantics of overloaded functions, so all the <a href="#Function_Overloading">reasons not to overload |
| functions</a> apply.</p> |
| |
| <p>The defaults for arguments in a virtual function call are |
| determined by the static type of the target object, and |
| there's no guarantee that all overrides of a given function |
| declare the same defaults.</p> |
| |
| <p>Default parameters are re-evaluated at each call site, |
| which can bloat the generated code. Readers may also expect |
| the default's value to be fixed at the declaration instead |
| of varying at each call.</p> |
| |
| <p>Function pointers are confusing in the presence of |
| default arguments, since the function signature often |
| doesn't match the call signature. Adding |
| function overloads avoids these problems.</p> |
| |
| <p class="decision"></p> |
| <p>Default arguments are banned on virtual functions, where |
| they don't work properly, and in cases where the specified |
| default might not evaluate to the same value depending on |
| when it was evaluated. (For example, don't write <code>void |
| f(int n = counter++);</code>.)</p> |
| |
| <p>In some other cases, default arguments can improve the |
| readability of their function declarations enough to |
| overcome the downsides above, so they are allowed. When in |
| doubt, use overloads.</p> |
| |
| <h3 id="trailing_return">Trailing Return Type Syntax</h3> |
| |
| <p>Use trailing return types only where using the ordinary syntax (leading |
| return types) is impractical or much less readable.</p> |
| |
| <p class="definition"></p> |
| <p>C++ allows two different forms of function declarations. In the older |
| form, the return type appears before the function name. For example:</p> |
| <pre>int foo(int x); |
| </pre> |
| <p>The newer form, introduced in C++11, uses the <code>auto</code> |
| keyword before the function name and a trailing return type after |
| the argument list. For example, the declaration above could |
| equivalently be written:</p> |
| <pre>auto foo(int x) -> int; |
| </pre> |
| <p>The trailing return type is in the function's scope. This doesn't |
| make a difference for a simple case like <code>int</code> but it matters |
| for more complicated cases, like types declared in class scope or |
| types written in terms of the function parameters.</p> |
| |
| <p class="pros"></p> |
| <p>Trailing return types are the only way to explicitly specify the |
| return type of a <a href="#Lambda_expressions">lambda expression</a>. |
| In some cases the compiler is able to deduce a lambda's return type, |
| but not in all cases. Even when the compiler can deduce it automatically, |
| sometimes specifying it explicitly would be clearer for readers. |
| </p> |
| <p>Sometimes it's easier and more readable to specify a return type |
| after the function's parameter list has already appeared. This is |
| particularly true when the return type depends on template parameters. |
| For example:</p> |
| <pre> template <typename T, typename U> |
| auto add(T t, U u) -> decltype(t + u); |
| </pre> |
| versus |
| <pre> template <typename T, typename U> |
| decltype(declval<T&>() + declval<U&>()) add(T t, U u); |
| </pre> |
| |
| <p class="cons"></p> |
| <p>Trailing return type syntax is relatively new and it has no |
| analogue in C++-like languages such as C and Java, so some readers may |
| find it unfamiliar.</p> |
| <p>Existing code bases have an enormous number of function |
| declarations that aren't going to get changed to use the new syntax, |
| so the realistic choices are using the old syntax only or using a mixture |
| of the two. Using a single version is better for uniformity of style.</p> |
| |
| <p class="decision"></p> |
| <p>In most cases, continue to use the older style of function |
| declaration where the return type goes before the function name. |
| Use the new trailing-return-type form only in cases where it's |
| required (such as lambdas) or where, by putting the type after the |
| function's parameter list, it allows you to write the type in a much |
| more readable way. The latter case should be rare; it's mostly an |
| issue in fairly complicated template code, which is |
| <a href="#Template_metaprogramming">discouraged in most cases</a>.</p> |
| |
| |
| <h2 id="Google-Specific_Magic">Google-Specific Magic</h2> |
| |
| |
| |
| <div> |
| <p>There are various tricks and utilities that |
| we use to make C++ code more robust, and various ways we use |
| C++ that may differ from what you see elsewhere.</p> |
| </div> |
| |
| |
| |
| <h3 id="Ownership_and_Smart_Pointers">Ownership and Smart Pointers</h3> |
| |
| <p>Prefer to have single, fixed owners for dynamically |
| allocated objects. Prefer to transfer ownership with smart |
| pointers.</p> |
| |
| <p class="definition"></p> |
| <p>"Ownership" is a bookkeeping technique for managing |
| dynamically allocated memory (and other resources). The |
| owner of a dynamically allocated object is an object or |
| function that is responsible for ensuring that it is |
| deleted when no longer needed. Ownership can sometimes be |
| shared, in which case the last owner is typically |
| responsible for deleting it. Even when ownership is not |
| shared, it can be transferred from one piece of code to |
| another.</p> |
| |
| <p>"Smart" pointers are classes that act like pointers, |
| e.g. by overloading the <code>*</code> and |
| <code>-></code> operators. Some smart pointer types |
| can be used to automate ownership bookkeeping, to ensure |
| these responsibilities are met. |
| <a href="http://en.cppreference.com/w/cpp/memory/unique_ptr"> |
| <code>std::unique_ptr</code></a> is a smart pointer type |
| introduced in C++11, which expresses exclusive ownership |
| of a dynamically allocated object; the object is deleted |
| when the <code>std::unique_ptr</code> goes out of scope. |
| It cannot be copied, but can be <em>moved</em> to |
| represent ownership transfer. |
| <a href="http://en.cppreference.com/w/cpp/memory/shared_ptr"> |
| <code>std::shared_ptr</code></a> is a smart pointer type |
| that expresses shared ownership of |
| a dynamically allocated object. <code>std::shared_ptr</code>s |
| can be copied; ownership of the object is shared among |
| all copies, and the object is deleted when the last |
| <code>std::shared_ptr</code> is destroyed. </p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>It's virtually impossible to manage dynamically |
| allocated memory without some sort of ownership |
| logic.</li> |
| |
| <li>Transferring ownership of an object can be cheaper |
| than copying it (if copying it is even possible).</li> |
| |
| <li>Transferring ownership can be simpler than |
| 'borrowing' a pointer or reference, because it reduces |
| the need to coordinate the lifetime of the object |
| between the two users.</li> |
| |
| <li>Smart pointers can improve readability by making |
| ownership logic explicit, self-documenting, and |
| unambiguous.</li> |
| |
| <li>Smart pointers can eliminate manual ownership |
| bookkeeping, simplifying the code and ruling out large |
| classes of errors.</li> |
| |
| <li>For const objects, shared ownership can be a simple |
| and efficient alternative to deep copying.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Ownership must be represented and transferred via |
| pointers (whether smart or plain). Pointer semantics |
| are more complicated than value semantics, especially |
| in APIs: you have to worry not just about ownership, |
| but also aliasing, lifetime, and mutability, among |
| other issues.</li> |
| |
| <li>The performance costs of value semantics are often |
| overestimated, so the performance benefits of ownership |
| transfer might not justify the readability and |
| complexity costs.</li> |
| |
| <li>APIs that transfer ownership force their clients |
| into a single memory management model.</li> |
| |
| <li>Code using smart pointers is less explicit about |
| where the resource releases take place.</li> |
| |
| <li><code>std::unique_ptr</code> expresses ownership |
| transfer using C++11's move semantics, which are |
| relatively new and may confuse some programmers.</li> |
| |
| <li>Shared ownership can be a tempting alternative to |
| careful ownership design, obfuscating the design of a |
| system.</li> |
| |
| <li>Shared ownership requires explicit bookkeeping at |
| run-time, which can be costly.</li> |
| |
| <li>In some cases (e.g. cyclic references), objects |
| with shared ownership may never be deleted.</li> |
| |
| <li>Smart pointers are not perfect substitutes for |
| plain pointers.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>If dynamic allocation is necessary, prefer to keep |
| ownership with the code that allocated it. If other code |
| needs access to the object, consider passing it a copy, |
| or passing a pointer or reference without transferring |
| ownership. Prefer to use <code>std::unique_ptr</code> to |
| make ownership transfer explicit. For example:</p> |
| |
| <pre>std::unique_ptr<Foo> FooFactory(); |
| void FooConsumer(std::unique_ptr<Foo> ptr); |
| </pre> |
| |
| |
| |
| <p>Do not design your code to use shared ownership |
| without a very good reason. One such reason is to avoid |
| expensive copy operations, but you should only do this if |
| the performance benefits are significant, and the |
| underlying object is immutable (i.e. |
| <code>std::shared_ptr<const Foo></code>). If you |
| do use shared ownership, prefer to use |
| <code>std::shared_ptr</code>.</p> |
| |
| <p>Never use <code>std::auto_ptr</code>. Instead, use |
| <code>std::unique_ptr</code>.</p> |
| |
| <h3 id="cpplint">cpplint</h3> |
| |
| <p>Use <code>cpplint.py</code> to detect style errors.</p> |
| |
| <p><code>cpplint.py</code> |
| is a tool that reads a source file and identifies many |
| style errors. It is not perfect, and has both false |
| positives and false negatives, but it is still a valuable |
| tool. False positives can be ignored by putting <code>// |
| NOLINT</code> at the end of the line or |
| <code>// NOLINTNEXTLINE</code> in the previous line.</p> |
| |
| |
| |
| <div> |
| <p>Some projects have instructions on |
| how to run <code>cpplint.py</code> from their project |
| tools. If the project you are contributing to does not, |
| you can download |
| <a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/cpplint/cpplint.py"> |
| <code>cpplint.py</code></a> separately.</p> |
| </div> |
| |
| |
| |
| <h2 id="Other_C++_Features">Other C++ Features</h2> |
| |
| <h3 id="Rvalue_references">Rvalue References</h3> |
| |
| <p>Use rvalue references to:</p> |
| <ul> |
| <li>Define move constructors and move assignment operators.</li> |
| |
| <li>Define <a href="#Function_Overloading">overload sets</a> with |
| const& and && variants if you have evidence that this |
| provides meaningfully better performance than passing by value, |
| or if you're writing low-overhead generic code that needs to support |
| arbitrary types. Beware combinatorial overload sets, that is, seldom |
| overload more than one parameter.</li> |
| |
| <li>Support 'perfect forwarding' in generic code.</li> |
| </ul> |
| |
| <p class="definition"></p> |
| <p> Rvalue references |
| are a type of reference that can only bind to temporary |
| objects. The syntax is similar to traditional reference |
| syntax. For example, <code>void f(std::string&& |
| s);</code> declares a function whose argument is an |
| rvalue reference to a std::string.</p> |
| |
| <p id="Forwarding_references"> When the token '&&' is applied to |
| an unqualified template argument in a function |
| parameter, special template argument deduction |
| rules apply. Such a reference is called forwarding reference.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Defining a move constructor (a constructor taking |
| an rvalue reference to the class type) makes it |
| possible to move a value instead of copying it. If |
| <code>v1</code> is a <code>std::vector<std::string></code>, |
| for example, then <code>auto v2(std::move(v1))</code> |
| will probably just result in some simple pointer |
| manipulation instead of copying a large amount of data. |
| In many cases this can result in a major performance |
| improvement.</li> |
| |
| <li>Rvalue references make it possible to implement |
| types that are movable but not copyable, which can be |
| useful for types that have no sensible definition of |
| copying but where you might still want to pass them as |
| function arguments, put them in containers, etc.</li> |
| |
| <li><code>std::move</code> is necessary to make |
| effective use of some standard-library types, such as |
| <code>std::unique_ptr</code>.</li> |
| |
| <li><a href="#Forwarding_references">Forwarding references</a> which |
| use the rvalue reference token, make it possible to write a |
| generic function wrapper that forwards its arguments to |
| another function, and works whether or not its |
| arguments are temporary objects and/or const. |
| This is called 'perfect forwarding'.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Rvalue references are not yet widely understood. Rules like reference |
| collapsing and the special deduction rule for forwarding references |
| are somewhat obscure.</li> |
| |
| <li>Rvalue references are often misused. Using rvalue |
| references is counter-intuitive in signatures where the argument is expected |
| to have a valid specified state after the function call, or where no move |
| operation is performed.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>You may use rvalue references to define move constructors and move |
| assignment operators (as described in |
| <a href="#Copyable_Movable_Types">Copyable and Movable Types</a>). See the |
| <a href="primer#copying_moving">C++ Primer</a> for more information about |
| move semantics and <code>std::move</code>.</p> |
| |
| <p>You may use rvalue references to define pairs of overloads, one taking |
| <code>Foo&&</code> and the other taking <code>const Foo&</code>. |
| Usually the preferred solution is just to pass by value, but an overloaded pair |
| of functions sometimes yields better performance and is sometimes necessary in |
| generic code that needs to support a wide variety of types. As always: if |
| you're writing more complicated code for the sake of performance, make sure you |
| have evidence that it actually helps.</p> |
| |
| <p>You may use forwarding references in conjunction with <code> |
| <a href="http://en.cppreference.com/w/cpp/utility/forward">std::forward</a></code>, |
| to support perfect forwarding.</p> |
| |
| <h3 id="Friends">Friends</h3> |
| |
| <p>We allow use of <code>friend</code> classes and functions, |
| within reason.</p> |
| |
| <p>Friends should usually be defined in the same file so |
| that the reader does not have to look in another file to |
| find uses of the private members of a class. A common use |
| of <code>friend</code> is to have a |
| <code>FooBuilder</code> class be a friend of |
| <code>Foo</code> so that it can construct the inner state |
| of <code>Foo</code> correctly, without exposing this |
| state to the world. In some cases it may be useful to |
| make a unittest class a friend of the class it tests.</p> |
| |
| <p>Friends extend, but do not break, the encapsulation |
| boundary of a class. In some cases this is better than |
| making a member public when you want to give only one |
| other class access to it. However, most classes should |
| interact with other classes solely through their public |
| members.</p> |
| |
| <h3 id="Exceptions">Exceptions</h3> |
| |
| <p>We do not use C++ exceptions.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Exceptions allow higher levels of an application to |
| decide how to handle "can't happen" failures in deeply |
| nested functions, without the obscuring and error-prone |
| bookkeeping of error codes.</li> |
| |
| |
| |
| <div> |
| <li>Exceptions are used by most other |
| modern languages. Using them in C++ would make it more |
| consistent with Python, Java, and the C++ that others |
| are familiar with.</li> |
| </div> |
| |
| <li>Some third-party C++ libraries use exceptions, and |
| turning them off internally makes it harder to |
| integrate with those libraries.</li> |
| |
| <li>Exceptions are the only way for a constructor to |
| fail. We can simulate this with a factory function or |
| an <code>Init()</code> method, but these require heap |
| allocation or a new "invalid" state, respectively.</li> |
| |
| <li>Exceptions are really handy in testing |
| frameworks.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>When you add a <code>throw</code> statement to an |
| existing function, you must examine all of its |
| transitive callers. Either they must make at least the |
| basic exception safety guarantee, or they must never |
| catch the exception and be happy with the program |
| terminating as a result. For instance, if |
| <code>f()</code> calls <code>g()</code> calls |
| <code>h()</code>, and <code>h</code> throws an |
| exception that <code>f</code> catches, <code>g</code> |
| has to be careful or it may not clean up properly.</li> |
| |
| <li>More generally, exceptions make the control flow of |
| programs difficult to evaluate by looking at code: |
| functions may return in places you don't expect. This |
| causes maintainability and debugging difficulties. You |
| can minimize this cost via some rules on how and where |
| exceptions can be used, but at the cost of more that a |
| developer needs to know and understand.</li> |
| |
| <li>Exception safety requires both RAII and different |
| coding practices. Lots of supporting machinery is |
| needed to make writing correct exception-safe code |
| easy. Further, to avoid requiring readers to understand |
| the entire call graph, exception-safe code must isolate |
| logic that writes to persistent state into a "commit" |
| phase. This will have both benefits and costs (perhaps |
| where you're forced to obfuscate code to isolate the |
| commit). Allowing exceptions would force us to always |
| pay those costs even when they're not worth it.</li> |
| |
| <li>Turning on exceptions adds data to each binary |
| produced, increasing compile time (probably slightly) |
| and possibly increasing address space pressure. |
| </li> |
| |
| <li>The availability of exceptions may encourage |
| developers to throw them when they are not appropriate |
| or recover from them when it's not safe to do so. For |
| example, invalid user input should not cause exceptions |
| to be thrown. We would need to make the style guide |
| even longer to document these restrictions!</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>On their face, the benefits of using exceptions |
| outweigh the costs, especially in new projects. However, |
| for existing code, the introduction of exceptions has |
| implications on all dependent code. If exceptions can be |
| propagated beyond a new project, it also becomes |
| problematic to integrate the new project into existing |
| exception-free code. Because most existing C++ code at |
| Google is not prepared to deal with exceptions, it is |
| comparatively difficult to adopt new code that generates |
| exceptions.</p> |
| |
| <p>Given that Google's existing code is not |
| exception-tolerant, the costs of using exceptions are |
| somewhat greater than the costs in a new project. The |
| conversion process would be slow and error-prone. We |
| don't believe that the available alternatives to |
| exceptions, such as error codes and assertions, introduce |
| a significant burden. </p> |
| |
| <p>Our advice against using exceptions is not predicated |
| on philosophical or moral grounds, but practical ones. |
| Because we'd like to use our open-source |
| projects at Google and it's difficult to do so if those |
| projects use exceptions, we need to advise against |
| exceptions in Google open-source projects as well. |
| Things would probably be different if we had to do it all |
| over again from scratch.</p> |
| |
| <p>This prohibition also applies to the exception handling related |
| features added in C++11, such as |
| <code>std::exception_ptr</code> and |
| <code>std::nested_exception</code>.</p> |
| |
| <p>There is an <a href="#Windows_Code">exception</a> to |
| this rule (no pun intended) for Windows code.</p> |
| |
| <h3 id="noexcept"><code>noexcept</code></h3> |
| |
| <p>Specify <code>noexcept</code> when it is useful and correct.</p> |
| |
| <p class="definition"></p> |
| <p>The <code>noexcept</code> specifier is used to specify whether |
| a function will throw exceptions or not. If an exception |
| escapes from a function marked <code>noexcept</code>, the program |
| crashes via <code>std::terminate</code>.</p> |
| |
| <p>The <code>noexcept</code> operator performs a compile-time |
| check that returns true if an expression is declared to not |
| throw any exceptions.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Specifying move constructors as <code>noexcept</code> |
| improves performance in some cases, e.g. |
| <code>std::vector<T>::resize()</code> moves rather than |
| copies the objects if T's move constructor is |
| <code>noexcept</code>.</li> |
| |
| <li>Specifying <code>noexcept</code> on a function can |
| trigger compiler optimizations in environments where |
| exceptions are enabled, e.g. compiler does not have to |
| generate extra code for stack-unwinding, if it knows |
| that no exceptions can be thrown due to a |
| <code>noexcept</code> specifier.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li> |
| |
| In projects following this guide |
| that have exceptions disabled it is hard |
| to ensure that <code>noexcept</code> |
| specifiers are correct, and hard to define what |
| correctness even means.</li> |
| |
| <li>It's hard, if not impossible, to undo <code>noexcept</code> |
| because it eliminates a guarantee that callers may be relying |
| on, in ways that are hard to detect.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>You may use <code>noexcept</code> when it is useful for |
| performance if it accurately reflects the intended semantics |
| of your function, i.e. that if an exception is somehow thrown |
| from within the function body then it represents a fatal error. |
| You can assume that <code>noexcept</code> on move constructors |
| has a meaningful performance benefit. If you think |
| there is significant performance benefit from specifying |
| <code>noexcept</code> on some other function, please discuss it |
| with |
| your project leads.</p> |
| |
| <p>Prefer unconditional <code>noexcept</code> if exceptions are |
| completely disabled (i.e. most Google C++ environments). |
| Otherwise, use conditional <code>noexcept</code> specifiers |
| with simple conditions, in ways that evaluate false only in |
| the few cases where the function could potentially throw. |
| The tests might include type traits check on whether the |
| involved operation might throw (e.g. |
| <code>std::is_nothrow_move_constructible</code> for |
| move-constructing objects), or on whether allocation can throw |
| (e.g. <code>absl::default_allocator_is_nothrow</code> for |
| standard default allocation). Note in many cases the only |
| possible cause for an exception is allocation failure (we |
| believe move constructors should not throw except due to |
| allocation failure), and there are many applications where it’s |
| appropriate to treat memory exhaustion as a fatal error rather |
| than an exceptional condition that your program should attempt |
| to recover from. Even for other |
| potential failures you should prioritize interface simplicity |
| over supporting all possible exception throwing scenarios: |
| instead of writing a complicated <code>noexcept</code> clause |
| that depends on whether a hash function can throw, for example, |
| simply document that your component doesn’t support hash |
| functions throwing and make it unconditionally |
| <code>noexcept</code>.</p> |
| |
| <h3 id="Run-Time_Type_Information__RTTI_">Run-Time Type |
| Information (RTTI)</h3> |
| |
| <p>Avoid using Run Time Type Information (RTTI).</p> |
| |
| <p class="definition"></p> |
| <p> RTTI allows a |
| programmer to query the C++ class of an object at run |
| time. This is done by use of <code>typeid</code> or |
| <code>dynamic_cast</code>.</p> |
| |
| <p class="pros"></p> |
| <p>The standard alternatives to RTTI (described below) |
| require modification or redesign of the class hierarchy |
| in question. Sometimes such modifications are infeasible |
| or undesirable, particularly in widely-used or mature |
| code.</p> |
| |
| <p>RTTI can be useful in some unit tests. For example, it |
| is useful in tests of factory classes where the test has |
| to verify that a newly created object has the expected |
| dynamic type. It is also useful in managing the |
| relationship between objects and their mocks.</p> |
| |
| <p>RTTI is useful when considering multiple abstract |
| objects. Consider</p> |
| |
| <pre>bool Base::Equal(Base* other) = 0; |
| bool Derived::Equal(Base* other) { |
| Derived* that = dynamic_cast<Derived*>(other); |
| if (that == nullptr) |
| return false; |
| ... |
| } |
| </pre> |
| |
| <p class="cons"></p> |
| <p>Querying the type of an object at run-time frequently |
| means a design problem. Needing to know the type of an |
| object at runtime is often an indication that the design |
| of your class hierarchy is flawed.</p> |
| |
| <p>Undisciplined use of RTTI makes code hard to maintain. |
| It can lead to type-based decision trees or switch |
| statements scattered throughout the code, all of which |
| must be examined when making further changes.</p> |
| |
| <p class="decision"></p> |
| <p>RTTI has legitimate uses but is prone to abuse, so you |
| must be careful when using it. You may use it freely in |
| unittests, but avoid it when possible in other code. In |
| particular, think twice before using RTTI in new code. If |
| you find yourself needing to write code that behaves |
| differently based on the class of an object, consider one |
| of the following alternatives to querying the type:</p> |
| |
| <ul> |
| <li>Virtual methods are the preferred way of executing |
| different code paths depending on a specific subclass |
| type. This puts the work within the object itself.</li> |
| |
| <li>If the work belongs outside the object and instead |
| in some processing code, consider a double-dispatch |
| solution, such as the Visitor design pattern. This |
| allows a facility outside the object itself to |
| determine the type of class using the built-in type |
| system.</li> |
| </ul> |
| |
| <p>When the logic of a program guarantees that a given |
| instance of a base class is in fact an instance of a |
| particular derived class, then a |
| <code>dynamic_cast</code> may be used freely on the |
| object. Usually one |
| can use a <code>static_cast</code> as an alternative in |
| such situations.</p> |
| |
| <p>Decision trees based on type are a strong indication |
| that your code is on the wrong track.</p> |
| |
| <pre class="badcode">if (typeid(*data) == typeid(D1)) { |
| ... |
| } else if (typeid(*data) == typeid(D2)) { |
| ... |
| } else if (typeid(*data) == typeid(D3)) { |
| ... |
| </pre> |
| |
| <p>Code such as this usually breaks when additional |
| subclasses are added to the class hierarchy. Moreover, |
| when properties of a subclass change, it is difficult to |
| find and modify all the affected code segments.</p> |
| |
| <p>Do not hand-implement an RTTI-like workaround. The |
| arguments against RTTI apply just as much to workarounds |
| like class hierarchies with type tags. Moreover, |
| workarounds disguise your true intent.</p> |
| |
| <h3 id="Casting">Casting</h3> |
| |
| <p>Use C++-style casts |
| like <code>static_cast<float>(double_value)</code>, or brace |
| initialization for conversion of arithmetic types like |
| <code>int64 y = int64{1} << 42</code>. Do not use |
| cast formats like |
| <code>int y = (int)x</code> or <code>int y = int(x)</code> (but the latter |
| is okay when invoking a constructor of a class type).</p> |
| |
| <p class="definition"></p> |
| <p> C++ introduced a |
| different cast system from C that distinguishes the types |
| of cast operations.</p> |
| |
| <p class="pros"></p> |
| <p>The problem with C casts is the ambiguity of the operation; |
| sometimes you are doing a <em>conversion</em> |
| (e.g., <code>(int)3.5</code>) and sometimes you are doing |
| a <em>cast</em> (e.g., <code>(int)"hello"</code>). Brace |
| initialization and C++ casts can often help avoid this |
| ambiguity. Additionally, C++ casts are more visible when searching for |
| them.</p> |
| |
| <p class="cons"></p> |
| <p>The C++-style cast syntax is verbose and cumbersome.</p> |
| |
| <p class="decision"></p> |
| <p>Do not use C-style casts. Instead, use these C++-style casts when |
| explicit type conversion is necessary. </p> |
| |
| <ul> |
| <li>Use brace initialization to convert arithmetic types |
| (e.g. <code>int64{x}</code>). This is the safest approach because code |
| will not compile if conversion can result in information loss. The |
| syntax is also concise.</li> |
| |
| |
| |
| <li>Use <code>static_cast</code> as the equivalent of a C-style cast |
| that does value conversion, when you need to |
| explicitly up-cast a pointer from a class to its superclass, or when |
| you need to explicitly cast a pointer from a superclass to a |
| subclass. In this last case, you must be sure your object is |
| actually an instance of the subclass.</li> |
| |
| |
| |
| <li>Use <code>const_cast</code> to remove the |
| <code>const</code> qualifier (see <a href="#Use_of_const">const</a>).</li> |
| |
| <li>Use <code>reinterpret_cast</code> to do unsafe conversions of |
| pointer types to and from integer and other pointer |
| types. Use this |
| only if you know what you are doing and you understand the aliasing |
| issues. Also, consider the alternative |
| <code>absl::bit_cast</code>.</li> |
| |
| <li>Use <code>absl::bit_cast</code> to interpret the raw bits of a |
| value using a different type of the same size (a type pun), such as |
| interpreting the bits of a <code>double</code> as |
| <code>int64</code>.</li> |
| </ul> |
| |
| <p>See the <a href="#Run-Time_Type_Information__RTTI_"> |
| RTTI section</a> for guidance on the use of |
| <code>dynamic_cast</code>.</p> |
| |
| <h3 id="Streams">Streams</h3> |
| |
| <p>Use streams where appropriate, and stick to "simple" |
| usages. Overload <code><<</code> for streaming only for types |
| representing values, and write only the user-visible value, not any |
| implementation details.</p> |
| |
| <p class="definition"></p> |
| <p>Streams are the standard I/O abstraction in C++, as |
| exemplified by the standard header <code><iostream></code>. |
| They are widely used in Google code, mostly for debug logging |
| and test diagnostics.</p> |
| |
| <p class="pros"></p> |
| <p>The <code><<</code> and <code>>></code> |
| stream operators provide an API for formatted I/O that |
| is easily learned, portable, reusable, and extensible. |
| <code>printf</code>, by contrast, doesn't even support |
| <code>std::string</code>, to say nothing of user-defined types, |
| and is very difficult to use portably. |
| <code>printf</code> also obliges you to choose among the |
| numerous slightly different versions of that function, |
| and navigate the dozens of conversion specifiers.</p> |
| |
| <p>Streams provide first-class support for console I/O |
| via <code>std::cin</code>, <code>std::cout</code>, |
| <code>std::cerr</code>, and <code>std::clog</code>. |
| The C APIs do as well, but are hampered by the need to |
| manually buffer the input. </p> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Stream formatting can be configured by mutating the |
| state of the stream. Such mutations are persistent, so |
| the behavior of your code can be affected by the entire |
| previous history of the stream, unless you go out of your |
| way to restore it to a known state every time other code |
| might have touched it. User code can not only modify the |
| built-in state, it can add new state variables and behaviors |
| through a registration system.</li> |
| |
| <li>It is difficult to precisely control stream output, due |
| to the above issues, the way code and data are mixed in |
| streaming code, and the use of operator overloading (which |
| may select a different overload than you expect).</li> |
| |
| <li>The practice of building up output through chains |
| of <code><<</code> operators interferes with |
| internationalization, because it bakes word order into the |
| code, and streams' support for localization is <a href="http://www.boost.org/doc/libs/1_48_0/libs/locale/doc/html/rationale.html#rationale_why"> |
| flawed</a>.</li> |
| |
| |
| |
| |
| |
| <li>The streams API is subtle and complex, so programmers must |
| develop experience with it in order to use it effectively.</li> |
| |
| <li>Resolving the many overloads of <code><<</code> is |
| extremely costly for the compiler. When used pervasively in a |
| large code base, it can consume as much as 20% of the parsing |
| and semantic analysis time.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Use streams only when they are the best tool for the job. |
| This is typically the case when the I/O is ad-hoc, local, |
| human-readable, and targeted at other developers rather than |
| end-users. Be consistent with the code around you, and with the |
| codebase as a whole; if there's an established tool for |
| your problem, use that tool instead. |
| In particular, |
| |
| logging libraries are usually a better |
| choice than <code>std::cerr</code> or <code>std::clog</code> |
| for diagnostic output, and the libraries in |
| |
| <code>absl/strings</code> |
| or the equivalent are usually a |
| better choice than <code>std::stringstream</code>.</p> |
| |
| <p>Avoid using streams for I/O that faces external users or |
| handles untrusted data. Instead, find and use the appropriate |
| templating libraries to handle issues like internationalization, |
| localization, and security hardening.</p> |
| |
| <p>If you do use streams, avoid the stateful parts of the |
| streams API (other than error state), such as <code>imbue()</code>, |
| <code>xalloc()</code>, and <code>register_callback()</code>. |
| Use explicit formatting functions (see e.g. |
| |
| <code>absl/strings</code>) |
| rather than |
| stream manipulators or formatting flags to control formatting |
| details such as number base, precision, or padding.</p> |
| |
| <p>Overload <code><<</code> as a streaming operator |
| for your type only if your type represents a value, and |
| <code><<</code> writes out a human-readable string |
| representation of that value. Avoid exposing implementation |
| details in the output of <code><<</code>; if you need to print |
| object internals for debugging, use named functions instead |
| (a method named <code>DebugString()</code> is the most common |
| convention).</p> |
| |
| <h3 id="Preincrement_and_Predecrement">Preincrement and Predecrement</h3> |
| |
| <p>Use prefix form (<code>++i</code>) of the increment and |
| decrement operators with iterators and other template |
| objects.</p> |
| |
| <p class="definition"></p> |
| <p> When a variable |
| is incremented (<code>++i</code> or <code>i++</code>) or |
| decremented (<code>--i</code> or <code>i--</code>) and |
| the value of the expression is not used, one must decide |
| whether to preincrement (decrement) or postincrement |
| (decrement).</p> |
| |
| <p class="pros"></p> |
| <p>When the return value is ignored, the "pre" form |
| (<code>++i</code>) is never less efficient than the |
| "post" form (<code>i++</code>), and is often more |
| efficient. This is because post-increment (or decrement) |
| requires a copy of <code>i</code> to be made, which is |
| the value of the expression. If <code>i</code> is an |
| iterator or other non-scalar type, copying <code>i</code> |
| could be expensive. Since the two types of increment |
| behave the same when the value is ignored, why not just |
| always pre-increment?</p> |
| |
| <p class="cons"></p> |
| <p>The tradition developed, in C, of using post-increment |
| when the expression value is not used, especially in |
| <code>for</code> loops. Some find post-increment easier |
| to read, since the "subject" (<code>i</code>) precedes |
| the "verb" (<code>++</code>), just like in English.</p> |
| |
| <p class="decision"></p> |
| <p> For simple scalar |
| (non-object) values there is no reason to prefer one form |
| and we allow either. For iterators and other template |
| types, use pre-increment.</p> |
| |
| <h3 id="Use_of_const">Use of const</h3> |
| |
| <p>In APIs, use <code>const</code> whenever it makes sense. |
| <code>constexpr</code> is a better choice for some uses of |
| const.</p> |
| |
| <p class="definition"></p> |
| <p> Declared variables and parameters can be preceded |
| by the keyword <code>const</code> to indicate the variables |
| are not changed (e.g., <code>const int foo</code>). Class |
| functions can have the <code>const</code> qualifier to |
| indicate the function does not change the state of the |
| class member variables (e.g., <code>class Foo { int |
| Bar(char c) const; };</code>).</p> |
| |
| <p class="pros"></p> |
| <p>Easier for people to understand how variables are being |
| used. Allows the compiler to do better type checking, |
| and, conceivably, generate better code. Helps people |
| convince themselves of program correctness because they |
| know the functions they call are limited in how they can |
| modify your variables. Helps people know what functions |
| are safe to use without locks in multi-threaded |
| programs.</p> |
| |
| <p class="cons"></p> |
| <p><code>const</code> is viral: if you pass a |
| <code>const</code> variable to a function, that function |
| must have <code>const</code> in its prototype (or the |
| variable will need a <code>const_cast</code>). This can |
| be a particular problem when calling library |
| functions.</p> |
| |
| <p class="decision"></p> |
| <p>We strongly recommend using <code>const</code> |
| in APIs (i.e. on function parameters, methods, and |
| non-local variables) wherever it is meaningful and accurate. This |
| provides consistent, mostly compiler-verified documentation |
| of what objects an operation can mutate. Having |
| a consistent and reliable way to distinguish reads from writes |
| is critical to writing thread-safe code, and is useful in |
| many other contexts as well. In particular:</p> |
| |
| <ul> |
| <li>If a function guarantees that it will not modify an argument |
| passed by reference or by pointer, the corresponding function parameter |
| should be a reference-to-const (<code>const T&</code>) or |
| pointer-to-const (<code>const T*</code>), respectively.</li> |
| |
| <li>For a function parameter passed by value, <code>const</code> has |
| no effect on the caller, thus is not recommended in function |
| declarations. See |
| |
| |
| <a href="https://abseil.io/tips/109">TotW #109</a>. |
| |
| |
| </li><li>Declare methods to be <code>const</code> unless they |
| alter the logical state of the object (or enable the user to modify |
| that state, e.g. by returning a non-const reference, but that's |
| rare), or they can't safely be invoked concurrently.</li> |
| </ul> |
| |
| <p>Using <code>const</code> on local variables is neither encouraged |
| nor discouraged.</p> |
| |
| <p>All of a class's <code>const</code> operations should be safe |
| to invoke concurrently with each other. If that's not feasible, the class must |
| be clearly documented as "thread-unsafe".</p> |
| |
| |
| <h4>Where to put the const</h4> |
| |
| <p>Some people favor the form <code>int const *foo</code> |
| to <code>const int* foo</code>. They argue that this is |
| more readable because it's more consistent: it keeps the |
| rule that <code>const</code> always follows the object |
| it's describing. However, this consistency argument |
| doesn't apply in codebases with few deeply-nested pointer |
| expressions since most <code>const</code> expressions |
| have only one <code>const</code>, and it applies to the |
| underlying value. In such cases, there's no consistency |
| to maintain. Putting the <code>const</code> first is |
| arguably more readable, since it follows English in |
| putting the "adjective" (<code>const</code>) before the |
| "noun" (<code>int</code>).</p> |
| |
| <p>That said, while we encourage putting |
| <code>const</code> first, we do not require it. But be |
| consistent with the code around you!</p> |
| |
| <h3 id="Use_of_constexpr">Use of constexpr</h3> |
| |
| <p>Use <code>constexpr</code> to define true |
| constants or to ensure constant initialization.</p> |
| |
| <p class="definition"></p> |
| <p> Some variables can be declared <code>constexpr</code> |
| to indicate the variables are true constants, i.e. fixed at |
| compilation/link time. Some functions and constructors |
| can be declared <code>constexpr</code> which enables them |
| to be used in defining a <code>constexpr</code> |
| variable.</p> |
| |
| <p class="pros"></p> |
| <p>Use of <code>constexpr</code> enables definition of |
| constants with floating-point expressions rather than |
| just literals; definition of constants of user-defined |
| types; and definition of constants with function |
| calls.</p> |
| |
| <p class="cons"></p> |
| <p>Prematurely marking something as constexpr may cause |
| migration problems if later on it has to be downgraded. |
| Current restrictions on what is allowed in constexpr |
| functions and constructors may invite obscure workarounds |
| in these definitions.</p> |
| |
| <p class="decision"></p> |
| <p><code>constexpr</code> definitions enable a more |
| robust specification of the constant parts of an |
| interface. Use <code>constexpr</code> to specify true |
| constants and the functions that support their |
| definitions. Avoid complexifying function definitions to |
| enable their use with <code>constexpr</code>. Do not use |
| <code>constexpr</code> to force inlining.</p> |
| |
| <h3 id="Integer_Types">Integer Types</h3> |
| |
| <p>Of the built-in C++ integer types, the only one used |
| is |
| <code>int</code>. If a program needs a variable of a |
| different size, use |
| a precise-width integer type from |
| <code><stdint.h></code>, such as |
| <code>int16_t</code>. If your variable represents a |
| value that could ever be greater than or equal to 2^31 |
| (2GiB), use a 64-bit type such as |
| <code>int64_t</code>. |
| Keep in mind that even if your value won't ever be too large |
| for an <code>int</code>, it may be used in intermediate |
| calculations which may require a larger type. When in doubt, |
| choose a larger type.</p> |
| |
| <p class="definition"></p> |
| <p> C++ does not specify the sizes of integer types |
| like <code>int</code>. Typically people assume |
| that <code>short</code> is 16 bits, |
| <code>int</code> is 32 bits, <code>long</code> is 32 bits |
| and <code>long long</code> is 64 bits.</p> |
| |
| <p class="pros"></p> |
| <p>Uniformity of declaration.</p> |
| |
| <p class="cons"></p> |
| <p>The sizes of integral types in C++ can vary based on |
| compiler and architecture.</p> |
| |
| <p class="decision"></p> |
| |
| <p> |
| <code><cstdint></code> defines types |
| like <code>int16_t</code>, <code>uint32_t</code>, |
| <code>int64_t</code>, etc. You should always use |
| those in preference to <code>short</code>, <code>unsigned |
| long long</code> and the like, when you need a guarantee |
| on the size of an integer. Of the C integer types, only |
| <code>int</code> should be used. When appropriate, you |
| are welcome to use standard types like |
| <code>size_t</code> and <code>ptrdiff_t</code>.</p> |
| |
| <p>We use <code>int</code> very often, for integers we |
| know are not going to be too big, e.g., loop counters. |
| Use plain old <code>int</code> for such things. You |
| should assume that an <code>int</code> is |
| |
| at least 32 bits, but don't |
| assume that it has more than 32 bits. If you need a 64-bit |
| integer type, use |
| <code>int64_t</code> |
| or |
| <code>uint64_t</code>.</p> |
| |
| <p>For integers we know can be "big", |
| use |
| <code>int64_t</code>. |
| </p> |
| |
| <p>You should not use the unsigned integer types such as |
| |
| <code>uint32_t</code>, unless there is a valid |
| reason such as representing a bit pattern rather than a |
| number, or you need defined overflow modulo 2^N. In |
| particular, do not use unsigned types to say a number |
| will never be negative. Instead, use |
| |
| assertions for this.</p> |
| |
| |
| |
| <p>If your code is a container that returns a size, be |
| sure to use a type that will accommodate any possible |
| usage of your container. When in doubt, use a larger type |
| rather than a smaller type.</p> |
| |
| <p>Use care when converting integer types. Integer conversions and |
| promotions can cause undefined behavior, leading to security bugs and |
| other problems.</p> |
| |
| <h4>On Unsigned Integers</h4> |
| |
| <p>Unsigned integers are good for representing bitfields and modular |
| arithmetic. Because of historical accident, the C++ standard also uses |
| unsigned integers to represent the size of containers - many members |
| of the standards body believe this to be a mistake, but it is |
| effectively impossible to fix at this point. The fact that unsigned |
| arithmetic doesn't model the behavior of a simple integer, but is |
| instead defined by the standard to model modular arithmetic (wrapping |
| around on overflow/underflow), means that a significant class of bugs |
| cannot be diagnosed by the compiler. In other cases, the defined |
| behavior impedes optimization.</p> |
| |
| <p>That said, mixing signedness of integer types is responsible for an |
| equally large class of problems. The best advice we can provide: try |
| to use iterators and containers rather than pointers and sizes, try |
| not to mix signedness, and try to avoid unsigned types (except for |
| representing bitfields or modular arithmetic). Do not use an unsigned |
| type merely to assert that a variable is non-negative.</p> |
| |
| <h3 id="64-bit_Portability">64-bit Portability</h3> |
| |
| <p>Code should be 64-bit and 32-bit friendly. Bear in mind |
| problems of printing, comparisons, and structure alignment.</p> |
| |
| <ul> |
| <li> |
| <p>Correct portable <code>printf()</code> conversion specifiers for |
| some integral typedefs rely on macro expansions that we find unpleasant to |
| use and impractical to require (the <code>PRI</code> macros from |
| <code><cinttypes></code>). Unless there is no reasonable alternative |
| for your particular case, try to avoid or even upgrade APIs that rely on the |
| <code>printf</code> family. Instead use a library supporting typesafe numeric |
| formatting, such as |
| |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/strings/str_cat.h"><code>StrCat</code></a> |
| |
| or |
| |
| |
| <a href="https://github.com/abseil/abseil-cpp/blob/master/absl/strings/substitute.h"><code>Substitute</code></a> |
| |
| for fast simple conversions, |
| |
| or <a href="#Streams"><code>std::ostream</code></a>.</p> |
| |
| <p>Unfortunately, the <code>PRI</code> macros are the only portable way to |
| specify a conversion for the standard bitwidth typedefs (e.g. |
| <code>int64_t</code>, <code>uint64_t</code>, <code>int32_t</code>, |
| <code>uint32_t</code>, etc). |
| |
| Where possible, avoid passing arguments of types specified by bitwidth |
| typedefs to <code>printf</code>-based APIs. Note that it is acceptable |
| to use typedefs for which printf has dedicated length modifiers, such as |
| <code>size_t</code> (<code>z</code>), |
| <code>ptrdiff_t</code> (<code>t</code>), and |
| <code>maxint_t</code> (<code>j</code>).</p> |
| </li> |
| |
| <li>Remember that <code>sizeof(void *)</code> != |
| <code>sizeof(int)</code>. Use <code>intptr_t</code> if |
| you want a pointer-sized integer.</li> |
| |
| <li>You may need to be careful with structure |
| alignments, particularly for structures being stored on |
| disk. Any class/structure with a |
| <code>int64_t</code>/<code>uint64_t</code> |
| member will by default end up being 8-byte aligned on a |
| 64-bit system. If you have such structures being shared |
| on disk between 32-bit and 64-bit code, you will need |
| to ensure that they are packed the same on both |
| architectures. |
| Most compilers offer a way to |
| alter structure alignment. For gcc, you can use |
| <code>__attribute__((packed))</code>. MSVC offers |
| <code>#pragma pack()</code> and |
| <code>__declspec(align())</code>.</li> |
| |
| <li> |
| <p>Use <a href="#Casting">braced-initialization</a> as needed to create |
| 64-bit constants. For example:</p> |
| |
| |
| <div> |
| <pre>int64_t my_value{0x123456789}; |
| uint64_t my_mask{3ULL << 48}; |
| </pre> |
| </div> |
| </li> |
| </ul> |
| |
| <h3 id="Preprocessor_Macros">Preprocessor Macros</h3> |
| |
| <p>Avoid defining macros, especially in headers; prefer |
| inline functions, enums, and <code>const</code> variables. |
| Name macros with a project-specific prefix. Do not use |
| macros to define pieces of a C++ API.</p> |
| |
| <p>Macros mean that the code you see is not the same as |
| the code the compiler sees. This can introduce unexpected |
| behavior, especially since macros have global scope.</p> |
| |
| <p>The problems introduced by macros are especially severe |
| when they are used to define pieces of a C++ API, |
| and still more so for public APIs. Every error message from |
| the compiler when developers incorrectly use that interface |
| now must explain how the macros formed the interface. |
| Refactoring and analysis tools have a dramatically harder |
| time updating the interface. As a consequence, we |
| specifically disallow using macros in this way. |
| For example, avoid patterns like:</p> |
| |
| <pre class="badcode">class WOMBAT_TYPE(Foo) { |
| // ... |
| |
| public: |
| EXPAND_PUBLIC_WOMBAT_API(Foo) |
| |
| EXPAND_WOMBAT_COMPARISONS(Foo, ==, <) |
| }; |
| </pre> |
| |
| <p>Luckily, macros are not nearly as necessary in C++ as |
| they are in C. Instead of using a macro to inline |
| performance-critical code, use an inline function. |
| Instead of using a macro to store a constant, use a |
| <code>const</code> variable. Instead of using a macro to |
| "abbreviate" a long variable name, use a reference. |
| Instead of using a macro to conditionally compile code |
| ... well, don't do that at all (except, of course, for |
| the <code>#define</code> guards to prevent double |
| inclusion of header files). It makes testing much more |
| difficult.</p> |
| |
| <p>Macros can do things these other techniques cannot, |
| and you do see them in the codebase, especially in the |
| lower-level libraries. And some of their special features |
| (like stringifying, concatenation, and so forth) are not |
| available through the language proper. But before using a |
| macro, consider carefully whether there's a non-macro way |
| to achieve the same result. If you need to use a macro to |
| define an interface, contact |
| your project leads to request |
| a waiver of this rule.</p> |
| |
| <p>The following usage pattern will avoid many problems |
| with macros; if you use macros, follow it whenever |
| possible:</p> |
| |
| <ul> |
| <li>Don't define macros in a <code>.h</code> file.</li> |
| |
| <li><code>#define</code> macros right before you use |
| them, and <code>#undef</code> them right after.</li> |
| |
| <li>Do not just <code>#undef</code> an existing macro |
| before replacing it with your own; instead, pick a name |
| that's likely to be unique.</li> |
| |
| <li>Try not to use macros that expand to unbalanced C++ |
| constructs, or at least document that behavior |
| well.</li> |
| |
| <li>Prefer not using <code>##</code> to generate |
| function/class/variable names.</li> |
| </ul> |
| |
| <p>Exporting macros from headers (i.e. defining them in a header |
| without <code>#undef</code>ing them before the end of the header) |
| is extremely strongly discouraged. If you do export a macro from a |
| header, it must have a globally unique name. To achieve this, it |
| must be named with a prefix consisting of your project's namespace |
| name (but upper case). </p> |
| |
| <h3 id="0_and_nullptr/NULL">0 and nullptr/NULL</h3> |
| |
| <p>Use <code>nullptr</code> for pointers, and <code>'\0'</code> for chars (and |
| not the <code>0</code> literal).</p> |
| |
| <p>For pointers (address values), use <code>nullptr</code>, as this |
| provides type-safety.</p> |
| |
| <p>For C++03 projects, prefer <code>NULL</code> to <code>0</code>. While the |
| values are equivalent, <code>NULL</code> looks more like a pointer to the |
| reader, and some C++ compilers provide special definitions of <code>NULL</code> |
| which enable them to give useful warnings. Never use <code>NULL</code> for |
| numeric (integer or floating-point) values.</p> |
| |
| <p>Use <code>'\0'</code> for the null character. Using the correct type makes |
| the code more readable.</p> |
| |
| <h3 id="sizeof">sizeof</h3> |
| |
| <p>Prefer <code>sizeof(<var>varname</var>)</code> to |
| <code>sizeof(<var>type</var>)</code>.</p> |
| |
| <p>Use <code>sizeof(<var>varname</var>)</code> when you |
| take the size of a particular variable. |
| <code>sizeof(<var>varname</var>)</code> will update |
| appropriately if someone changes the variable type either |
| now or later. You may use |
| <code>sizeof(<var>type</var>)</code> for code unrelated |
| to any particular variable, such as code that manages an |
| external or internal data format where a variable of an |
| appropriate C++ type is not convenient.</p> |
| |
| <pre>struct data; |
| memset(&data, 0, sizeof(data)); |
| </pre> |
| |
| <pre class="badcode">memset(&data, 0, sizeof(Struct)); |
| </pre> |
| |
| <pre>if (raw_size < sizeof(int)) { |
| LOG(ERROR) << "compressed record not big enough for count: " << raw_size; |
| return false; |
| } |
| </pre> |
| |
| <a name="auto"></a> |
| <h3 id="Type_deduction">Type deduction</h3> |
| |
| <p>Use type deduction only if it makes the code clearer to readers who aren't |
| familiar with the project, or if it makes the code safer. Do not use it |
| merely to avoid the inconvenience of writing an explicit type.</p> |
| |
| <p class="definition"></p> |
| |
| <p>There are several contexts in which C++ allows (or even requires) types to |
| be deduced by the compiler, rather than spelled out explicitly in the code:</p> |
| <dl> |
| <dt><a href="https://en.cppreference.com/w/cpp/language/template_argument_deduction">Function template argument deduction</a></dt> |
| <dd>A function template can be invoked without explicit template arguments. |
| The compiler deduces those arguments from the types of the function |
| arguments: |
| <pre class="neutralcode">template <typename T> |
| void f(T t); |
| |
| f(0); // Invokes f<int>(0)</pre> |
| </dd> |
| <dt><a href="https://en.cppreference.com/w/cpp/language/auto"><code>auto</code> variable declarations</a></dt> |
| <dd>A variable declaration can use the <code>auto</code> keyword in place |
| of the type. The compiler deduces the type from the variable's |
| initializer, following the same rules as function template argument |
| deduction with the same initializer (so long as you don't use curly braces |
| instead of parentheses). |
| <pre class="neutralcode">auto a = 42; // a is an int |
| auto& b = a; // b is an int& |
| auto c = b; // c is an int |
| auto d{42}; // d is an int, not a std::initializer_list<int> |
| </pre> |
| <code>auto</code> can be qualified with <code>const</code>, and can be |
| used as part of a pointer or reference type, but it can't be used as a |
| template argument. A rare variant of this syntax uses |
| <code>decltype(auto)</code> instead of <code>auto</code>, in which case |
| the deduced type is the result of applying |
| <a href="https://en.cppreference.com/w/cpp/language/decltype"><code>decltype</code></a> |
| to the initializer. |
| </dd> |
| <dt><a href="https://en.cppreference.com/w/cpp/language/function#Return_type_deduction">Function return type deduction</a></dt> |
| <dd><code>auto</code> (and <code>decltype(auto)</code>) can also be used in |
| place of a function return type. The compiler deduces the return type from |
| the <code>return</code> statements in the function body, following the same |
| rules as for variable declarations: |
| <pre class="neutralcode">auto f() { return 0; } // The return type of f is int</pre> |
| <a href="#Lambda_expressions">Lambda expression</a> return types can be |
| deduced in the same way, but this is triggered by omitting the return type, |
| rather than by an explicit <code>auto</code>. Confusingly, |
| <a href="trailing_return">trailing return type</a> syntax for functions |
| also uses <code>auto</code> in the return-type position, but that doesn't |
| rely on type deduction; it's just an alternate syntax for an explicit |
| return type. |
| </dd> |
| <dt><a href="https://isocpp.org/wiki/faq/cpp14-language#generic-lambdas">Generic lambdas</a></dt> |
| <dd>A lambda expression can use the <code>auto</code> keyword in place of |
| one or more of its parameter types. This causes the lambda's call operator |
| to be a function template instead of an ordinary function, with a separate |
| template parameter for each <code>auto</code> function parameter: |
| <pre class="neutralcode">// Sort `vec` in increasing order |
| std::sort(vec.begin(), vec.end(), [](auto lhs, auto rhs) { return lhs > rhs; });</pre> |
| </dd> |
| <dt><a href="https://isocpp.org/wiki/faq/cpp14-language#lambda-captures">Lambda init captures</a></dt> |
| <dd>Lambda captures can have explicit initializers, which can be used to |
| declare wholly new variables rather than only capturing existing ones: |
| <pre class="neutralcode">[x = 42, y = "foo"] { ... } // x is an int, and y is a const char*</pre> |
| This syntax doesn't allow the type to be specified; instead, it's deduced |
| using the rules for <code>auto</code> variables. |
| </dd> |
| <dt><a href="https://en.cppreference.com/w/cpp/language/class_template_argument_deduction">Class template argument deduction</a></dt> |
| <dd>See <a href="#CTAD">below</a>.</dd> |
| <dt><a href="https://en.cppreference.com/w/cpp/language/structured_binding">Structured bindings</a></dt> |
| <dd>When declaring a tuple, struct, or array using <code>auto</code>, you can |
| specify names for the individual elements instead of a name for the whole |
| object; these names are called "structured bindings", and the whole |
| declaration is called a "structured binding declaration". This syntax |
| provides no way of specifying the type of either the enclosing object |
| or the individual names: |
| <pre class="neutralcode">auto [iter, success] = my_map.insert({key, value}); |
| if (!success) { |
| iter->second = value; |
| }</pre> |
| The <code>auto</code> can also be qualified with <code>const</code>, |
| <code>&</code>, and <code>&&</code>, but note that these qualifiers |
| technically apply to the anonymous tuple/struct/array, rather than the |
| individual bindings. The rules that determine the types of the bindings |
| are quite complex; the results tend to be unsurprising, except that |
| the binding types typically won't be references even if the declaration |
| declares a reference (but they will usually behave like references anyway). |
| </dd> |
| |
| <p>(These summaries omit many details and caveats; see the links for further |
| information.)</p> |
| |
| <p class="pros"></p> |
| |
| <ul> |
| <li>C++ type names can be long and cumbersome, especially when they |
| involve templates or namespaces.</li> |
| <li>When a C++ type name is repeated within a single declaration or a |
| small code region, the repetition may not be aiding readability.</li> |
| <li>It is sometimes safer to let the type be deduced, since that avoids |
| the possibility of unintended copies or type conversions.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <p>C++ code is usually clearer when types are explicit, |
| especially when type deduction would depend on information from |
| distant parts of the code. In expressions like:</p> |
| |
| <pre class="badcode">auto foo = x.add_foo(); |
| auto i = y.Find(key); |
| </pre> |
| |
| <p>it may not be obvious what the resulting types are if the type |
| of <code>y</code> isn't very well known, or if <code>y</code> was |
| declared many lines earlier.</p> |
| |
| <p>Programmers have to understand when type deduction will or won't |
| produce a reference type, or they'll get copies when they didn't |
| mean to.</p> |
| |
| <p>If a deduced type is used as part of an interface, then a |
| programmer might change its type while only intending to |
| change its value, leading to a more radical API change |
| than intended.</p> |
| |
| <p class="decision"></p> |
| |
| <p>The fundamental rule is: use type deduction only to make the code |
| clearer or safer, and do not use it merely to avoid the |
| inconvenience of writing an explicit type. When judging whether the |
| code is clearer, keep in mind that your readers are not necessarily |
| on your team, or familiar with your project, so types that you and |
| your reviewer experience as as unnecessary clutter will very often |
| provide useful information to others. For example, you can assume that |
| the return type of <code>make_unique<Foo>()</code> is obvious, |
| but the return type of <code>MyWidgetFactory()</code> probably isn't.</p> |
| |
| <p>These principles applies to all forms of type deduction, but the |
| details vary, as described in the following sections.</p> |
| |
| <h4>Function template argument deduction</h4> |
| |
| <p>Function template argument deduction is almost always OK. Type deduction |
| is the expected default way of interacting with function templates, |
| because it allows function templates to act like infinite sets of ordinary |
| function overloads. Consequently, function templates are almost always |
| designed so that template argument deduction is clear and safe, or |
| doesn't compile.</p> |
| |
| <h4>Local variable type deduction</h4> |
| |
| <p>For local variables, you can use type deduction to make the code clearer |
| by eliminating type information that is obvious or irrelevant, so that |
| the reader can focus on the meaningful parts of the code: |
| </p><pre class="neutralcode">std::unique_ptr<WidgetWithBellsAndWhistles> widget_ptr = |
| absl::make_unique<WidgetWithBellsAndWhistles>(arg1, arg2); |
| absl::flat_hash_map<std::string, |
| std::unique_ptr<WidgetWithBellsAndWhistles>>::const_iterator |
| it = my_map_.find(key); |
| std::array<int, 0> numbers = {4, 8, 15, 16, 23, 42};</pre> |
| |
| <pre class="goodcode">auto widget_ptr = absl::make_unique<WidgetWithBellsAndWhistles>(arg1, arg2); |
| auto it = my_map_.find(key); |
| std::array numbers = {4, 8, 15, 16, 23, 42};</pre> |
| |
| <p>Types sometimes contain a mixture of useful information and boilerplate, |
| such as <code>it</code> in the example above: it's obvious that the |
| type is an iterator, and in many contexts the container type and even the |
| key type aren't relevant, but the type of the values is probably useful. |
| In such situations, it's often possible to define local variables with |
| explicit types that convey the relevant information: |
| </p><pre class="goodcode">auto it = my_map_.find(key); |
| if (it != my_map_.end()) { |
| WidgetWithBellsAndWhistles& widget = *it->second; |
| // Do stuff with `widget` |
| }</pre> |
| If the type is a template instance, and the parameters are |
| boilerplate but the template itself is informative, you can use |
| class template argument deduction to suppress the boilerplate. However, |
| cases where this actually provides a meaningful benefit are quite rare. |
| Note that class template argument deduction is also subject to a |
| <a href="#CTAD">separate style rule</a>. |
| |
| <p>Do not use <code>decltype(auto)</code> if a simpler option will work, |
| because it's a fairly obscure feature, so it has a high cost in code |
| clarity.</p> |
| |
| <h4>Return type deduction</h4> |
| |
| <p>Use return type deduction (for both functions and lambdas) only if the |
| function body has a very small number of <code>return</code> statements, |
| and very little other code, because otherwise the reader may not be able |
| to tell at a glance what the return type is. Furthermore, use it only |
| if the function or lambda has a very narrow scope, because functions with |
| deduced return types don't define abstraction boundaries: the implementation |
| <em>is</em> the interface. In particular, public functions in header files |
| should almost never have deduced return types.</p> |
| |
| <h4>Parameter type deduction</h4> |
| |
| <p><code>auto</code> parameter types for lambdas should be used with caution, |
| because the actual type is determined by the code that calls the lambda, |
| rather than by the definition of the lambda. Consequently, an explicit |
| type will almost always be clearer unless the lambda is explicitly called |
| very close to where it's defined (so that the reader can easily see both), |
| or the lambda is passed to an interface so well-known that it's |
| obvious what arguments it will eventually be called with (e.g. |
| the <code>std::sort</code> example above).</p> |
| |
| <h4>Lambda init captures</h4> |
| |
| <p>Init captures are covered by a <a href="#Lambda_expressions">more specific |
| style rule</a>, which largely supersedes the general rules for |
| type deduction.</p> |
| |
| <h4>Structured bindings</h4> |
| |
| <p>Unlike other forms of type deduction, structured bindings can actually |
| give the reader additional information, by giving meaningful names to the |
| elements of a larger object. This means that a structured binding declaration |
| may provide a net readability improvement over an explicit type, even in cases |
| where <code>auto</code> would not. Structured bindings are especially |
| beneficial when the object is a pair or tuple (as in the <code>insert</code> |
| example above), because they don't have meaningful field names to begin with, |
| but note that you generally <a href="#Structs_vs._Tuples">shouldn't use |
| pairs or tuples</a> unless a pre-existing API like <code>insert</code> |
| forces you to.</p> |
| |
| <p>If the object being bound is a struct, it may sometimes be helpful to |
| provide names that are more specific to your usage, but keep in mind that |
| this may also mean the names are less recognizable to your reader than the |
| field names. We recommend using a comment to indicate the name of the |
| underlying field, if it doesn't match the name of the binding, using the |
| same syntax as for function parameter comments: |
| </p><pre>auto [/*field_name1=*/ bound_name1, /*field_name2=*/ bound_name2] = ...</pre> |
| As with function parameter comments, this can enable tools to detect if |
| you get the order of the fields wrong. |
| |
| <h3 id="CTAD">Class template argument deduction</h3> |
| |
| <p>Use class template argument deduction only with templates that have |
| explicitly opted into supporting it.</p> |
| |
| <p class="definition"></p> |
| <p><a href="https://en.cppreference.com/w/cpp/language/class_template_argument_deduction">Class |
| template argument deduction</a> (often abbreviated "CTAD") occurs when |
| a variable is declared with a type that names a template, and the template |
| argument list is not provided (not even empty angle brackets): |
| </p><pre class="neutralcode">std::array a = {1, 2, 3}; // `a` is a std::array<int, 3></pre> |
| The compiler deduces the arguments from the initializer using the |
| template's "deduction guides", which can be explicit or implicit. |
| |
| <p>Explicit deduction guides look like function declarations with trailing |
| return types, except that there's no leading <code>auto</code>, and the |
| function name is the name of the template. For example, the above example |
| relies on this deduction guide for <code>std::array</code>: |
| </p><pre class="neutralcode">namespace std { |
| template <class T, class... U> |
| array(T, U...) -> std::array<T, 1 + sizeof...(U)>; |
| }</pre> |
| Constructors in a primary template (as opposed to a template specialization) |
| also implicitly define deduction guides. |
| |
| <p>When you declare a variable that relies on CTAD, the compiler selects |
| a deduction guide using the rules of constructor overload resolution, |
| and that guide's return type becomes the type of the variable.</p> |
| |
| <p class="pros"></p> |
| <p>CTAD can sometimes allow you to omit boilerplate from your code.</p> |
| |
| <p class="cons"></p> |
| <p>The implicit deduction guides that are generated from constructors |
| may have undesirable behavior, or be outright incorrect. This is |
| particularly problematic for constructors written before CTAD was |
| introduced in C++17, because the authors of those constructors had no |
| way of knowing about (much less fixing) any problems that their |
| constructors would cause for CTAD. Furthermore, adding explicit deduction |
| guides to fix those problems might break any existing code that relies on |
| the implicit deduction guides.</p> |
| |
| <p>CTAD also suffers from many of the same drawbacks as <code>auto</code>, |
| because they are both mechanisms for deducing all or part of a variable's |
| type from its initializer. CTAD does give the reader more information |
| than <code>auto</code>, but it also doesn't give the reader an obvious |
| cue that information has been omitted.</p> |
| |
| <p class="decision"></p> |
| <p>Do not use CTAD with a given template unless the template's maintainers |
| have opted into supporting use of CTAD by providing at least one explicit |
| deduction guide (all templates in the <code>std</code> namespace are |
| also presumed to have opted in). This should be enforced with a compiler |
| warning if available.</p> |
| |
| <p>Uses of CTAD must also follow the general rules on |
| <a href="#Type_deduction">Type deduction</a>.</p> |
| |
| <h3 id="Lambda_expressions">Lambda expressions</h3> |
| |
| <p>Use lambda expressions where appropriate. Prefer explicit captures |
| when the lambda will escape the current scope.</p> |
| |
| <p class="definition"></p> |
| <p> Lambda expressions are a concise way of creating anonymous |
| function objects. They're often useful when passing |
| functions as arguments. For example:</p> |
| |
| <pre>std::sort(v.begin(), v.end(), [](int x, int y) { |
| return Weight(x) < Weight(y); |
| }); |
| </pre> |
| |
| <p> They further allow capturing variables from the enclosing scope either |
| explicitly by name, or implicitly using a default capture. Explicit captures |
| require each variable to be listed, as |
| either a value or reference capture:</p> |
| |
| <pre>int weight = 3; |
| int sum = 0; |
| // Captures `weight` by value and `sum` by reference. |
| std::for_each(v.begin(), v.end(), [weight, &sum](int x) { |
| sum += weight * x; |
| }); |
| </pre> |
| |
| |
| <p>Default captures implicitly capture any variable referenced in the |
| lambda body, including <code>this</code> if any members are used:</p> |
| |
| <pre>const std::vector<int> lookup_table = ...; |
| std::vector<int> indices = ...; |
| // Captures `lookup_table` by reference, sorts `indices` by the value |
| // of the associated element in `lookup_table`. |
| std::sort(indices.begin(), indices.end(), [&](int a, int b) { |
| return lookup_table[a] < lookup_table[b]; |
| }); |
| </pre> |
| |
| <p>A variable capture can also have an explicit initializer, which can |
| be used for capturing move-only variables by value, or for other situations |
| not handled by ordinary reference or value captures: |
| </p><pre>std::unique_ptr<Foo> foo = ...; |
| [foo = std::move(foo)] () { |
| ... |
| }</pre> |
| Such captures (often called "init captures" or "generalized lambda captures") |
| need not actually "capture" anything from the enclosing scope, or even have |
| a name from the enclosing scope; this syntax is a fully general way to define |
| members of a lambda object: |
| <pre class="neutralcode">[foo = std::vector<int>({1, 2, 3})] () { |
| ... |
| }</pre> |
| The type of a capture with an initializer is deduced using the same rules |
| as <code>auto</code>. |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Lambdas are much more concise than other ways of |
| defining function objects to be passed to STL |
| algorithms, which can be a readability |
| improvement.</li> |
| |
| <li>Appropriate use of default captures can remove |
| redundancy and highlight important exceptions from |
| the default.</li> |
| |
| <li>Lambdas, <code>std::function</code>, and |
| <code>std::bind</code> can be used in combination as a |
| general purpose callback mechanism; they make it easy |
| to write functions that take bound functions as |
| arguments.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Variable capture in lambdas can be a source of dangling-pointer |
| bugs, particularly if a lambda escapes the current scope.</li> |
| |
| <li>Default captures by value can be misleading because they do not prevent |
| dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep |
| copy, so it often has the same lifetime issues as capture by reference. |
| This is especially confusing when capturing 'this' by value, since the use |
| of 'this' is often implicit.</li> |
| |
| <li>Captures actually declare new variables (whether or not the captures have |
| initializers), but they look nothing like any other variable declaration |
| syntax in C++. In particular, there's no place for the variable's type, |
| or even an <code>auto</code> placeholder (although init captures can |
| indicate it indirectly, e.g. with a cast). This can make it difficult to |
| even recognize them as declarations.</li> |
| |
| <li>Init captures inherently rely on <a href="#Type_deduction">type |
| deduction</a>, and suffer from many of the same drawbacks as |
| <code>auto</code>, with the additional problem that the syntax doesn't |
| even cue the reader that deduction is taking place.</li> |
| |
| <li>It's possible for use of lambdas to get out of |
| hand; very long nested anonymous functions can make |
| code harder to understand.</li> |
| |
| </ul> |
| |
| <p class="decision"></p> |
| <ul> |
| <li>Use lambda expressions where appropriate, with formatting as |
| described <a href="#Formatting_Lambda_Expressions">below</a>.</li> |
| <li>Prefer explicit captures if the lambda may escape the current scope. |
| For example, instead of: |
| <pre class="badcode">{ |
| Foo foo; |
| ... |
| executor->Schedule([&] { Frobnicate(foo); }) |
| ... |
| } |
| // BAD! The fact that the lambda makes use of a reference to `foo` and |
| // possibly `this` (if `Frobnicate` is a member function) may not be |
| // apparent on a cursory inspection. If the lambda is invoked after |
| // the function returns, that would be bad, because both `foo` |
| // and the enclosing object could have been destroyed. |
| </pre> |
| prefer to write: |
| <pre>{ |
| Foo foo; |
| ... |
| executor->Schedule([&foo] { Frobnicate(foo); }) |
| ... |
| } |
| // BETTER - The compile will fail if `Frobnicate` is a member |
| // function, and it's clearer that `foo` is dangerously captured by |
| // reference. |
| </pre> |
| </li> |
| <li>Use default capture by reference ([&]) only when the |
| lifetime of the lambda is obviously shorter than any potential |
| captures. |
| </li> |
| <li>Use default capture by value ([=]) only as a means of binding a |
| few variables for a short lambda, where the set of captured |
| variables is obvious at a glance. Prefer not to write long or |
| complex lambdas with default capture by value. |
| </li> |
| <li>Use captures only to actually capture variables from the enclosing scope. |
| Do not use captures with initializers to introduce new names, or |
| to substantially change the meaning of an existing name. Instead, |
| declare a new variable in the conventional way and then capture it, |
| or avoid the lambda shorthand and define a function object explicitly.</li> |
| <li>See the section on <a href="#Type_deduction">type deduction</a> |
| for guidance on specifying the parameter and return types.</li> |
| |
| </ul> |
| |
| <h3 id="Template_metaprogramming">Template metaprogramming</h3> |
| |
| <p>Avoid complicated template programming.</p> |
| |
| <p class="definition"></p> |
| <p>Template metaprogramming refers to a family of techniques that |
| exploit the fact that the C++ template instantiation mechanism is |
| Turing complete and can be used to perform arbitrary compile-time |
| computation in the type domain.</p> |
| |
| <p class="pros"></p> |
| <p>Template metaprogramming allows extremely flexible interfaces that |
| are type safe and high performance. Facilities like |
| |
| <a href="https://code.google.com/p/googletest/">Google Test</a>, |
| <code>std::tuple</code>, <code>std::function</code>, and |
| Boost.Spirit would be impossible without it.</p> |
| |
| <p class="cons"></p> |
| <p>The techniques used in template metaprogramming are often obscure |
| to anyone but language experts. Code that uses templates in |
| complicated ways is often unreadable, and is hard to debug or |
| maintain.</p> |
| |
| <p>Template metaprogramming often leads to extremely poor compile |
| time error messages: even if an interface is simple, the complicated |
| implementation details become visible when the user does something |
| wrong.</p> |
| |
| <p>Template metaprogramming interferes with large scale refactoring by |
| making the job of refactoring tools harder. First, the template code |
| is expanded in multiple contexts, and it's hard to verify that the |
| transformation makes sense in all of them. Second, some refactoring |
| tools work with an AST that only represents the structure of the code |
| after template expansion. It can be difficult to automatically work |
| back to the original source construct that needs to be |
| rewritten.</p> |
| |
| <p class="decision"></p> |
| <p>Template metaprogramming sometimes allows cleaner and easier-to-use |
| interfaces than would be possible without it, but it's also often a |
| temptation to be overly clever. It's best used in a small number of |
| low level components where the extra maintenance burden is spread out |
| over a large number of uses.</p> |
| |
| <p>Think twice before using template metaprogramming or other |
| complicated template techniques; think about whether the average |
| member of your team will be able to understand your code well enough |
| to maintain it after you switch to another project, or whether a |
| non-C++ programmer or someone casually browsing the code base will be |
| able to understand the error messages or trace the flow of a function |
| they want to call. If you're using recursive template instantiations |
| or type lists or metafunctions or expression templates, or relying on |
| SFINAE or on the <code>sizeof</code> trick for detecting function |
| overload resolution, then there's a good chance you've gone too |
| far.</p> |
| |
| <p>If you use template metaprogramming, you should expect to put |
| considerable effort into minimizing and isolating the complexity. You |
| should hide metaprogramming as an implementation detail whenever |
| possible, so that user-facing headers are readable, and you should |
| make sure that tricky code is especially well commented. You should |
| carefully document how the code is used, and you should say something |
| about what the "generated" code looks like. Pay extra attention to the |
| error messages that the compiler emits when users make mistakes. The |
| error messages are part of your user interface, and your code should |
| be tweaked as necessary so that the error messages are understandable |
| and actionable from a user point of view.</p> |
| |
| <h3 id="Boost">Boost</h3> |
| |
| <p>Use only approved libraries from the Boost library |
| collection.</p> |
| |
| <p class="definition"></p> |
| <p> The |
| <a href="https://www.boost.org/"> |
| Boost library collection</a> is a popular collection of |
| peer-reviewed, free, open-source C++ libraries.</p> |
| |
| <p class="pros"></p> |
| <p>Boost code is generally very high-quality, is widely |
| portable, and fills many important gaps in the C++ |
| standard library, such as type traits and better binders.</p> |
| |
| <p class="cons"></p> |
| <p>Some Boost libraries encourage coding practices which can |
| hamper readability, such as metaprogramming and other |
| advanced template techniques, and an excessively |
| "functional" style of programming. </p> |
| |
| <p class="decision"></p> |
| |
| |
| |
| <div> |
| <p>In order to maintain a high level of readability for |
| all contributors who might read and maintain code, we |
| only allow an approved subset of Boost features. |
| Currently, the following libraries are permitted:</p> |
| |
| <ul> |
| <li> |
| <a href="https://www.boost.org/libs/utility/call_traits.htm"> |
| Call Traits</a> from <code>boost/call_traits.hpp</code></li> |
| |
| <li><a href="https://www.boost.org/libs/utility/compressed_pair.htm"> |
| Compressed Pair</a> from <code>boost/compressed_pair.hpp</code></li> |
| |
| <li><a href="https://www.boost.org/libs/graph/"> |
| The Boost Graph Library (BGL)</a> from <code>boost/graph</code>, |
| except serialization (<code>adj_list_serialize.hpp</code>) and |
| parallel/distributed algorithms and data structures |
| (<code>boost/graph/parallel/*</code> and |
| <code>boost/graph/distributed/*</code>).</li> |
| |
| <li><a href="https://www.boost.org/libs/property_map/"> |
| Property Map</a> from <code>boost/property_map</code>, except |
| parallel/distributed property maps (<code>boost/property_map/parallel/*</code>).</li> |
| |
| <li><a href="https://www.boost.org/libs/iterator/"> |
| Iterator</a> from <code>boost/iterator</code></li> |
| |
| <li>The part of <a href="https://www.boost.org/libs/polygon/"> |
| Polygon</a> that deals with Voronoi diagram |
| construction and doesn't depend on the rest of |
| Polygon: |
| <code>boost/polygon/voronoi_builder.hpp</code>, |
| <code>boost/polygon/voronoi_diagram.hpp</code>, and |
| <code>boost/polygon/voronoi_geometry_type.hpp</code></li> |
| |
| <li><a href="https://www.boost.org/libs/bimap/"> |
| Bimap</a> from <code>boost/bimap</code></li> |
| |
| <li><a href="https://www.boost.org/libs/math/doc/html/dist.html"> |
| Statistical Distributions and Functions</a> from |
| <code>boost/math/distributions</code></li> |
| |
| <li><a href="https://www.boost.org/libs/math/doc/html/special.html"> |
| Special Functions</a> from <code>boost/math/special_functions</code></li> |
| |
| <li><a href="https://www.boost.org/libs/math/doc/html/root_finding.html"> |
| Root Finding Functions</a> from <code>boost/math/tools</code></li> |
| |
| <li><a href="https://www.boost.org/libs/multi_index/"> |
| Multi-index</a> from <code>boost/multi_index</code></li> |
| |
| <li><a href="https://www.boost.org/libs/heap/"> |
| Heap</a> from <code>boost/heap</code></li> |
| |
| <li>The flat containers from |
| <a href="https://www.boost.org/libs/container/">Container</a>: |
| <code>boost/container/flat_map</code>, and |
| <code>boost/container/flat_set</code></li> |
| |
| <li><a href="https://www.boost.org/libs/intrusive/">Intrusive</a> |
| from <code>boost/intrusive</code>.</li> |
| |
| <li><a href="https://www.boost.org/libs/sort/">The |
| <code>boost/sort</code> library</a>.</li> |
| |
| <li><a href="https://www.boost.org/libs/preprocessor/">Preprocessor</a> |
| from <code>boost/preprocessor</code>.</li> |
| </ul> |
| |
| <p>We are actively considering adding other Boost |
| features to the list, so this list may be expanded in |
| the future.</p> |
| </div> |
| |
| |
| |
| <h3 id="std_hash">std::hash</h3> |
| |
| <p>Do not define specializations of <code>std::hash</code>.</p> |
| |
| <p class="definition"></p> |
| <p><code>std::hash<T></code> is the function object that the |
| C++11 hash containers use to hash keys of type <code>T</code>, |
| unless the user explicitly specifies a different hash function. For |
| example, <code>std::unordered_map<int, std::string></code> is a hash |
| map that uses <code>std::hash<int></code> to hash its keys, |
| whereas <code>std::unordered_map<int, std::string, MyIntHash></code> |
| uses <code>MyIntHash</code>.</p> |
| |
| <p><code>std::hash</code> is defined for all integral, floating-point, |
| pointer, and <code>enum</code> types, as well as some standard library |
| types such as <code>string</code> and <code>unique_ptr</code>. Users |
| can enable it to work for their own types by defining specializations |
| of it for those types.</p> |
| |
| <p class="pros"></p> |
| <p><code>std::hash</code> is easy to use, and simplifies the code |
| since you don't have to name it explicitly. Specializing |
| <code>std::hash</code> is the standard way of specifying how to |
| hash a type, so it's what outside resources will teach, and what |
| new engineers will expect.</p> |
| |
| <p class="cons"></p> |
| <p><code>std::hash</code> is hard to specialize. It requires a lot |
| of boilerplate code, and more importantly, it combines responsibility |
| for identifying the hash inputs with responsibility for executing the |
| hashing algorithm itself. The type author has to be responsible for |
| the former, but the latter requires expertise that a type author |
| usually doesn't have, and shouldn't need. The stakes here are high |
| because low-quality hash functions can be security vulnerabilities, |
| due to the emergence of |
| <a href="https://emboss.github.io/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/"> |
| hash flooding attacks</a>.</p> |
| |
| <p>Even for experts, <code>std::hash</code> specializations are |
| inordinately difficult to implement correctly for compound types, |
| because the implementation cannot recursively call <code>std::hash</code> |
| on data members. High-quality hash algorithms maintain large |
| amounts of internal state, and reducing that state to the |
| <code>size_t</code> bytes that <code>std::hash</code> |
| returns is usually the slowest part of the computation, so it |
| should not be done more than once.</p> |
| |
| <p>Due to exactly that issue, <code>std::hash</code> does not work |
| with <code>std::pair</code> or <code>std::tuple</code>, and the |
| language does not allow us to extend it to support them.</p> |
| |
| <p class="decision"></p> |
| <p>You can use <code>std::hash</code> with the types that it supports |
| "out of the box", but do not specialize it to support additional types. |
| If you need a hash table with a key type that <code>std::hash</code> |
| does not support, consider using legacy hash containers (e.g. |
| <code>hash_map</code>) for now; they use a different default hasher, |
| which is unaffected by this prohibition.</p> |
| |
| <p>If you want to use the standard hash containers anyway, you will |
| need to specify a custom hasher for the key type, e.g.</p> |
| <pre>std::unordered_map<MyKeyType, Value, MyKeyTypeHasher> my_map; |
| </pre><p> |
| Consult with the type's owners to see if there is an existing hasher |
| that you can use; otherwise work with them to provide one, |
| or roll your own.</p> |
| |
| <p>We are planning to provide a hash function that can work with any type, |
| using a new customization mechanism that doesn't have the drawbacks of |
| <code>std::hash</code>.</p> |
| |
| |
| |
| <h3 id="Other_Features"><a name="C++11">Other C++ Features</a></h3> |
| |
| <p>As with <a href="#Boost">Boost</a>, some modern C++ |
| extensions encourage coding practices that hamper |
| readability—for example by removing |
| checked redundancy (such as type names) that may be |
| helpful to readers, or by encouraging template |
| metaprogramming. Other extensions duplicate functionality |
| available through existing mechanisms, which may lead to confusion |
| and conversion costs.</p> |
| |
| <p class="decision"></p> |
| <p>In addition to what's described in the rest of the style |
| guide, the following C++ features may not be used:</p> |
| |
| <ul> |
| |
| |
| <li>Compile-time rational numbers |
| (<code><ratio></code>), because of concerns that |
| it's tied to a more template-heavy interface |
| style.</li> |
| |
| <li>The <code><cfenv></code> and |
| <code><fenv.h></code> headers, because many |
| compilers do not support those features reliably.</li> |
| |
| <li>The <code><filesystem></code> header, which |
| |
| does not have sufficient support for testing, and suffers |
| from inherent security vulnerabilities.</li> |
| |
| |
| </ul> |
| |
| <h3 id="Nonstandard_Extensions">Nonstandard Extensions</h3> |
| |
| <p>Nonstandard extensions to C++ may not be used unless otherwise specified.</p> |
| |
| <p class="definition"></p> |
| <p>Compilers support various extensions that are not part of standard C++. Such |
| extensions include GCC's <code>__attribute__</code>, intrinsic functions such |
| as <code>__builtin_prefetch</code>, designated initializers (e.g. |
| <code>Foo f = {.field = 3}</code>), inline assembly, <code>__COUNTER__</code>, |
| <code>__PRETTY_FUNCTION__</code>, compound statement expressions (e.g. |
| <code>foo = ({ int x; Bar(&x); x })</code>, variable-length arrays and |
| <code>alloca()</code>, and the "<a href="https://en.wikipedia.org/wiki/Elvis_operator">Elvis Operator</a>" |
| <code>a?:b</code>.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Nonstandard extensions may provide useful features that do not exist |
| in standard C++. For example, some people think that designated |
| initializers are more readable than standard C++ features like |
| constructors.</li> |
| <li>Important performance guidance to the compiler can only be specified |
| using extensions.</li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>Nonstandard extensions do not work in all compilers. Use of nonstandard |
| extensions reduces portability of code.</li> |
| <li>Even if they are supported in all targeted compilers, the extensions |
| are often not well-specified, and there may be subtle behavior differences |
| between compilers.</li> |
| <li>Nonstandard extensions add to the language features that a reader must |
| know to understand the code.</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Do not use nonstandard extensions. You may use portability wrappers that |
| are implemented using nonstandard extensions, so long as those wrappers |
| |
| are provided by a designated project-wide |
| portability header.</p> |
| |
| <h3 id="Aliases">Aliases</h3> |
| |
| <p>Public aliases are for the benefit of an API's user, and should be clearly documented.</p> |
| |
| <p class="definition"></p> |
| <p>There are several ways to create names that are aliases of other entities:</p> |
| <pre>typedef Foo Bar; |
| using Bar = Foo; |
| using other_namespace::Foo; |
| </pre> |
| |
| <p>In new code, <code>using</code> is preferable to <code>typedef</code>, |
| because it provides a more consistent syntax with the rest of C++ and works |
| with templates.</p> |
| |
| <p>Like other declarations, aliases declared in a header file are part of that |
| header's public API unless they're in a function definition, in the private portion of a class, |
| or in an explicitly-marked internal namespace. Aliases in such areas or in .cc files are |
| implementation details (because client code can't refer to them), and are not restricted by this |
| rule.</p> |
| |
| <p class="pros"></p> |
| <ul> |
| <li>Aliases can improve readability by simplifying a long or complicated name.</li> |
| <li>Aliases can reduce duplication by naming in one place a type used repeatedly in an API, |
| which <em>might</em> make it easier to change the type later. |
| </li> |
| </ul> |
| |
| <p class="cons"></p> |
| <ul> |
| <li>When placed in a header where client code can refer to them, aliases increase the |
| number of entities in that header's API, increasing its complexity.</li> |
| <li>Clients can easily rely on unintended details of public aliases, making |
| changes difficult.</li> |
| <li>It can be tempting to create a public alias that is only intended for use |
| in the implementation, without considering its impact on the API, or on maintainability.</li> |
| <li>Aliases can create risk of name collisions</li> |
| <li>Aliases can reduce readability by giving a familiar construct an unfamiliar name</li> |
| <li>Type aliases can create an unclear API contract: |
| it is unclear whether the alias is guaranteed to be identical to the type it aliases, |
| to have the same API, or only to be usable in specified narrow ways</li> |
| </ul> |
| |
| <p class="decision"></p> |
| <p>Don't put an alias in your public API just to save typing in the implementation; |
| do so only if you intend it to be used by your clients.</p> |
| <p>When defining a public alias, document the intent of |
| the new name, including whether it is guaranteed to always be the same as the type |
| it's currently aliased to, or whether a more limited compatibility is |
| intended. This lets the user know whether they can treat the types as |
| substitutable or whether more specific rules must be followed, and can help the |
| implementation retain some degree of freedom to change the alias.</p> |
| <p>Don't put namespace aliases in your public API. (See also <a href="#Namespaces">Namespaces</a>). |
| </p> |
| |
| <p>For example, these aliases document how they are intended to be used in client code:</p> |
| <pre>namespace mynamespace { |
| // Used to store field measurements. DataPoint may change from Bar* to some internal type. |
| // Client code should treat it as an opaque pointer. |
| using DataPoint = foo::Bar*; |
| |
| // A set of measurements. Just an alias for user convenience. |
| using TimeSeries = std::unordered_set<DataPoint, std::hash<DataPoint>, DataPointComparator>; |
| } // namespace mynamespace |
| </pre> |
| |
| <p>These aliases don't document intended use, and half of them aren't meant for client use:</p> |
| |
| <pre class="badcode">namespace mynamespace { |
| // Bad: none of these say how they should be used. |
| using DataPoint = foo::Bar*; |
| using std::unordered_set; // Bad: just for local convenience |
| using std::hash; // Bad: just for local convenience |
| typedef unordered_set<DataPoint, hash<DataPoint>, DataPointComparator> TimeSeries; |
| } // namespace mynamespace |
| </pre> |
| |
| <p>However, local convenience aliases are fine in function definitions, private sections of |
| classes, explicitly marked internal namespaces, and in .cc files:</p> |
| |
| <pre>// In a .cc file |
| using foo::Bar; |
| </pre> |
| |
| <h2 id="Naming">Naming</h2> |
| |
| <p>The most important consistency rules are those that govern |
| naming. The style of a name immediately informs us what sort of |
| thing the named entity is: a type, a variable, a function, a |
| constant, a macro, etc., without requiring us to search for the |
| declaration of that entity. The pattern-matching engine in our |
| brains relies a great deal on these naming rules. |
| </p> |
| |
| <p>Naming rules are pretty arbitrary, but |
| we feel that |
| consistency is more important than individual preferences in this |
| area, so regardless of whether you find them sensible or not, |
| the rules are the rules.</p> |
| |
| <h3 id="General_Naming_Rules">General Naming Rules</h3> |
| |
| <p>Optimize for readability using names that would be clear |
| even to people on a different team.</p> |
| |
| <p>Use names that describe the purpose or intent of the object. |
| Do not worry about saving horizontal space as it is far |
| more important to make your code immediately |
| understandable by a new reader. Minimize the use of |
| abbreviations that would likely be unknown to someone outside |
| your project (especially acronyms and initialisms). Do not |
| abbreviate by deleting letters within a word. As a rule of thumb, |
| an abbreviation is probably OK if it's listed in |
| Wikipedia. Generally speaking, descriptiveness should be |
| proportional to the name's scope of visibility. For example, |
| <code>n</code> may be a fine name within a 5-line function, |
| but within the scope of a class, it's likely too vague.</p> |
| |
| <pre>class MyClass { |
| public: |
| int CountFooErrors(const std::vector<Foo>& foos) { |
| int n = 0; // Clear meaning given limited scope and context |
| for (const auto& foo : foos) { |
| ... |
| ++n; |
| } |
| return n; |
| } |
| void DoSomethingImportant() { |
| std::string fqdn = ...; // Well-known abbreviation for Fully Qualified Domain Name |
| } |
| private: |
| const int kMaxAllowedConnections = ...; // Clear meaning within context |
| }; |
| </pre> |
| |
| <pre class="badcode">class MyClass { |
| public: |
| int CountFooErrors(const std::vector<Foo>& foos) { |
| int total_number_of_foo_errors = 0; // Overly verbose given limited scope and context |
| for (int foo_index = 0; foo_index < foos.size(); ++foo_index) { // Use idiomatic `i` |
| ... |
| ++total_number_of_foo_errors; |
| } |
| return total_number_of_foo_errors; |
| } |
| void DoSomethingImportant() { |
| int cstmr_id = ...; // Deletes internal letters |
| } |
| private: |
| const int kNum = ...; // Unclear meaning within broad scope |
| }; |
| </pre> |
| |
| <p>Note that certain universally-known abbreviations are OK, such as |
| <code>i</code> for an iteration variable and <code>T</code> for a |
| template parameter.</p> |
| |
| <p>For the purposes of the naming rules below, a "word" is anything that you |
| would write in English without internal spaces. This includes abbreviations and |
| acronyms; e.g., for "<a href="https://en.wikipedia.org/wiki/Camel_case">camel |
| case</a>" or "Pascal case," in which the first letter of each word is |
| capitalized, use a name like <code>StartRpc()</code>, not |
| <code>StartRPC()</code>.</p> |
| |
| <p>Template parameters should follow the naming style for their |
| category: type template parameters should follow the rules for |
| <a href="#Type_Names">type names</a>, and non-type template |
| parameters should follow the rules for <a href="#Variable_Names"> |
| variable names</a>. |
| |
| </p><h3 id="File_Names">File Names</h3> |
| |
| <p>Filenames should be all lowercase and can include |
| underscores (<code>_</code>) or dashes (<code>-</code>). |
| Follow the convention that your |
| |
| project uses. If there is no consistent |
| local pattern to follow, prefer "_".</p> |
| |
| <p>Examples of acceptable file names:</p> |
| |
| <ul> |
| <li><code>my_useful_class.cc</code></li> |
| <li><code>my-useful-class.cc</code></li> |
| <li><code>myusefulclass.cc</code></li> |
| <li><code>myusefulclass_test.cc // _unittest and _regtest are deprecated.</code></li> |
| </ul> |
| |
| <p>C++ files should end in <code>.cc</code> and header files should end in |
| <code>.h</code>. Files that rely on being textually included at specific points |
| should end in <code>.inc</code> (see also the section on |
| <a href="#Self_contained_Headers">self-contained headers</a>).</p> |
| |
| <p>Do not use filenames that already exist in |
| <code>/usr/include</code>, such as <code>db.h</code>.</p> |
| |
| <p>In general, make your filenames very specific. For |
| example, use <code>http_server_logs.h</code> rather than |
| <code>logs.h</code>. A very common case is to have a pair |
| of files called, e.g., <code>foo_bar.h</code> and |
| <code>foo_bar.cc</code>, defining a class called |
| <code>FooBar</code>.</p> |
| |
| <h3 id="Type_Names">Type Names</h3> |
| |
| <p>Type names start with a capital letter and have a capital |
| letter for each new word, with no underscores: |
| <code>MyExcitingClass</code>, <code>MyExcitingEnum</code>.</p> |
| |
| <p>The names of all types — classes, structs, type aliases, |
| enums, and type template parameters — have the same naming convention. |
| Type names should start with a capital letter and have a capital letter |
| for each new word. No underscores. For example:</p> |
| |
| <pre>// classes and structs |
| class UrlTable { ... |
| class UrlTableTester { ... |
| struct UrlTableProperties { ... |
| |
| // typedefs |
| typedef hash_map<UrlTableProperties *, std::string> PropertiesMap; |
| |
| // using aliases |
| using PropertiesMap = hash_map<UrlTableProperties *, std::string>; |
| |
| // enums |
| enum UrlTableErrors { ... |
| </pre> |
| |
| <h3 id="Variable_Names">Variable Names</h3> |
| |
| <p>The names of variables (including function parameters) and data members are |
| all lowercase, with underscores between words. Data members of classes (but not |
| structs) additionally have trailing underscores. For instance: |
| <code>a_local_variable</code>, <code>a_struct_data_member</code>, |
| <code>a_class_data_member_</code>.</p> |
| |
| <h4>Common Variable names</h4> |
| |
| <p>For example:</p> |
| |
| <pre>std::string table_name; // OK - lowercase with underscore. |
| </pre> |
| |
| <pre class="badcode">std::string tableName; // Bad - mixed case. |
| </pre> |
| |
| <h4>Class Data Members</h4> |
| |
| <p>Data members of classes, both static and non-static, are |
| named like ordinary nonmember variables, but with a |
| trailing underscore.</p> |
| |
| <pre>class TableInfo { |
| ... |
| private: |
| std::string table_name_; // OK - underscore at end. |
| static Pool<TableInfo>* pool_; // OK. |
| }; |
| </pre> |
| |
| <h4>Struct Data Members</h4> |
| |
| <p>Data members of structs, both static and non-static, |
| are named like ordinary nonmember variables. They do not have |
| the trailing underscores that data members in classes have.</p> |
| |
| <pre>struct UrlTableProperties { |
| std::string name; |
| int num_entries; |
| static Pool<UrlTableProperties>* pool; |
| }; |
| </pre> |
| |
| |
| <p>See <a href="#Structs_vs._Classes">Structs vs. |
| Classes</a> for a discussion of when to use a struct |
| versus a class.</p> |
| |
| <h3 id="Constant_Names">Constant Names</h3> |
| |
| <p>Variables declared constexpr or const, and whose value is fixed for |
| the duration of the program, are named with a leading "k" followed |
| by mixed case. Underscores can be used as separators in the rare cases |
| where capitalization cannot be used for separation. For example:</p> |
| |
| <pre>const int kDaysInAWeek = 7; |
| const int kAndroid8_0_0 = 24; // Android 8.0.0 |
| </pre> |
| |
| <p>All such variables with static storage duration (i.e. statics and globals, |
| see <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration"> |
| Storage Duration</a> for details) should be named this way. This |
| convention is optional for variables of other storage classes, e.g. automatic |
| variables, otherwise the usual variable naming rules apply.</p> |
| |
| <h3 id="Function_Names">Function Names</h3> |
| |
| <p>Regular functions have mixed case; accessors and mutators may be named |
| like variables.</p> |
| |
| <p>Ordinarily, functions should start with a capital letter and have a |
| capital letter for each new word.</p> |
| |
| <pre>AddTableEntry() |
| DeleteUrl() |
| OpenFileOrDie() |
| </pre> |
| |
| <p>(The same naming rule applies to class- and namespace-scope |
| constants that are exposed as part of an API and that are intended to look |
| like functions, because the fact that they're objects rather than functions |
| is an unimportant implementation detail.)</p> |
| |
| <p>Accessors and mutators (get and set functions) may be named like |
| variables. These often correspond to actual member variables, but this is |
| not required. For example, <code>int count()</code> and <code>void |
| set_count(int count)</code>.</p> |
| |
| <h3 id="Namespace_Names">Namespace Names</h3> |
| |
| Namespace names are all lower-case. Top-level namespace names are |
| based on the project name |
| . Avoid collisions |
| between nested namespaces and well-known top-level namespaces. |
| |
| <p>The name of a top-level namespace should usually be the |
| name of the project or team whose code is contained in that |
| namespace. The code in that namespace should usually be in |
| a directory whose basename matches the namespace name (or in |
| subdirectories thereof).</p> |
| |
| |
| |
| <p>Keep in mind that the <a href="#General_Naming_Rules">rule |
| against abbreviated names</a> applies to namespaces just as much |
| as variable names. Code inside the namespace seldom needs to |
| mention the namespace name, so there's usually no particular need |
| for abbreviation anyway.</p> |
| |
| <p>Avoid nested namespaces that match well-known top-level |
| namespaces. Collisions between namespace names can lead to surprising |
| build breaks because of name lookup rules. In particular, do not |
| create any nested <code>std</code> namespaces. Prefer unique project |
| identifiers |
| (<code>websearch::index</code>, <code>websearch::index_util</code>) |
| over collision-prone names like <code>websearch::util</code>.</p> |
| |
| <p>For <code>internal</code> namespaces, be wary of other code being |
| added to the same <code>internal</code> namespace causing a collision |
| (internal helpers within a team tend to be related and may lead to |
| collisions). In such a situation, using the filename to make a unique |
| internal name is helpful |
| (<code>websearch::index::frobber_internal</code> for use |
| in <code>frobber.h</code>)</p> |
| |
| <h3 id="Enumerator_Names">Enumerator Names</h3> |
| |
| <p>Enumerators (for both scoped and unscoped enums) should be named <i>either</i> like |
| <a href="#Constant_Names">constants</a> or like |
| <a href="#Macro_Names">macros</a>: either <code>kEnumName</code> or |
| <code>ENUM_NAME</code>.</p> |
| |
| <p>Preferably, the individual enumerators should be named |
| like <a href="#Constant_Names">constants</a>. However, it |
| is also acceptable to name them like |
| <a href="#Macro_Names">macros</a>. The enumeration name, |
| <code>UrlTableErrors</code> (and |
| <code>AlternateUrlTableErrors</code>), is a type, and |
| therefore mixed case.</p> |
| |
| <pre>enum UrlTableErrors { |
| kOk = 0, |
| kErrorOutOfMemory, |
| kErrorMalformedInput, |
| }; |
| enum AlternateUrlTableErrors { |
| OK = 0, |
| OUT_OF_MEMORY = 1, |
| MALFORMED_INPUT = 2, |
| }; |
| </pre> |
| |
| <p>Until January 2009, the style was to name enum values |
| like <a href="#Macro_Names">macros</a>. This caused |
| problems with name collisions between enum values and |
| macros. Hence, the change to prefer constant-style naming |
| was put in place. New code should prefer constant-style |
| naming if possible. However, there is no reason to change |
| old code to use constant-style names, unless the old |
| names are actually causing a compile-time problem.</p> |
| |
| |
| |
| <h3 id="Macro_Names">Macro Names</h3> |
| |
| <p>You're not really going to <a href="#Preprocessor_Macros"> |
| define a macro</a>, are you? If you do, they're like this: |
| <code>MY_MACRO_THAT_SCARES_SMALL_CHILDREN_AND_ADULTS_ALIKE</code>. |
| </p> |
| |
| <p>Please see the <a href="#Preprocessor_Macros">description |
| of macros</a>; in general macros should <em>not</em> be used. |
| However, if they are absolutely needed, then they should be |
| named with all capitals and underscores.</p> |
| |
| <pre>#define ROUND(x) ... |
| #define PI_ROUNDED 3.0 |
| </pre> |
| |
| <h3 id="Exceptions_to_Naming_Rules">Exceptions to Naming Rules</h3> |
| |
| <p>If you are naming something that is analogous to an |
| existing C or C++ entity then you can follow the existing |
| naming convention scheme.</p> |
| |
| <dl> |
| <dt><code>bigopen()</code></dt> |
| <dd>function name, follows form of <code>open()</code></dd> |
| |
| <dt><code>uint</code></dt> |
| <dd><code>typedef</code></dd> |
| |
| <dt><code>bigpos</code></dt> |
| <dd><code>struct</code> or <code>class</code>, follows |
| form of <code>pos</code></dd> |
| |
| <dt><code>sparse_hash_map</code></dt> |
| <dd>STL-like entity; follows STL naming conventions</dd> |
| |
| <dt><code>LONGLONG_MAX</code></dt> |
| <dd>a constant, as in <code>INT_MAX</code></dd> |
| </dl> |
| |
| <h2 id="Comments">Comments</h2> |
| |
| <p>Comments are absolutely vital to keeping our code readable. The following rules describe what you |
| should comment and where. But remember: while comments are very important, the best code is |
| self-documenting. Giving sensible names to types and variables is much better than using obscure |
| names that you must then explain through comments.</p> |
| |
| <p>When writing your comments, write for your audience: the |
| next |
| contributor who will need to |
| understand your code. Be generous — the next |
| one may be you!</p> |
| |
| <h3 id="Comment_Style">Comment Style</h3> |
| |
| <p>Use either the <code>//</code> or <code>/* */</code> |
| syntax, as long as you are consistent.</p> |
| |
| <p>You can use either the <code>//</code> or the <code>/* |
| */</code> syntax; however, <code>//</code> is |
| <em>much</em> more common. Be consistent with how you |
| comment and what style you use where.</p> |
| |
| <h3 id="File_Comments">File Comments</h3> |
| |
| <div> |
| <p>Start each file with license boilerplate.</p> |
| </div> |
| |
| <p>File comments describe the contents of a file. If a file declares, |
| implements, or tests exactly one abstraction that is documented by a comment |
| at the point of declaration, file comments are not required. All other files |
| must have file comments.</p> |
| |
| <h4>Legal Notice and Author |
| Line</h4> |
| |
| |
| |
| <div> |
| <p>Every file should contain license |
| boilerplate. Choose the appropriate boilerplate for the |
| license used by the project (for example, Apache 2.0, |
| BSD, LGPL, GPL).</p> |
| </div> |
| |
| <p>If you make significant changes to a file with an |
| author line, consider deleting the author line. |
| New files should usually not contain copyright notice or |
| author line.</p> |
| |
| <h4>File Contents</h4> |
| |
| <p>If a <code>.h</code> declares multiple abstractions, the file-level comment |
| should broadly describe the contents of the file, and how the abstractions are |
| related. A 1 or 2 sentence file-level comment may be sufficient. The detailed |
| documentation about individual abstractions belongs with those abstractions, |
| not at the file level.</p> |
| |
| <p>Do not duplicate comments in both the <code>.h</code> and the |
| <code>.cc</code>. Duplicated comments diverge.</p> |
| |
| <h3 id="Class_Comments">Class Comments</h3> |
| |
| <p>Every non-obvious class declaration should have an accompanying |
| comment that describes what it is for and how it should be used.</p> |
| |
| <pre>// Iterates over the contents of a GargantuanTable. |
| // Example: |
| // GargantuanTableIterator* iter = table->NewIterator(); |
| // for (iter->Seek("foo"); !iter->done(); iter->Next()) { |
| // process(iter->key(), iter->value()); |
| // } |
| // delete iter; |
| class GargantuanTableIterator { |
| ... |
| }; |
| </pre> |
| |
| <p>The class comment should provide the reader with enough information to know |
| how and when to use the class, as well as any additional considerations |
| necessary to correctly use the class. Document the synchronization assumptions |
| the class makes, if any. If an instance of the class can be accessed by |
| multiple threads, take extra care to document the rules and invariants |
| surrounding multithreaded use.</p> |
| |
| <p>The class comment is often a good place for a small example code snippet |
| demonstrating a simple and focused usage of the class.</p> |
| |
| <p>When sufficiently separated (e.g. <code>.h</code> and <code>.cc</code> |
| files), comments describing the use of the class should go together with its |
| interface definition; comments about the class operation and implementation |
| should accompany the implementation of the class's methods.</p> |
| |
| <h3 id="Function_Comments">Function Comments</h3> |
| |
| <p>Declaration comments describe use of the function (when it is |
| non-obvious); comments at the definition of a function describe |
| operation.</p> |
| |
| <h4>Function Declarations</h4> |
| |
| <p>Almost every function declaration should have comments immediately |
| preceding it that describe what the function does and how to use |
| it. These comments may be omitted only if the function is simple and |
| obvious (e.g. simple accessors for obvious properties of the |
| class). These comments should open with descriptive verbs in the |
| indicative mood ("Opens the file") rather than verbs in the imperative |
| ("Open the file"). The comment describes the function; it does not |
| tell the function what to do. In general, these comments do not |
| describe how the function performs its task. Instead, that should be |
| left to comments in the function definition.</p> |
| |
| <p>Types of things to mention in comments at the function |
| declaration:</p> |
| |
| <ul> |
| <li>What the inputs and outputs are.</li> |
| |
| <li>For class member functions: whether the object |
| remembers reference arguments beyond the duration of |
| the method call, and whether it will free them or |
| not.</li> |
| |
| <li>If the function allocates memory that the caller |
| must free.</li> |
| |
| <li>Whether any of the arguments can be a null |
| pointer.</li> |
| |
| <li>If there are any performance implications of how a |
| function is used.</li> |
| |
| <li>If the function is re-entrant. What are its |
| synchronization assumptions?</li> |
| </ul> |
| |
| <p>Here is an example:</p> |
| |
| <pre>// Returns an iterator for this table. It is the client's |
| // responsibility to delete the iterator when it is done with it, |
| // and it must not use the iterator once the GargantuanTable object |
| // on which the iterator was created has been deleted. |
| // |
| // The iterator is initially positioned at the beginning of the table. |
| // |
| // This method is equivalent to: |
| // Iterator* iter = table->NewIterator(); |
| // iter->Seek(""); |
| // return iter; |
| // If you are going to immediately seek to another place in the |
| // returned iterator, it will be faster to use NewIterator() |
| // and avoid the extra seek. |
| Iterator* GetIterator() const; |
| </pre> |
| |
| <p>However, do not be unnecessarily verbose or state the |
| completely obvious.</p> |
| |
| <p>When documenting function overrides, focus on the |
| specifics of the override itself, rather than repeating |
| the comment from the overridden function. In many of these |
| cases, the override needs no additional documentation and |
| thus no comment is required.</p> |
| |
| <p>When commenting constructors and destructors, remember |
| that the person reading your code knows what constructors |
| and destructors are for, so comments that just say |
| something like "destroys this object" are not useful. |
| Document what constructors do with their arguments (for |
| example, if they take ownership of pointers), and what |
| cleanup the destructor does. If this is trivial, just |
| skip the comment. It is quite common for destructors not |
| to have a header comment.</p> |
| |
| <h4>Function Definitions</h4> |
| |
| <p>If there is anything tricky about how a function does |
| its job, the function definition should have an |
| explanatory comment. For example, in the definition |
| comment you might describe any coding tricks you use, |
| give an overview of the steps you go through, or explain |
| why you chose to implement the function in the way you |
| did rather than using a viable alternative. For instance, |
| you might mention why it must acquire a lock for the |
| first half of the function but why it is not needed for |
| the second half.</p> |
| |
| <p>Note you should <em>not</em> just repeat the comments |
| given with the function declaration, in the |
| <code>.h</code> file or wherever. It's okay to |
| recapitulate briefly what the function does, but the |
| focus of the comments should be on how it does it.</p> |
| |
| <h3 id="Variable_Comments">Variable Comments</h3> |
| |
| <p>In general the actual name of the variable should be |
| descriptive enough to give a good idea of what the variable |
| is used for. In certain cases, more comments are required.</p> |
| |
| <h4>Class Data Members</h4> |
| |
| <p>The purpose of each class data member (also called an instance |
| variable or member variable) must be clear. If there are any |
| invariants (special values, relationships between members, lifetime |
| requirements) not clearly expressed by the type and name, they must be |
| commented. However, if the type and name suffice (<code>int |
| num_events_;</code>), no comment is needed.</p> |
| |
| <p>In particular, add comments to describe the existence and meaning |
| of sentinel values, such as nullptr or -1, when they are not |
| obvious. For example:</p> |
| |
| <pre>private: |
| // Used to bounds-check table accesses. -1 means |
| // that we don't yet know how many entries the table has. |
| int num_total_entries_; |
| </pre> |
| |
| <h4>Global Variables</h4> |
| |
| <p>All global variables should have a comment describing what they |
| are, what they are used for, and (if unclear) why it needs to be |
| global. For example:</p> |
| |
| <pre>// The total number of tests cases that we run through in this regression test. |
| const int kNumTestCases = 6; |
| </pre> |
| |
| <h3 id="Implementation_Comments">Implementation Comments</h3> |
| |
| <p>In your implementation you should have comments in tricky, |
| non-obvious, interesting, or important parts of your code.</p> |
| |
| <h4>Explanatory Comments</h4> |
| |
| <p>Tricky or complicated code blocks should have comments |
| before them. Example:</p> |
| |
| <pre>// Divide result by two, taking into account that x |
| // contains the carry from the add. |
| for (int i = 0; i < result->size(); ++i) { |
| x = (x << 8) + (*result)[i]; |
| (*result)[i] = x >> 1; |
| x &= 1; |
| } |
| </pre> |
| |
| <h4>Line-end Comments</h4> |
| |
| <p>Also, lines that are non-obvious should get a comment |
| at the end of the line. These end-of-line comments should |
| be separated from the code by 2 spaces. Example:</p> |
| |
| <pre>// If we have enough memory, mmap the data portion too. |
| mmap_budget = max<int64>(0, mmap_budget - index_->length()); |
| if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock)) |
| return; // Error already logged. |
| </pre> |
| |
| <p>Note that there are both comments that describe what |
| the code is doing, and comments that mention that an |
| error has already been logged when the function |
| returns.</p> |
| |
| <h4 id="Function_Argument_Comments" class="stylepoint_subsection">Function Argument Comments</h4> |
| |
| <p>When the meaning of a function argument is nonobvious, consider |
| one of the following remedies:</p> |
| |
| <ul> |
| <li>If the argument is a literal constant, and the same constant is |
| used in multiple function calls in a way that tacitly assumes they're |
| the same, you should use a named constant to make that constraint |
| explicit, and to guarantee that it holds.</li> |
| |
| <li>Consider changing the function signature to replace a <code>bool</code> |
| argument with an <code>enum</code> argument. This will make the argument |
| values self-describing.</li> |
| |
| <li>For functions that have several configuration options, consider |
| defining a single class or struct to hold all the options |
| , |
| and pass an instance of that. |
| This approach has several advantages. Options are referenced by name |
| at the call site, which clarifies their meaning. It also reduces |
| function argument count, which makes function calls easier to read and |
| write. As an added benefit, you don't have to change call sites when |
| you add another option. |
| </li> |
| |
| <li>Replace large or complex nested expressions with named variables.</li> |
| |
| <li>As a last resort, use comments to clarify argument meanings at the |
| call site. </li> |
| </ul> |
| |
| Consider the following example: |
| |
| <pre class="badcode">// What are these arguments? |
| const DecimalNumber product = CalculateProduct(values, 7, false, nullptr); |
| </pre> |
| |
| <p>versus:</p> |
| |
| <pre>ProductOptions options; |
| options.set_precision_decimals(7); |
| options.set_use_cache(ProductOptions::kDontUseCache); |
| const DecimalNumber product = |
| CalculateProduct(values, options, /*completion_callback=*/nullptr); |
| </pre> |
| |
| <h4 id="Implementation_Comment_Donts">Don'ts</h4> |
| |
| <p>Do not state the obvious. In particular, don't literally describe what |
| code does, unless the behavior is nonobvious to a reader who understands |
| C++ well. Instead, provide higher level comments that describe <i>why</i> |
| the code does what it does, or make the code self describing.</p> |
| |
| Compare this: |
| |
| <pre class="badcode">// Find the element in the vector. <-- Bad: obvious! |
| auto iter = std::find(v.begin(), v.end(), element); |
| if (iter != v.end()) { |
| Process(element); |
| } |
| </pre> |
| |
| To this: |
| |
| <pre>// Process "element" unless it was already processed. |
| auto iter = std::find(v.begin(), v.end(), element); |
| if (iter != v.end()) { |
| Process(element); |
| } |
| </pre> |
| |
| Self-describing code doesn't need a comment. The comment from |
| the example above would be obvious: |
| |
| <pre>if (!IsAlreadyProcessed(element)) { |
| Process(element); |
| } |
| </pre> |
| |
| <h3 id="Punctuation,_Spelling_and_Grammar">Punctuation, Spelling, and Grammar</h3> |
| |
| <p>Pay attention to punctuation, spelling, and grammar; it is |
| easier to read well-written comments than badly written |
| ones.</p> |
| |
| <p>Comments should be as readable as narrative text, with |
| proper capitalization and punctuation. In many cases, |
| complete sentences are more readable than sentence |
| fragments. Shorter comments, such as comments at the end |
| of a line of code, can sometimes be less formal, but you |
| should be consistent with your style.</p> |
| |
| <p>Although it can be frustrating to have a code reviewer |
| point out that you are using a comma when you should be |
| using a semicolon, it is very important that source code |
| maintain a high level of clarity and readability. Proper |
| punctuation, spelling, and grammar help with that |
| goal.</p> |
| |
| <h3 id="TODO_Comments">TODO Comments</h3> |
| |
| <p>Use <code>TODO</code> comments for code that is temporary, |
| a short-term solution, or good-enough but not perfect.</p> |
| |
| <p><code>TODO</code>s should include the string |
| <code>TODO</code> in all caps, followed by the |
| |
| name, e-mail address, bug ID, or other |
| identifier |
| of the person or issue with the best context |
| about the problem referenced by the <code>TODO</code>. The |
| main purpose is to have a consistent <code>TODO</code> that |
| can be searched to find out how to get more details upon |
| request. A <code>TODO</code> is not a commitment that the |
| person referenced will fix the problem. Thus when you create |
| a <code>TODO</code> with a name, it is almost always your |
| name that is given.</p> |
| |
| |
| |
| <div> |
| <pre>// TODO(kl@gmail.com): Use a "*" here for concatenation operator. |
| // TODO(Zeke) change this to use relations. |
| // TODO(bug 12345): remove the "Last visitors" feature |
| </pre> |
| </div> |
| |
| <p>If your <code>TODO</code> is of the form "At a future |
| date do something" make sure that you either include a |
| very specific date ("Fix by November 2005") or a very |
| specific event ("Remove this code when all clients can |
| handle XML responses.").</p> |
| |
| <h2 id="Formatting">Formatting</h2> |
| |
| <p>Coding style and formatting are pretty arbitrary, but a |
| |
| project is much easier to follow |
| if everyone uses the same style. Individuals may not agree with every |
| aspect of the formatting rules, and some of the rules may take |
| some getting used to, but it is important that all |
| |
| project contributors follow the |
| style rules so that |
| they can all read and understand |
| everyone's code easily.</p> |
| |
| |
| |
| <div> |
| <p>To help you format code correctly, we've created a |
| <a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/google-c-style.el"> |
| settings file for emacs</a>.</p> |
| </div> |
| |
| <h3 id="Line_Length">Line Length</h3> |
| |
| <p>Each line of text in your code should be at most 80 |
| characters long.</p> |
| |
| |
| |
| <div> |
| <p>We recognize that this rule is |
| controversial, but so much existing code already adheres |
| to it, and we feel that consistency is important.</p> |
| </div> |
| |
| <p class="pros"></p> |
| <p>Those who favor this rule |
| argue that it is rude to force them to resize |
| their windows and there is no need for anything longer. |
| Some folks are used to having several code windows |
| side-by-side, and thus don't have room to widen their |
| windows in any case. People set up their work environment |
| assuming a particular maximum window width, and 80 |
| columns has been the traditional standard. Why change |
| it?</p> |
| |
| <p class="cons"></p> |
| <p>Proponents of change argue that a wider line can make |
| code more readable. The 80-column limit is an hidebound |
| throwback to 1960s mainframes; modern equipment has wide screens that |
| can easily show longer lines.</p> |
| |
| <p class="decision"></p> |
| <p> 80 characters is the maximum.</p> |
| |
| <p>A line may exceed 80 characters if it is</p> |
| |
| <ul> |
| <li>a comment line which is not feasible to split without harming |
| readability, ease of cut and paste or auto-linking -- e.g. if a line |
| contains an example command or a literal URL longer than 80 characters.</li> |
| |
| <li>a raw-string literal with content that exceeds 80 characters. Except for |
| test code, such literals should appear near the top of a file.</li> |
| |
| <li>an include statement.</li> |
| |
| <li>a <a href="#The__define_Guard">header guard</a></li> |
| |
| <li>a using-declaration</li> |
| </ul> |
| |
| <h3 id="Non-ASCII_Characters">Non-ASCII Characters</h3> |
| |
| <p>Non-ASCII characters should be rare, and must use UTF-8 |
| formatting.</p> |
| |
| <p>You shouldn't hard-code user-facing text in source, |
| even English, so use of non-ASCII characters should be |
| rare. However, in certain cases it is appropriate to |
| include such words in your code. For example, if your |
| code parses data files from foreign sources, it may be |
| appropriate to hard-code the non-ASCII string(s) used in |
| those data files as delimiters. More commonly, unittest |
| code (which does not need to be localized) might |
| contain non-ASCII strings. In such cases, you should use |
| UTF-8, since that is an encoding |
| understood by most tools able to handle more than just |
| ASCII.</p> |
| |
| <p>Hex encoding is also OK, and encouraged where it |
| enhances readability — for example, |
| <code>"\xEF\xBB\xBF"</code>, or, even more simply, |
| <code>u8"\uFEFF"</code>, is the Unicode zero-width |
| no-break space character, which would be invisible if |
| included in the source as straight UTF-8.</p> |
| |
| <p>Use the <code>u8</code> prefix |
| to guarantee that a string literal containing |
| <code>\uXXXX</code> escape sequences is encoded as UTF-8. |
| Do not use it for strings containing non-ASCII characters |
| encoded as UTF-8, because that will produce incorrect |
| output if the compiler does not interpret the source file |
| as UTF-8. </p> |
| |
| <p>You shouldn't use the C++11 <code>char16_t</code> and |
| <code>char32_t</code> character types, since they're for |
| non-UTF-8 text. For similar reasons you also shouldn't |
| use <code>wchar_t</code> (unless you're writing code that |
| interacts with the Windows API, which uses |
| <code>wchar_t</code> extensively).</p> |
| |
| <h3 id="Spaces_vs._Tabs">Spaces vs. Tabs</h3> |
| |
| <p>Use only spaces, and indent 2 spaces at a time.</p> |
| |
| <p>We use spaces for indentation. Do not use tabs in your |
| code. You should set your editor to emit spaces when you |
| hit the tab key.</p> |
| |
| <h3 id="Function_Declarations_and_Definitions">Function Declarations and Definitions</h3> |
| |
| <p>Return type on the same line as function name, parameters |
| on the same line if they fit. Wrap parameter lists which do |
| not fit on a single line as you would wrap arguments in a |
| <a href="#Function_Calls">function call</a>.</p> |
| |
| <p>Functions look like this:</p> |
| |
| |
| <pre>ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) { |
| DoSomething(); |
| ... |
| } |
| </pre> |
| |
| <p>If you have too much text to fit on one line:</p> |
| |
| <pre>ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2, |
| Type par_name3) { |
| DoSomething(); |
| ... |
| } |
| </pre> |
| |
| <p>or if you cannot fit even the first parameter:</p> |
| |
| <pre>ReturnType LongClassName::ReallyReallyReallyLongFunctionName( |
| Type par_name1, // 4 space indent |
| Type par_name2, |
| Type par_name3) { |
| DoSomething(); // 2 space indent |
| ... |
| } |
| </pre> |
| |
| <p>Some points to note:</p> |
| |
| <ul> |
| <li>Choose good parameter names.</li> |
| |
| <li>A parameter name may be omitted only if the parameter is not used in the |
| function's definition.</li> |
| |
| <li>If you cannot fit the return type and the function |
| name on a single line, break between them.</li> |
| |
| <li>If you break after the return type of a function |
| declaration or definition, do not indent.</li> |
| |
| <li>The open parenthesis is always on the same line as |
| the function name.</li> |
| |
| <li>There is never a space between the function name |
| and the open parenthesis.</li> |
| |
| <li>There is never a space between the parentheses and |
| the parameters.</li> |
| |
| <li>The open curly brace is always on the end of the last line of the function |
| declaration, not the start of the next line.</li> |
| |
| <li>The close curly brace is either on the last line by |
| itself or on the same line as the open curly brace.</li> |
| |
| <li>There should be a space between the close |
| parenthesis and the open curly brace.</li> |
| |
| <li>All parameters should be aligned if possible.</li> |
| |
| <li>Default indentation is 2 spaces.</li> |
| |
| <li>Wrapped parameters have a 4 space indent.</li> |
| </ul> |
| |
| <p>Unused parameters that are obvious from context may be omitted:</p> |
| |
| <pre>class Foo { |
| public: |
| Foo(const Foo&) = delete; |
| Foo& operator=(const Foo&) = delete; |
| }; |
| </pre> |
| |
| <p>Unused parameters that might not be obvious should comment out the variable |
| name in the function definition:</p> |
| |
| <pre>class Shape { |
| public: |
| virtual void Rotate(double radians) = 0; |
| }; |
| |
| class Circle : public Shape { |
| public: |
| void Rotate(double radians) override; |
| }; |
| |
| void Circle::Rotate(double /*radians*/) {} |
| </pre> |
| |
| <pre class="badcode">// Bad - if someone wants to implement later, it's not clear what the |
| // variable means. |
| void Circle::Rotate(double) {} |
| </pre> |
| |
| <p>Attributes, and macros that expand to attributes, appear at the very |
| beginning of the function declaration or definition, before the |
| return type:</p> |
| <pre>ABSL_MUST_USE_RESULT bool IsOk(); |
| </pre> |
| |
| <h3 id="Formatting_Lambda_Expressions">Lambda Expressions</h3> |
| |
| <p>Format parameters and bodies as for any other function, and capture |
| lists like other comma-separated lists.</p> |
| |
| <p>For by-reference captures, do not leave a space between the |
| ampersand (&) and the variable name.</p> |
| <pre>int x = 0; |
| auto x_plus_n = [&x](int n) -> int { return x + n; } |
| </pre> |
| <p>Short lambdas may be written inline as function arguments.</p> |
| <pre>std::set<int> blacklist = {7, 8, 9}; |
| std::vector<int> digits = {3, 9, 1, 8, 4, 7, 1}; |
| digits.erase(std::remove_if(digits.begin(), digits.end(), [&blacklist](int i) { |
| return blacklist.find(i) != blacklist.end(); |
| }), |
| digits.end()); |
| </pre> |
| |
| <h3 id="Floating_Literals">Floating-point Literals</h3> |
| |
| <p>Floating-point literals should always have a radix point, with digits on both |
| sides, even if they use exponential notation. Readability is improved if all |
| floating-point literals take this familiar form, as this helps ensure that they |
| are not mistaken for integer literals, and that the |
| <code>E</code>/<code>e</code> of the exponential notation is not mistaken for a |
| hexadecimal digit. It is fine to initialize a floating-point variable with an |
| integer literal (assuming the variable type can exactly represent that integer), |
| but note that a number in exponential notation is never an integer literal. |
| </p> |
| |
| <pre class="badcode">float f = 1.f; |
| long double ld = -.5L; |
| double d = 1248e6; |
| </pre> |
| |
| <pre class="goodcode">float f = 1.0f; |
| float f2 = 1; // Also OK |
| long double ld = -0.5L; |
| double d = 1248.0e6; |
| </pre> |
| |
| |
| <h3 id="Function_Calls">Function Calls</h3> |
| |
| <p>Either write the call all on a single line, wrap the |
| arguments at the parenthesis, or start the arguments on a new |
| line indented by four spaces and continue at that 4 space |
| indent. In the absence of other considerations, use the |
| minimum number of lines, including placing multiple arguments |
| on each line where appropriate.</p> |
| |
| <p>Function calls have the following format:</p> |
| <pre>bool result = DoSomething(argument1, argument2, argument3); |
| </pre> |
| |
| <p>If the arguments do not all fit on one line, they |
| should be broken up onto multiple lines, with each |
| subsequent line aligned with the first argument. Do not |
| add spaces after the open paren or before the close |
| paren:</p> |
| <pre>bool result = DoSomething(averyveryveryverylongargument1, |
| argument2, argument3); |
| </pre> |
| |
| <p>Arguments may optionally all be placed on subsequent |
| lines with a four space indent:</p> |
| <pre>if (...) { |
| ... |
| ... |
| if (...) { |
| bool result = DoSomething( |
| argument1, argument2, // 4 space indent |
| argument3, argument4); |
| ... |
| } |
| </pre> |
| |
| <p>Put multiple arguments on a single line to reduce the |
| number of lines necessary for calling a function unless |
| there is a specific readability problem. Some find that |
| formatting with strictly one argument on each line is |
| more readable and simplifies editing of the arguments. |
| However, we prioritize for the reader over the ease of |
| editing arguments, and most readability problems are |
| better addressed with the following techniques.</p> |
| |
| <p>If having multiple arguments in a single line decreases |
| readability due to the complexity or confusing nature of the |
| expressions that make up some arguments, try creating |
| variables that capture those arguments in a descriptive name:</p> |
| <pre>int my_heuristic = scores[x] * y + bases[x]; |
| bool result = DoSomething(my_heuristic, x, y, z); |
| </pre> |
| |
| <p>Or put the confusing argument on its own line with |
| an explanatory comment:</p> |
| <pre>bool result = DoSomething(scores[x] * y + bases[x], // Score heuristic. |
| x, y, z); |
| </pre> |
| |
| <p>If there is still a case where one argument is |
| significantly more readable on its own line, then put it on |
| its own line. The decision should be specific to the argument |
| which is made more readable rather than a general policy.</p> |
| |
| <p>Sometimes arguments form a structure that is important |
| for readability. In those cases, feel free to format the |
| arguments according to that structure:</p> |
| <pre>// Transform the widget by a 3x3 matrix. |
| my_widget.Transform(x1, x2, x3, |
| y1, y2, y3, |
| z1, z2, z3); |
| </pre> |
| |
| <h3 id="Braced_Initializer_List_Format">Braced Initializer List Format</h3> |
| |
| <p>Format a <a href="#Braced_Initializer_List">braced initializer list</a> |
| exactly like you would format a function call in its place.</p> |
| |
| <p>If the braced list follows a name (e.g. a type or |
| variable name), format as if the <code>{}</code> were the |
| parentheses of a function call with that name. If there |
| is no name, assume a zero-length name.</p> |
| |
| <pre>// Examples of braced init list on a single line. |
| return {foo, bar}; |
| functioncall({foo, bar}); |
| std::pair<int, int> p{foo, bar}; |
| |
| // When you have to wrap. |
| SomeFunction( |
| {"assume a zero-length name before {"}, |
| some_other_function_parameter); |
| SomeType variable{ |
| some, other, values, |
| {"assume a zero-length name before {"}, |
| SomeOtherType{ |
| "Very long string requiring the surrounding breaks.", |
| some, other values}, |
| SomeOtherType{"Slightly shorter string", |
| some, other, values}}; |
| SomeType variable{ |
| "This is too long to fit all in one line"}; |
| MyType m = { // Here, you could also break before {. |
| superlongvariablename1, |
| superlongvariablename2, |
| {short, interior, list}, |
| {interiorwrappinglist, |
| interiorwrappinglist2}}; |
| </pre> |
| |
| <h3 id="Conditionals">Conditionals</h3> |
| |
| <p>Prefer no spaces inside parentheses. The <code>if</code> |
| and <code>else</code> keywords belong on separate lines.</p> |
| |
| <p>There are two acceptable formats for a basic |
| conditional statement. One includes spaces between the |
| parentheses and the condition, and one does not.</p> |
| |
| <p>The most common form is without spaces. Either is |
| fine, but <em>be consistent</em>. If you are modifying a |
| file, use the format that is already present. If you are |
| writing new code, use the format that the other files in |
| that directory or project use. If in doubt and you have |
| no personal preference, do not add the spaces.</p> |
| |
| <pre>if (condition) { // no spaces inside parentheses |
| ... // 2 space indent. |
| } else if (...) { // The else goes on the same line as the closing brace. |
| ... |
| } else { |
| ... |
| } |
| </pre> |
| |
| <p>If you prefer you may add spaces inside the |
| parentheses:</p> |
| |
| <pre>if ( condition ) { // spaces inside parentheses - rare |
| ... // 2 space indent. |
| } else { // The else goes on the same line as the closing brace. |
| ... |
| } |
| </pre> |
| |
| <p>Note that in all cases you must have a space between |
| the <code>if</code> and the open parenthesis. You must |
| also have a space between the close parenthesis and the |
| curly brace, if you're using one.</p> |
| |
| <pre class="badcode">if(condition) { // Bad - space missing after IF. |
| if (condition){ // Bad - space missing before {. |
| if(condition){ // Doubly bad. |
| </pre> |
| |
| <pre>if (condition) { // Good - proper space after IF and before {. |
| </pre> |
| |
| <p>Short conditional statements may be written on one |
| line if this enhances readability. You may use this only |
| when the line is brief and the statement does not use the |
| <code>else</code> clause.</p> |
| |
| <pre>if (x == kFoo) return new Foo(); |
| if (x == kBar) return new Bar(); |
| </pre> |
| |
| <p>This is not allowed when the if statement has an |
| <code>else</code>:</p> |
| |
| <pre class="badcode">// Not allowed - IF statement on one line when there is an ELSE clause |
| if (x) DoThis(); |
| else DoThat(); |
| </pre> |
| |
| <p>In general, curly braces are not required for |
| single-line statements, but they are allowed if you like |
| them; conditional or loop statements with complex |
| conditions or statements may be more readable with curly |
| braces. Some |
| projects require that an |
| <code>if</code> must always have an accompanying |
| brace.</p> |
| |
| <pre>if (condition) |
| DoSomething(); // 2 space indent. |
| |
| if (condition) { |
| DoSomething(); // 2 space indent. |
| } |
| </pre> |
| |
| <p>However, if one part of an |
| <code>if</code>-<code>else</code> statement uses curly |
| braces, the other part must too:</p> |
| |
| <pre class="badcode">// Not allowed - curly on IF but not ELSE |
| if (condition) { |
| foo; |
| } else |
| bar; |
| |
| // Not allowed - curly on ELSE but not IF |
| if (condition) |
| foo; |
| else { |
| bar; |
| } |
| </pre> |
| |
| <pre>// Curly braces around both IF and ELSE required because |
| // one of the clauses used braces. |
| if (condition) { |
| foo; |
| } else { |
| bar; |
| } |
| </pre> |
| |
| <h3 id="Loops_and_Switch_Statements">Loops and Switch Statements</h3> |
| |
| <p>Switch statements may use braces for blocks. Annotate |
| non-trivial fall-through between cases. |
| Braces are optional for single-statement loops. |
| Empty loop bodies should use either empty braces or <code>continue</code>.</p> |
| |
| <p><code>case</code> blocks in <code>switch</code> |
| statements can have curly braces or not, depending on |
| your preference. If you do include curly braces they |
| should be placed as shown below.</p> |
| |
| <p>If not conditional on an enumerated value, switch |
| statements should always have a <code>default</code> case |
| (in the case of an enumerated value, the compiler will |
| warn you if any values are not handled). If the default |
| case should never execute, treat this as an error. For example: |
| |
| </p> |
| |
| <div> |
| <pre>switch (var) { |
| case 0: { // 2 space indent |
| ... // 4 space indent |
| break; |
| } |
| case 1: { |
| ... |
| break; |
| } |
| default: { |
| assert(false); |
| } |
| } |
| </pre> |
| </div> |
| |
| <p>Fall-through from one case label to |
| another must be annotated using the |
| <code>ABSL_FALLTHROUGH_INTENDED;</code> macro (defined in |
| |
| <code>absl/base/macros.h</code>). |
| <code>ABSL_FALLTHROUGH_INTENDED;</code> should be placed at a |
| point of execution where a fall-through to the next case |
| label occurs. A common exception is consecutive case |
| labels without intervening code, in which case no |
| annotation is needed.</p> |
| |
| <pre>switch (x) { |
| case 41: // No annotation needed here. |
| case 43: |
| if (dont_be_picky) { |
| // Use this instead of or along with annotations in comments. |
| ABSL_FALLTHROUGH_INTENDED; |
| } else { |
| CloseButNoCigar(); |
| break; |
| } |
| case 42: |
| DoSomethingSpecial(); |
| ABSL_FALLTHROUGH_INTENDED; |
| default: |
| DoSomethingGeneric(); |
| break; |
| } |
| </pre> |
| |
| <p> Braces are optional for single-statement loops.</p> |
| |
| <pre>for (int i = 0; i < kSomeNumber; ++i) |
| printf("I love you\n"); |
| |
| for (int i = 0; i < kSomeNumber; ++i) { |
| printf("I take it back\n"); |
| } |
| </pre> |
| |
| |
| <p>Empty loop bodies should use either an empty pair of braces or |
| <code>continue</code> with no braces, rather than a single semicolon.</p> |
| |
| <pre>while (condition) { |
| // Repeat test until it returns false. |
| } |
| for (int i = 0; i < kSomeNumber; ++i) {} // Good - one newline is also OK. |
| while (condition) continue; // Good - continue indicates no logic. |
| </pre> |
| |
| <pre class="badcode">while (condition); // Bad - looks like part of do/while loop. |
| </pre> |
| |
| <h3 id="Pointer_and_Reference_Expressions">Pointer and Reference Expressions</h3> |
| |
| <p>No spaces around period or arrow. Pointer operators do not |
| have trailing spaces.</p> |
| |
| <p>The following are examples of correctly-formatted |
| pointer and reference expressions:</p> |
| |
| <pre>x = *p; |
| p = &x; |
| x = r.y; |
| x = r->y; |
| </pre> |
| |
| <p>Note that:</p> |
| |
| <ul> |
| <li>There are no spaces around the period or arrow when |
| accessing a member.</li> |
| |
| <li>Pointer operators have no space after the |
| <code>*</code> or <code>&</code>.</li> |
| </ul> |
| |
| <p>When declaring a pointer variable or argument, you may |
| place the asterisk adjacent to either the type or to the |
| variable name:</p> |
| |
| <pre>// These are fine, space preceding. |
| char *c; |
| const std::string &str; |
| |
| // These are fine, space following. |
| char* c; |
| const std::string& str; |
| </pre> |
| |
| <p>You should do this consistently within a single |
| file, |
| so, when modifying an existing file, use the style in |
| that file.</p> |
| |
| It is allowed (if unusual) to declare multiple variables in the same |
| declaration, but it is disallowed if any of those have pointer or |
| reference decorations. Such declarations are easily misread. |
| <pre>// Fine if helpful for readability. |
| int x, y; |
| </pre> |
| <pre class="badcode">int x, *y; // Disallowed - no & or * in multiple declaration |
| char * c; // Bad - spaces on both sides of * |
| const std::string & str; // Bad - spaces on both sides of & |
| </pre> |
| |
| <h3 id="Boolean_Expressions">Boolean Expressions</h3> |
| |
| <p>When you have a boolean expression that is longer than the |
| <a href="#Line_Length">standard line length</a>, be |
| consistent in how you break up the lines.</p> |
| |
| <p>In this example, the logical AND operator is always at |
| the end of the lines:</p> |
| |
| <pre>if (this_one_thing > this_other_thing && |
| a_third_thing == a_fourth_thing && |
| yet_another && last_one) { |
| ... |
| } |
| </pre> |
| |
| <p>Note that when the code wraps in this example, both of |
| the <code>&&</code> logical AND operators are at |
| the end of the line. This is more common in Google code, |
| though wrapping all operators at the beginning of the |
| line is also allowed. Feel free to insert extra |
| parentheses judiciously because they can be very helpful |
| in increasing readability when used |
| appropriately. Also note that you should always use |
| the punctuation operators, such as |
| <code>&&</code> and <code>~</code>, rather than |
| the word operators, such as <code>and</code> and |
| <code>compl</code>.</p> |
| |
| <h3 id="Return_Values">Return Values</h3> |
| |
| <p>Do not needlessly surround the <code>return</code> |
| expression with parentheses.</p> |
| |
| <p>Use parentheses in <code>return expr;</code> only |
| where you would use them in <code>x = expr;</code>.</p> |
| |
| <pre>return result; // No parentheses in the simple case. |
| // Parentheses OK to make a complex expression more readable. |
| return (some_long_condition && |
| another_condition); |
| </pre> |
| |
| <pre class="badcode">return (value); // You wouldn't write var = (value); |
| return(result); // return is not a function! |
| </pre> |
| |
| |
| |
| <h3 id="Variable_and_Array_Initialization">Variable and Array Initialization</h3> |
| |
| <p>Your choice of <code>=</code>, <code>()</code>, or |
| <code>{}</code>.</p> |
| |
| <p>You may choose between <code>=</code>, |
| <code>()</code>, and <code>{}</code>; the following are |
| all correct:</p> |
| |
| <pre>int x = 3; |
| int x(3); |
| int x{3}; |
| std::string name = "Some Name"; |
| std::string name("Some Name"); |
| std::string name{"Some Name"}; |
| </pre> |
| |
| <p>Be careful when using a braced initialization list <code>{...}</code> |
| on a type with an <code>std::initializer_list</code> constructor. |
| A nonempty <i>braced-init-list</i> prefers the |
| <code>std::initializer_list</code> constructor whenever |
| possible. Note that empty braces <code>{}</code> are special, and |
| will call a default constructor if available. To force the |
| non-<code>std::initializer_list</code> constructor, use parentheses |
| instead of braces.</p> |
| |
| <pre>std::vector<int> v(100, 1); // A vector containing 100 items: All 1s. |
| std::vector<int> v{100, 1}; // A vector containing 2 items: 100 and 1. |
| </pre> |
| |
| <p>Also, the brace form prevents narrowing of integral |
| types. This can prevent some types of programming |
| errors.</p> |
| |
| <pre>int pi(3.14); // OK -- pi == 3. |
| int pi{3.14}; // Compile error: narrowing conversion. |
| </pre> |
| |
| <h3 id="Preprocessor_Directives">Preprocessor Directives</h3> |
| |
| <p>The hash mark that starts a preprocessor directive should |
| always be at the beginning of the line.</p> |
| |
| <p>Even when preprocessor directives are within the body |
| of indented code, the directives should start at the |
| beginning of the line.</p> |
| |
| <pre>// Good - directives at beginning of line |
| if (lopsided_score) { |
| #if DISASTER_PENDING // Correct -- Starts at beginning of line |
| DropEverything(); |
| # if NOTIFY // OK but not required -- Spaces after # |
| NotifyClient(); |
| # endif |
| #endif |
| BackToNormal(); |
| } |
| </pre> |
| |
| <pre class="badcode">// Bad - indented directives |
| if (lopsided_score) { |
| #if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line |
| DropEverything(); |
| #endif // Wrong! Do not indent "#endif" |
| BackToNormal(); |
| } |
| </pre> |
| |
| <h3 id="Class_Format">Class Format</h3> |
| |
| <p>Sections in <code>public</code>, <code>protected</code> and |
| <code>private</code> order, each indented one space.</p> |
| |
| <p>The basic format for a class definition (lacking the |
| comments, see <a href="#Class_Comments">Class |
| Comments</a> for a discussion of what comments are |
| needed) is:</p> |
| |
| <pre>class MyClass : public OtherClass { |
| public: // Note the 1 space indent! |
| MyClass(); // Regular 2 space indent. |
| explicit MyClass(int var); |
| ~MyClass() {} |
| |
| void SomeFunction(); |
| void SomeFunctionThatDoesNothing() { |
| } |
| |
| void set_some_var(int var) { some_var_ = var; } |
| int some_var() const { return some_var_; } |
| |
| private: |
| bool SomeInternalFunction(); |
| |
| int some_var_; |
| int some_other_var_; |
| }; |
| </pre> |
| |
| <p>Things to note:</p> |
| |
| <ul> |
| <li>Any base class name should be on the same line as |
| the subclass name, subject to the 80-column limit.</li> |
| |
| <li>The <code>public:</code>, <code>protected:</code>, |
| and <code>private:</code> keywords should be indented |
| one space.</li> |
| |
| <li>Except for the first instance, these keywords |
| should be preceded by a blank line. This rule is |
| optional in small classes.</li> |
| |
| <li>Do not leave a blank line after these |
| keywords.</li> |
| |
| <li>The <code>public</code> section should be first, |
| followed by the <code>protected</code> and finally the |
| <code>private</code> section.</li> |
| |
| <li>See <a href="#Declaration_Order">Declaration |
| Order</a> for rules on ordering declarations within |
| each of these sections.</li> |
| </ul> |
| |
| <h3 id="Constructor_Initializer_Lists">Constructor Initializer Lists</h3> |
| |
| <p>Constructor initializer lists can be all on one line or |
| with subsequent lines indented four spaces.</p> |
| |
| <p>The acceptable formats for initializer lists are:</p> |
| |
| <pre>// When everything fits on one line: |
| MyClass::MyClass(int var) : some_var_(var) { |
| DoSomething(); |
| } |
| |
| // If the signature and initializer list are not all on one line, |
| // you must wrap before the colon and indent 4 spaces: |
| MyClass::MyClass(int var) |
| : some_var_(var), some_other_var_(var + 1) { |
| DoSomething(); |
| } |
| |
| // When the list spans multiple lines, put each member on its own line |
| // and align them: |
| MyClass::MyClass(int var) |
| : some_var_(var), // 4 space indent |
| some_other_var_(var + 1) { // lined up |
| DoSomething(); |
| } |
| |
| // As with any other code block, the close curly can be on the same |
| // line as the open curly, if it fits. |
| MyClass::MyClass(int var) |
| : some_var_(var) {} |
| </pre> |
| |
| <h3 id="Namespace_Formatting">Namespace Formatting</h3> |
| |
| <p>The contents of namespaces are not indented.</p> |
| |
| <p><a href="#Namespaces">Namespaces</a> do not add an |
| extra level of indentation. For example, use:</p> |
| |
| <pre>namespace { |
| |
| void foo() { // Correct. No extra indentation within namespace. |
| ... |
| } |
| |
| } // namespace |
| </pre> |
| |
| <p>Do not indent within a namespace:</p> |
| |
| <pre class="badcode">namespace { |
| |
| // Wrong! Indented when it should not be. |
| void foo() { |
| ... |
| } |
| |
| } // namespace |
| </pre> |
| |
| <p>When declaring nested namespaces, put each namespace |
| on its own line.</p> |
| |
| <pre>namespace foo { |
| namespace bar { |
| </pre> |
| |
| <h3 id="Horizontal_Whitespace">Horizontal Whitespace</h3> |
| |
| <p>Use of horizontal whitespace depends on location. Never put |
| trailing whitespace at the end of a line.</p> |
| |
| <h4>General</h4> |
| |
| <pre>void f(bool b) { // Open braces should always have a space before them. |
| ... |
| int i = 0; // Semicolons usually have no space before them. |
| // Spaces inside braces for braced-init-list are optional. If you use them, |
| // put them on both sides! |
| int x[] = { 0 }; |
| int x[] = {0}; |
| |
| // Spaces around the colon in inheritance and initializer lists. |
| class Foo : public Bar { |
| public: |
| // For inline function implementations, put spaces between the braces |
| // and the implementation itself. |
| Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces. |
| void Reset() { baz_ = 0; } // Spaces separating braces from implementation. |
| ... |
| </pre> |
| |
| <p>Adding trailing whitespace can cause extra work for |
| others editing the same file, when they merge, as can |
| removing existing trailing whitespace. So: Don't |
| introduce trailing whitespace. Remove it if you're |
| already changing that line, or do it in a separate |
| clean-up |
| operation (preferably when no-one |
| else is working on the file).</p> |
| |
| <h4>Loops and Conditionals</h4> |
| |
| <pre>if (b) { // Space after the keyword in conditions and loops. |
| } else { // Spaces around else. |
| } |
| while (test) {} // There is usually no space inside parentheses. |
| switch (i) { |
| for (int i = 0; i < 5; ++i) { |
| // Loops and conditions may have spaces inside parentheses, but this |
| // is rare. Be consistent. |
| switch ( i ) { |
| if ( test ) { |
| for ( int i = 0; i < 5; ++i ) { |
| // For loops always have a space after the semicolon. They may have a space |
| // before the semicolon, but this is rare. |
| for ( ; i < 5 ; ++i) { |
| ... |
| |
| // Range-based for loops always have a space before and after the colon. |
| for (auto x : counts) { |
| ... |
| } |
| switch (i) { |
| case 1: // No space before colon in a switch case. |
| ... |
| case 2: break; // Use a space after a colon if there's code after it. |
| </pre> |
| |
| <h4>Operators</h4> |
| |
| <pre>// Assignment operators always have spaces around them. |
| x = 0; |
| |
| // Other binary operators usually have spaces around them, but it's |
| // OK to remove spaces around factors. Parentheses should have no |
| // internal padding. |
| v = w * x + y / z; |
| v = w*x + y/z; |
| v = w * (x + z); |
| |
| // No spaces separating unary operators and their arguments. |
| x = -5; |
| ++x; |
| if (x && !y) |
| ... |
| </pre> |
| |
| <h4>Templates and Casts</h4> |
| |
| <pre>// No spaces inside the angle brackets (< and >), before |
| // <, or between >( in a cast |
| std::vector<std::string> x; |
| y = static_cast<char*>(x); |
| |
| // Spaces between type and pointer are OK, but be consistent. |
| std::vector<char *> x; |
| </pre> |
| |
| <h3 id="Vertical_Whitespace">Vertical Whitespace</h3> |
| |
| <p>Minimize use of vertical whitespace.</p> |
| |
| <p>This is more a principle than a rule: don't use blank lines when |
| you don't have to. In particular, don't put more than one or two blank |
| lines between functions, resist starting functions with a blank line, |
| don't end functions with a blank line, and be sparing with your use of |
| blank lines. A blank line within a block of code serves like a |
| paragraph break in prose: visually separating two thoughts.</p> |
| |
| <p>The basic principle is: The more code that fits on one screen, the |
| easier it is to follow and understand the control flow of the |
| program. Use whitespace purposefully to provide separation in that |
| flow.</p> |
| |
| <p>Some rules of thumb to help when blank lines may be |
| useful:</p> |
| |
| <ul> |
| <li>Blank lines at the beginning or end of a function |
| do not help readability.</li> |
| |
| <li>Blank lines inside a chain of if-else blocks may |
| well help readability.</li> |
| |
| <li>A blank line before a comment line usually helps |
| readability — the introduction of a new comment suggests |
| the start of a new thought, and the blank line makes it clear |
| that the comment goes with the following thing instead of the |
| preceding.</li> |
| </ul> |
| |
| <h2 id="Exceptions_to_the_Rules">Exceptions to the Rules</h2> |
| |
| <p>The coding conventions described above are mandatory. |
| However, like all good rules, these sometimes have exceptions, |
| which we discuss here.</p> |
| |
| |
| |
| <div> |
| <h3 id="Existing_Non-conformant_Code">Existing Non-conformant Code</h3> |
| |
| <p>You may diverge from the rules when dealing with code that |
| does not conform to this style guide.</p> |
| |
| <p>If you find yourself modifying code that was written |
| to specifications other than those presented by this |
| guide, you may have to diverge from these rules in order |
| to stay consistent with the local conventions in that |
| code. If you are in doubt about how to do this, ask the |
| original author or the person currently responsible for |
| the code. Remember that <em>consistency</em> includes |
| local consistency, too.</p> |
| |
| </div> |
| |
| |
| |
| <h3 id="Windows_Code">Windows Code</h3> |
| |
| <p> Windows |
| programmers have developed their own set of coding |
| conventions, mainly derived from the conventions in Windows |
| headers and other Microsoft code. We want to make it easy |
| for anyone to understand your code, so we have a single set |
| of guidelines for everyone writing C++ on any platform.</p> |
| |
| <p>It is worth reiterating a few of the guidelines that |
| you might forget if you are used to the prevalent Windows |
| style:</p> |
| |
| <ul> |
| <li>Do not use Hungarian notation (for example, naming |
| an integer <code>iNum</code>). Use the Google naming |
| conventions, including the <code>.cc</code> extension |
| for source files.</li> |
| |
| <li>Windows defines many of its own synonyms for |
| primitive types, such as <code>DWORD</code>, |
| <code>HANDLE</code>, etc. It is perfectly acceptable, |
| and encouraged, that you use these types when calling |
| Windows API functions. Even so, keep as close as you |
| can to the underlying C++ types. For example, use |
| <code>const TCHAR *</code> instead of |
| <code>LPCTSTR</code>.</li> |
| |
| <li>When compiling with Microsoft Visual C++, set the |
| compiler to warning level 3 or higher, and treat all |
| warnings as errors.</li> |
| |
| <li>Do not use <code>#pragma once</code>; instead use |
| the standard Google include guards. The path in the |
| include guards should be relative to the top of your |
| project tree.</li> |
| |
| <li>In fact, do not use any nonstandard extensions, |
| like <code>#pragma</code> and <code>__declspec</code>, |
| unless you absolutely must. Using |
| <code>__declspec(dllimport)</code> and |
| <code>__declspec(dllexport)</code> is allowed; however, |
| you must use them through macros such as |
| <code>DLLIMPORT</code> and <code>DLLEXPORT</code>, so |
| that someone can easily disable the extensions if they |
| share the code.</li> |
| </ul> |
| |
| <p>However, there are just a few rules that we |
| occasionally need to break on Windows:</p> |
| |
| <ul> |
| <li>Normally we <a href="#Multiple_Inheritance">strongly discourage |
| the use of multiple implementation inheritance</a>; |
| however, it is required when using COM and some ATL/WTL |
| classes. You may use multiple implementation |
| inheritance to implement COM or ATL/WTL classes and |
| interfaces.</li> |
| |
| <li>Although you should not use exceptions in your own |
| code, they are used extensively in the ATL and some |
| STLs, including the one that comes with Visual C++. |
| When using the ATL, you should define |
| <code>_ATL_NO_EXCEPTIONS</code> to disable exceptions. |
| You should investigate whether you can also disable |
| exceptions in your STL, but if not, it is OK to turn on |
| exceptions in the compiler. (Note that this is only to |
| get the STL to compile. You should still not write |
| exception handling code yourself.)</li> |
| |
| <li>The usual way of working with precompiled headers |
| is to include a header file at the top of each source |
| file, typically with a name like <code>StdAfx.h</code> |
| or <code>precompile.h</code>. To make your code easier |
| to share with other projects, avoid including this file |
| explicitly (except in <code>precompile.cc</code>), and |
| use the <code>/FI</code> compiler option to include the |
| file automatically.</li> |
| |
| <li>Resource headers, which are usually named |
| <code>resource.h</code> and contain only macros, do not |
| need to conform to these style guidelines.</li> |
| </ul> |
| |
| <h2 id="Parting_Words">Parting Words</h2> |
| |
| <p>Use common sense and <em>BE CONSISTENT</em>.</p> |
| |
| <p>If you are editing code, take a few minutes to look at the |
| code around you and determine its style. If they use spaces |
| around their <code>if</code> clauses, you should, too. If their |
| comments have little boxes of stars around them, make your |
| comments have little boxes of stars around them too.</p> |
| |
| <p>The point of having style guidelines is to have a common |
| vocabulary of coding so people can concentrate on what you are |
| saying, rather than on how you are saying it. We present global |
| style rules here so people know the vocabulary. But local style |
| is also important. If code you add to a file looks drastically |
| different from the existing code around it, the discontinuity |
| throws readers out of their rhythm when they go to read it. Try |
| to avoid this.</p> |
| |
| |
| |
| <p>OK, enough writing about writing code; the code itself is much |
| more interesting. Have fun!</p> |
| |
| <hr> |
| </div> |
| </body> |
| </html> |