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 <h1>Google C++ Style Guide</h1>
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 <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&#8217;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&#8217;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>&lt;PROJECT&gt;</i>_<i>&lt;PATH&gt;</i>_<i>&lt;FILE&gt;</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>&lt;unistd.h&gt;</code>,
     <code>&lt;stdlib.h&gt;</code>.</li>

   <li>A blank line</li>

   <li>C++ standard library headers (without file extension), e.g.
     <code>&lt;algorithm&gt;</code>, <code>&lt;cstddef&gt;</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 &lt;sys/types.h&gt;
 #include &lt;unistd.h&gt;

 #include &lt;string&gt;
 #include &lt;vector&gt;

 #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 &lt;initializer_list&gt;
 #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&lt;int&gt; v;
 v.push_back(1);  // Prefer initializing using brace initialization.
 v.push_back(2);
 </pre>

 <pre>std::vector&lt;int&gt; 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 &lt; 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 &lt; 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&lt;int, 3&gt; 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&amp; kBar = StrCat("a", "b", "c");

 void bar() {
   // bad: non-trivial destructor
   static std::map&lt;int, int&gt; 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&amp; 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&amp; 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&amp; 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&lt;int&gt;</code> is an example of a movable but not
 copyable type (since the value of the source
 <code>std::unique_ptr&lt;int&gt;</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&amp; other) = default;
   Copyable&amp; operator=(const Copyable&amp; other) = default;

   // The implicit move operations are suppressed by the declarations above.
 };

 class MoveOnly {
  public:
   MoveOnly(MoveOnly&amp;&amp; other);
   MoveOnly&amp; operator=(MoveOnly&amp;&amp; other);

   // The copy operations are implicitly deleted, but you can
   // spell that out explicitly if you want:
   MoveOnly(const MoveOnly&amp;) = delete;
   MoveOnly&amp; operator=(const MoveOnly&amp;) = delete;
 };

 class NotCopyableOrMovable {
  public:
   // Not copyable or movable
   NotCopyableOrMovable(const NotCopyableOrMovable&amp;) = delete;
   NotCopyableOrMovable&amp; operator=(const NotCopyableOrMovable&amp;)
       = delete;

   // The move operations are implicitly disabled, but you can
   // spell that out explicitly if you want:
   NotCopyableOrMovable(NotCopyableOrMovable&amp;&amp;) = delete;
   NotCopyableOrMovable&amp; operator=(NotCopyableOrMovable&amp;&amp;)
       = 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&lt;X&gt;</code>.
   While C++14's introduction of <code>std::get&lt;Type&gt;</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>&lt;</code>,
 <code>=</code>, and <code>&lt;&lt;</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 &lt; bar</code>
   may do one thing, while <code>&amp;foo &lt; &amp;bar</code>
   does something totally different.</li>

   <li>Certain operator overloads are inherently hazardous.
   Overloading unary <code>&amp;</code> can cause the same
   code to have different meanings depending on whether
   the overload declaration is visible. Overloads of
   <code>&amp;&amp;</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>&lt;</code>, overload all the comparison operators,
 and make sure <code>&lt;</code> and <code>&gt;</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 &lt; b</code> compiles but
 <code>b &lt; 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>&lt;&lt;</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>&lt;</code>.</p>

 <p>Do not overload <code>&amp;&amp;</code>, <code>||</code>,
 <code>,</code> (comma), or unary <code>&amp;</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>&lt;&lt;</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 &amp;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 &amp;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&amp;</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&amp;</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&amp;</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&amp;</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 &amp;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) -&gt; 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 &lt;typename T, typename U&gt;
     auto add(T t, U u) -&gt; decltype(t + u);
   </pre>
   versus
   <pre>    template &lt;typename T, typename U&gt;
     decltype(declval&lt;T&amp;&gt;() + declval&lt;U&amp;&gt;()) 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>-&gt;</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&lt;Foo&gt; FooFactory();
 void FooConsumer(std::unique_ptr&lt;Foo&gt; 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&lt;const Foo&gt;</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&amp; and &amp;&amp; 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&amp;&amp;
 s);</code> declares a function whose argument is an
 rvalue reference to a std::string.</p>

 <p id="Forwarding_references"> When the token '&amp;&amp;' 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&lt;std::string&gt;</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&amp;&amp;</code> and the other taking <code>const Foo&amp;</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&lt;T&gt;::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&#8217;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&#8217;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&lt;Derived*&gt;(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&lt;float&gt;(double_value)</code>, or brace
 initialization for conversion of arithmetic types like
 <code>int64 y = int64{1} &lt;&lt; 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>&lt;&lt;</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>&lt;iostream&gt;</code>.
 They are widely used in Google code, mostly for debug logging
 and test diagnostics.</p>

 <p class="pros"></p>
 <p>The <code>&lt;&lt;</code> and <code>&gt;&gt;</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>&lt;&lt;</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>&lt;&lt;</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>&lt;&lt;</code> as a streaming operator
 for your type only if your type represents a value, and
 <code>&lt;&lt;</code> writes out a human-readable string
 representation of that value. Avoid exposing implementation
 details in the output of <code>&lt;&lt;</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&amp;</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>&lt;stdint.h&gt;</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>&lt;cstdint&gt;</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>&lt;cinttypes&gt;</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 &lt;&lt; 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, ==, &lt;)
 };
 </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(&amp;data, 0, sizeof(data));
 </pre>

 <pre class="badcode">memset(&amp;data, 0, sizeof(Struct));
 </pre>

 <pre>if (raw_size &lt; sizeof(int)) {
   LOG(ERROR) &lt;&lt; "compressed record not big enough for count: " &lt;&lt; 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 &lt;typename T&gt;
 void f(T t);

 f(0);  // Invokes f&lt;int&gt;(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&amp; b = a;  // b is an int&amp;
 auto c = b;   // c is an int
 auto d{42};   // d is an int, not a std::initializer_list&lt;int&gt;
 </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 &gt; 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-&gt;second = value;
 }</pre>
     The <code>auto</code> can also be qualified with <code>const</code>,
     <code>&amp;</code>, and <code>&amp;&amp;</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&lt;Foo&gt;()</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&lt;WidgetWithBellsAndWhistles&gt; widget_ptr =
     absl::make_unique&lt;WidgetWithBellsAndWhistles&gt;(arg1, arg2);
 absl::flat_hash_map&lt;std::string,
                     std::unique_ptr&lt;WidgetWithBellsAndWhistles&gt;&gt;::const_iterator
     it = my_map_.find(key);
 std::array&lt;int, 0&gt; numbers = {4, 8, 15, 16, 23, 42};</pre>

   <pre class="goodcode">auto widget_ptr = absl::make_unique&lt;WidgetWithBellsAndWhistles&gt;(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&amp; widget = *it-&gt;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&lt;int, 3&gt;</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 &lt;class T, class... U&gt;
 array(T, U...) -&gt; std::array&lt;T, 1 + sizeof...(U)&gt;;
 }</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) &lt; 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, &amp;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&lt;int&gt; lookup_table = ...;
 std::vector&lt;int&gt; indices = ...;
 // Captures `lookup_table` by reference, sorts `indices` by the value
 // of the associated element in `lookup_table`.
 std::sort(indices.begin(), indices.end(), [&amp;](int a, int b) {
   return lookup_table[a] &lt; 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&lt;Foo&gt; 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&lt;int&gt;({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-&gt;Schedule([&amp;] { 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-&gt;Schedule([&amp;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 ([&amp;]) 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&lt;T&gt;</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&lt;int, std::string&gt;</code> is a hash
 map that uses <code>std::hash&lt;int&gt;</code> to hash its keys,
 whereas <code>std::unordered_map&lt;int, std::string, MyIntHash&gt;</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&lt;MyKeyType, Value, MyKeyTypeHasher&gt; 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&#8212;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>&lt;ratio&gt;</code>), because of concerns that
   it's tied to a more template-heavy interface
   style.</li>

   <li>The <code>&lt;cfenv&gt;</code> and
   <code>&lt;fenv.h&gt;</code> headers, because many
   compilers do not support those features reliably.</li>

   <li>The <code>&lt;filesystem&gt;</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(&amp;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&lt;DataPoint, std::hash&lt;DataPoint&gt;, DataPointComparator&gt;;
 }  // 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&lt;DataPoint, hash&lt;DataPoint&gt;, DataPointComparator&gt; 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&lt;Foo&gt;&amp; foos) {
     int n = 0;  // Clear meaning given limited scope and context
     for (const auto&amp; 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&lt;Foo&gt;&amp; foos) {
     int total_number_of_foo_errors = 0;  // Overly verbose given limited scope and context
     for (int foo_index = 0; foo_index &lt; 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 &#8212; classes, structs, type aliases,
 enums, and type template parameters &#8212; 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&lt;UrlTableProperties *, std::string&gt; PropertiesMap;

 // using aliases
 using PropertiesMap = hash_map&lt;UrlTableProperties *, std::string&gt;;

 // 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&lt;TableInfo&gt;* 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&lt;UrlTableProperties&gt;* 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 &#8212; 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-&gt;NewIterator();
 //    for (iter-&gt;Seek("foo"); !iter-&gt;done(); iter-&gt;Next()) {
 //      process(iter-&gt;key(), iter-&gt;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-&gt;NewIterator();
 //    iter-&gt;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 &lt; result-&gt;size(); ++i) {
   x = (x &lt;&lt; 8) + (*result)[i];
   (*result)[i] = x &gt;&gt; 1;
   x &amp;= 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&lt;int64&gt;(0, mmap_budget - index_-&gt;length());
 if (mmap_budget &gt;= data_size_ &amp;&amp; !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.  &lt;-- 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 &#8212; 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&amp;) = delete;
   Foo&amp; operator=(const Foo&amp;) = 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 (&amp;) and the variable name.</p>
 <pre>int x = 0;
 auto x_plus_n = [&amp;x](int n) -&gt; int { return x + n; }
 </pre>
 <p>Short lambdas may be written inline as function arguments.</p>
 <pre>std::set&lt;int&gt; blacklist = {7, 8, 9};
 std::vector&lt;int&gt; digits = {3, 9, 1, 8, 4, 7, 1};
 digits.erase(std::remove_if(digits.begin(), digits.end(), [&amp;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&lt;int, int&gt; 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 &lt; kSomeNumber; ++i)
   printf("I love you\n");

 for (int i = 0; i &lt; 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 &lt; 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 = &amp;x;
 x = r.y;
 x = r-&gt;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>&amp;</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 &amp;str;

 // These are fine, space following.
 char* c;
 const std::string&amp; 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 &amp; or * in multiple declaration
 char * c;  // Bad - spaces on both sides of *
 const std::string &amp; str;  // Bad - spaces on both sides of &amp;
 </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 &gt; this_other_thing &amp;&amp;
     a_third_thing == a_fourth_thing &amp;&amp;
     yet_another &amp;&amp; last_one) {
   ...
 }
 </pre>

 <p>Note that when the code wraps in this example, both of
 the <code>&amp;&amp;</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>&amp;&amp;</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 &amp;&amp;
         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&lt;int&gt; v(100, 1);  // A vector containing 100 items: All 1s.
 std::vector&lt;int&gt; 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 &lt; 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 &lt; 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 &lt; 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 &amp;&amp; !y)
   ...
 </pre>

 <h4>Templates and Casts</h4>

 <pre>// No spaces inside the angle brackets (&lt; and &gt;), before
 // &lt;, or between &gt;( in a cast
 std::vector&lt;std::string&gt; x;
 y = static_cast&lt;char*&gt;(x);

 // Spaces between type and pointer are OK, but be consistent.
 std::vector&lt;char *&gt; 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 &#8212; 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>
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