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| <div class="section" id="step-10-adding-generator-expressions"> | |
| <span id="guide:tutorial/Adding Generator Expressions"></span><h1>Step 10: Adding Generator Expressions<a class="headerlink" href="#step-10-adding-generator-expressions" title="Permalink to this headline">¶</a></h1> | |
| <p><span class="target" id="index-0-manual:cmake-generator-expressions(7)"></span><a class="reference internal" href="../../manual/cmake-generator-expressions.7.html#manual:cmake-generator-expressions(7)" title="cmake-generator-expressions(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">Generator</span> <span class="pre">expressions</span></code></a> are evaluated | |
| during build system generation to produce information specific to each build | |
| configuration.</p> | |
| <p><span class="target" id="index-1-manual:cmake-generator-expressions(7)"></span><a class="reference internal" href="../../manual/cmake-generator-expressions.7.html#manual:cmake-generator-expressions(7)" title="cmake-generator-expressions(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">Generator</span> <span class="pre">expressions</span></code></a> are allowed in | |
| the context of many target properties, such as <span class="target" id="index-0-prop_tgt:LINK_LIBRARIES"></span><a class="reference internal" href="../../prop_tgt/LINK_LIBRARIES.html#prop_tgt:LINK_LIBRARIES" title="LINK_LIBRARIES"><code class="xref cmake cmake-prop_tgt docutils literal notranslate"><span class="pre">LINK_LIBRARIES</span></code></a>, | |
| <span class="target" id="index-0-prop_tgt:INCLUDE_DIRECTORIES"></span><a class="reference internal" href="../../prop_tgt/INCLUDE_DIRECTORIES.html#prop_tgt:INCLUDE_DIRECTORIES" title="INCLUDE_DIRECTORIES"><code class="xref cmake cmake-prop_tgt docutils literal notranslate"><span class="pre">INCLUDE_DIRECTORIES</span></code></a>, <span class="target" id="index-0-prop_tgt:COMPILE_DEFINITIONS"></span><a class="reference internal" href="../../prop_tgt/COMPILE_DEFINITIONS.html#prop_tgt:COMPILE_DEFINITIONS" title="COMPILE_DEFINITIONS"><code class="xref cmake cmake-prop_tgt docutils literal notranslate"><span class="pre">COMPILE_DEFINITIONS</span></code></a> and others. | |
| They may also be used when using commands to populate those properties, such as | |
| <span class="target" id="index-0-command:target_link_libraries"></span><a class="reference internal" href="../../command/target_link_libraries.html#command:target_link_libraries" title="target_link_libraries"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_link_libraries()</span></code></a>, <span class="target" id="index-0-command:target_include_directories"></span><a class="reference internal" href="../../command/target_include_directories.html#command:target_include_directories" title="target_include_directories"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_include_directories()</span></code></a>, | |
| <span class="target" id="index-0-command:target_compile_definitions"></span><a class="reference internal" href="../../command/target_compile_definitions.html#command:target_compile_definitions" title="target_compile_definitions"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_compile_definitions()</span></code></a> and others.</p> | |
| <p><span class="target" id="index-2-manual:cmake-generator-expressions(7)"></span><a class="reference internal" href="../../manual/cmake-generator-expressions.7.html#manual:cmake-generator-expressions(7)" title="cmake-generator-expressions(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">Generator</span> <span class="pre">expressions</span></code></a> may be used | |
| to enable conditional linking, conditional definitions used when compiling, | |
| conditional include directories and more. The conditions may be based on the | |
| build configuration, target properties, platform information or any other | |
| queryable information.</p> | |
| <p>There are different types of | |
| <span class="target" id="index-3-manual:cmake-generator-expressions(7)"></span><a class="reference internal" href="../../manual/cmake-generator-expressions.7.html#manual:cmake-generator-expressions(7)" title="cmake-generator-expressions(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">generator</span> <span class="pre">expressions</span></code></a> including | |
| Logical, Informational, and Output expressions.</p> | |
| <p>Logical expressions are used to create conditional output. The basic | |
| expressions are the <code class="docutils literal notranslate"><span class="pre">0</span></code> and <code class="docutils literal notranslate"><span class="pre">1</span></code> expressions. A <code class="docutils literal notranslate"><span class="pre">$<0:...></span></code> results in the | |
| empty string, and <code class="docutils literal notranslate"><span class="pre"><1:...></span></code> results in the content of <code class="docutils literal notranslate"><span class="pre">...</span></code>. They can also | |
| be nested.</p> | |
| <p>A common usage of | |
| <span class="target" id="index-4-manual:cmake-generator-expressions(7)"></span><a class="reference internal" href="../../manual/cmake-generator-expressions.7.html#manual:cmake-generator-expressions(7)" title="cmake-generator-expressions(7)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">generator</span> <span class="pre">expressions</span></code></a> is to | |
| conditionally add compiler flags, such as those for language levels or | |
| warnings. A nice pattern is to associate this information to an <code class="docutils literal notranslate"><span class="pre">INTERFACE</span></code> | |
| target allowing this information to propagate. Let's start by constructing an | |
| <code class="docutils literal notranslate"><span class="pre">INTERFACE</span></code> target and specifying the required C++ standard level of <code class="docutils literal notranslate"><span class="pre">11</span></code> | |
| instead of using <span class="target" id="index-0-variable:CMAKE_CXX_STANDARD"></span><a class="reference internal" href="../../variable/CMAKE_CXX_STANDARD.html#variable:CMAKE_CXX_STANDARD" title="CMAKE_CXX_STANDARD"><code class="xref cmake cmake-variable docutils literal notranslate"><span class="pre">CMAKE_CXX_STANDARD</span></code></a>.</p> | |
| <p>So the following code:</p> | |
| <div class="literal-block-wrapper docutils container" id="cmakelists-txt-cxx-standard-variable-remove"> | |
| <div class="code-block-caption"><span class="caption-text">CMakeLists.txt</span><a class="headerlink" href="#cmakelists-txt-cxx-standard-variable-remove" title="Permalink to this code">¶</a></div> | |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="c"># specify the C++ standard</span> | |
| <span class="nf">set(</span><span class="no">CMAKE_CXX_STANDARD</span><span class="w"> </span><span class="m">11</span><span class="nf">)</span><span class="w"></span> | |
| <span class="nf">set(</span><span class="no">CMAKE_CXX_STANDARD_REQUIRED</span><span class="w"> </span><span class="nb">True</span><span class="nf">)</span><span class="w"></span> | |
| </pre></div> | |
| </div> | |
| </div> | |
| <p>Would be replaced with:</p> | |
| <div class="literal-block-wrapper docutils container" id="cmakelists-txt-cxx-std-feature"> | |
| <div class="code-block-caption"><span class="caption-text">CMakeLists.txt</span><a class="headerlink" href="#cmakelists-txt-cxx-std-feature" title="Permalink to this code">¶</a></div> | |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">add_library(</span><span class="nb">tutorial_compiler_flags</span><span class="w"> </span><span class="no">INTERFACE</span><span class="nf">)</span><span class="w"></span> | |
| <span class="nf">target_compile_features(</span><span class="nb">tutorial_compiler_flags</span><span class="w"> </span><span class="no">INTERFACE</span><span class="w"> </span><span class="nb">cxx_std_11</span><span class="nf">)</span><span class="w"></span> | |
| </pre></div> | |
| </div> | |
| </div> | |
| <p><strong>Note</strong>: This upcoming section will require a change to the | |
| <span class="target" id="index-0-command:cmake_minimum_required"></span><a class="reference internal" href="../../command/cmake_minimum_required.html#command:cmake_minimum_required" title="cmake_minimum_required"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">cmake_minimum_required()</span></code></a> usage in the code. The Generator Expression | |
| that is about to be used was introduced in <cite>3.15</cite>. Update the call to require | |
| that more recent version:</p> | |
| <div class="literal-block-wrapper docutils container" id="cmakelists-txt-version-update"> | |
| <div class="code-block-caption"><span class="caption-text">CMakeLists.txt</span><a class="headerlink" href="#cmakelists-txt-version-update" title="Permalink to this code">¶</a></div> | |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">cmake_minimum_required(</span><span class="no">VERSION</span><span class="w"> </span><span class="m">3.15</span><span class="nf">)</span><span class="w"></span> | |
| </pre></div> | |
| </div> | |
| </div> | |
| <p>Next we add the desired compiler warning flags that we want for our project. As | |
| warning flags vary based on the compiler we use the <code class="docutils literal notranslate"><span class="pre">COMPILE_LANG_AND_ID</span></code> | |
| generator expression to control which flags to apply given a language and a set | |
| of compiler ids as seen below:</p> | |
| <div class="literal-block-wrapper docutils container" id="cmakelists-txt-target-compile-options-genex"> | |
| <div class="code-block-caption"><span class="caption-text">CMakeLists.txt</span><a class="headerlink" href="#cmakelists-txt-target-compile-options-genex" title="Permalink to this code">¶</a></div> | |
| <div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">set(</span><span class="nb">gcc_like_cxx</span><span class="w"> </span><span class="s">"$<COMPILE_LANG_AND_ID:CXX,ARMClang,AppleClang,Clang,GNU,LCC>"</span><span class="nf">)</span><span class="w"></span> | |
| <span class="nf">set(</span><span class="nb">msvc_cxx</span><span class="w"> </span><span class="s">"$<COMPILE_LANG_AND_ID:CXX,MSVC>"</span><span class="nf">)</span><span class="w"></span> | |
| <span class="nf">target_compile_options(</span><span class="nb">tutorial_compiler_flags</span><span class="w"> </span><span class="no">INTERFACE</span><span class="w"></span> | |
| <span class="w"> </span><span class="s">"$<${gcc_like_cxx}:$<BUILD_INTERFACE:-Wall;-Wextra;-Wshadow;-Wformat=2;-Wunused>>"</span><span class="w"></span> | |
| <span class="w"> </span><span class="s">"$<${msvc_cxx}:$<BUILD_INTERFACE:-W3>>"</span><span class="w"></span> | |
| <span class="nf">)</span><span class="w"></span> | |
| </pre></div> | |
| </div> | |
| </div> | |
| <p>Looking at this we see that the warning flags are encapsulated inside a | |
| <code class="docutils literal notranslate"><span class="pre">BUILD_INTERFACE</span></code> condition. This is done so that consumers of our installed | |
| project will not inherit our warning flags.</p> | |
| <p><strong>Exercise</strong>: Modify <code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code> so that all targets have | |
| a <span class="target" id="index-1-command:target_link_libraries"></span><a class="reference internal" href="../../command/target_link_libraries.html#command:target_link_libraries" title="target_link_libraries"><code class="xref cmake cmake-command docutils literal notranslate"><span class="pre">target_link_libraries()</span></code></a> call to <code class="docutils literal notranslate"><span class="pre">tutorial_compiler_flags</span></code>.</p> | |
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