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  <section id="step-7-adding-system-introspection">
<span id="guide:tutorial/Adding System Introspection"></span><h1>Step 7: Adding System Introspection<a class="headerlink" href="#step-7-adding-system-introspection" title="Permalink to this heading">¶</a></h1>
<p>Let us consider adding some code to our project that depends on features the
target platform may not have. For this example, we will add some code that
depends on whether or not the target platform has the <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code>
functions. Of course almost every platform has these functions but for this
tutorial assume that they are not common.</p>
<section id="exercise-1-assessing-dependency-availability">
<h2>Exercise 1 - Assessing Dependency Availability<a class="headerlink" href="#exercise-1-assessing-dependency-availability" title="Permalink to this heading">¶</a></h2>
<section id="goal">
<h3>Goal<a class="headerlink" href="#goal" title="Permalink to this heading">¶</a></h3>
<p>Change implementation based on available system dependencies.</p>
</section>
<section id="helpful-resources">
<h3>Helpful Resources<a class="headerlink" href="#helpful-resources" title="Permalink to this heading">¶</a></h3>
<ul class="simple">
<li><p><span class="target" id="index-0-module:CheckCXXSourceCompiles"></span><a class="reference internal" href="../../module/CheckCXXSourceCompiles.html#module:CheckCXXSourceCompiles" title="CheckCXXSourceCompiles"><code class="xref cmake cmake-module docutils literal notranslate"><span class="pre">CheckCXXSourceCompiles</span></code></a></p></li>
<li><p><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></p></li>
</ul>
</section>
<section id="files-to-edit">
<h3>Files to Edit<a class="headerlink" href="#files-to-edit" title="Permalink to this heading">¶</a></h3>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">MathFunctions/mysqrt.cxx</span></code></p></li>
</ul>
</section>
<section id="getting-started">
<h3>Getting Started<a class="headerlink" href="#getting-started" title="Permalink to this heading">¶</a></h3>
<p>The starting source code is provided in the <code class="docutils literal notranslate"><span class="pre">Step7</span></code> directory. In this
exercise, complete <code class="docutils literal notranslate"><span class="pre">TODO</span> <span class="pre">1</span></code> through <code class="docutils literal notranslate"><span class="pre">TODO</span> <span class="pre">5</span></code>.</p>
<p>Start by editing <code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>. Include the
<span class="target" id="index-1-module:CheckCXXSourceCompiles"></span><a class="reference internal" href="../../module/CheckCXXSourceCompiles.html#module:CheckCXXSourceCompiles" title="CheckCXXSourceCompiles"><code class="xref cmake cmake-module docutils literal notranslate"><span class="pre">CheckCXXSourceCompiles</span></code></a> module. Then, use
<code class="docutils literal notranslate"><span class="pre">check_cxx_source_compiles</span></code> to determine whether <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> are
available from <code class="docutils literal notranslate"><span class="pre">cmath</span></code>. If they are available, use
<span class="target" id="index-1-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> to specify <code class="docutils literal notranslate"><span class="pre">HAVE_LOG</span></code> and <code class="docutils literal notranslate"><span class="pre">HAVE_EXP</span></code>
as compile definitions.</p>
<p>In the <code class="docutils literal notranslate"><span class="pre">MathFunctions/mysqrt.cxx</span></code>, include <code class="docutils literal notranslate"><span class="pre">cmath</span></code>. Then, if the system has
<code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code>, use them to compute the square root.</p>
</section>
<section id="build-and-run">
<h3>Build and Run<a class="headerlink" href="#build-and-run" title="Permalink to this heading">¶</a></h3>
<p>Make a new directory called <code class="docutils literal notranslate"><span class="pre">Step7_build</span></code>. Run the
<span class="target" id="index-0-manual:cmake(1)"></span><a class="reference internal" href="../../manual/cmake.1.html#manual:cmake(1)" title="cmake(1)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake</span></code></a> executable or the
<span class="target" id="index-0-manual:cmake-gui(1)"></span><a class="reference internal" href="../../manual/cmake-gui.1.html#manual:cmake-gui(1)" title="cmake-gui(1)"><code class="xref cmake cmake-manual docutils literal notranslate"><span class="pre">cmake-gui</span></code></a> to configure the project and then build it
with your chosen build tool and run the <code class="docutils literal notranslate"><span class="pre">Tutorial</span></code> executable.</p>
<p>This can look like the following:</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="go">mkdir Step7_build</span>
<span class="go">cd Step7_build</span>
<span class="go">cmake ../Step7</span>
<span class="go">cmake --build .</span>
</pre></div>
</div>
<p>Which function gives better results now, <code class="docutils literal notranslate"><span class="pre">sqrt</span></code> or <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code>?</p>
</section>
<section id="solution">
<h3>Solution<a class="headerlink" href="#solution" title="Permalink to this heading">¶</a></h3>
<p>In this exercise we will use functions from the
<span class="target" id="index-2-module:CheckCXXSourceCompiles"></span><a class="reference internal" href="../../module/CheckCXXSourceCompiles.html#module:CheckCXXSourceCompiles" title="CheckCXXSourceCompiles"><code class="xref cmake cmake-module docutils literal notranslate"><span class="pre">CheckCXXSourceCompiles</span></code></a> module so first we must include it in
<code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>.</p>
<details><summary>TODO 1: Click to show/hide answer</summary><div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-include-check-cxx-source-compiles">
<div class="code-block-caption"><span class="caption-text">TODO 1: MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-include-check-cxx-source-compiles" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">include(</span><span class="nb">CheckCXXSourceCompiles</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
</details><p>Then test for the availability of
<code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> using <code class="docutils literal notranslate"><span class="pre">check_cxx_compiles_source</span></code>. This function
lets us try compiling simple code with the required dependency prior to
the true source code compilation. The resulting variables <code class="docutils literal notranslate"><span class="pre">HAVE_LOG</span></code>
and <code class="docutils literal notranslate"><span class="pre">HAVE_EXP</span></code> represent whether those dependencies are available.</p>
<details><summary>TODO 2: Click to show/hide answer</summary><div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-check-cxx-source-compiles">
<div class="code-block-caption"><span class="caption-text">TODO 2: MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-check-cxx-source-compiles" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">check_cxx_source_compiles(</span><span class="s">&quot;</span>
<span class="s">  #include &lt;cmath&gt;</span>
<span class="s">  int main() {</span>
<span class="s">    std::log(1.0);</span>
<span class="s">    return 0;</span>
<span class="s">  }</span>
<span class="s">&quot;</span><span class="w"> </span><span class="no">HAVE_LOG</span><span class="nf">)</span><span class="w"></span>
<span class="nf">check_cxx_source_compiles(</span><span class="s">&quot;</span>
<span class="s">  #include &lt;cmath&gt;</span>
<span class="s">  int main() {</span>
<span class="s">    std::exp(1.0);</span>
<span class="s">    return 0;</span>
<span class="s">  }</span>
<span class="s">&quot;</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
</details><p>Next, we need to pass these CMake variables to our source code. This way,
our source code can tell what resources are available. If both <code class="docutils literal notranslate"><span class="pre">log</span></code> and
<code class="docutils literal notranslate"><span class="pre">exp</span></code> are available, use <span class="target" id="index-2-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> to specify
<code class="docutils literal notranslate"><span class="pre">HAVE_LOG</span></code> and <code class="docutils literal notranslate"><span class="pre">HAVE_EXP</span></code> as <code class="docutils literal notranslate"><span class="pre">PRIVATE</span></code> compile definitions.</p>
<details><summary>TODO 3: Click to show/hide answer</summary><div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-target-compile-definitions">
<div class="code-block-caption"><span class="caption-text">TODO 3: MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-target-compile-definitions" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">if(</span><span class="no">HAVE_LOG</span><span class="w"> </span><span class="no">AND</span><span class="w"> </span><span class="no">HAVE_EXP</span><span class="nf">)</span><span class="w"></span>
<span class="w">  </span><span class="nf">target_compile_definitions(</span><span class="nb">MathFunctions</span><span class="w"></span>
<span class="w">                             </span><span class="no">PRIVATE</span><span class="w"> </span><span class="s">&quot;HAVE_LOG&quot;</span><span class="w"> </span><span class="s">&quot;HAVE_EXP&quot;</span><span class="nf">)</span><span class="w"></span>
<span class="nf">endif()</span><span class="w"></span>
</pre></div>
</div>
</div>
</details><p>Since we may be using <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code>, we need to modify
<code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code> to include <code class="docutils literal notranslate"><span class="pre">cmath</span></code>.</p>
<details><summary>TODO 4: Click to show/hide answer</summary><div class="literal-block-wrapper docutils container" id="mathfunctions-mysqrt-cxx-include-cmath">
<div class="code-block-caption"><span class="caption-text">TODO 4: MathFunctions/mysqrt.cxx</span><a class="headerlink" href="#mathfunctions-mysqrt-cxx-include-cmath" title="Permalink to this code">¶</a></div>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;cmath&gt;</span><span class="cp"></span>
</pre></div>
</div>
</div>
</details><p>If <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> are available on the system, then use them to
compute the square root in the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function. The <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function in
<code class="docutils literal notranslate"><span class="pre">MathFunctions/mysqrt.cxx</span></code> will look as follows:</p>
<details><summary>TODO 5: Click to show/hide answer</summary><div class="literal-block-wrapper docutils container" id="mathfunctions-mysqrt-cxx-ifdef">
<div class="code-block-caption"><span class="caption-text">TODO 5: MathFunctions/mysqrt.cxx</span><a class="headerlink" href="#mathfunctions-mysqrt-cxx-ifdef" title="Permalink to this code">¶</a></div>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#if defined(HAVE_LOG) &amp;&amp; defined(HAVE_EXP)</span>
<span class="w">  </span><span class="kt">double</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">exp</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">log</span><span class="p">(</span><span class="n">x</span><span class="p">)</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="mf">0.5</span><span class="p">);</span><span class="w"></span>
<span class="w">  </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;Computing sqrt of &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; to be &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">result</span><span class="w"></span>
<span class="w">            </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; using log and exp&quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span><span class="w"></span>
<span class="cp">#else</span>
<span class="w">  </span><span class="kt">double</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">x</span><span class="p">;</span><span class="w"></span>
</pre></div>
</div>
</div>
</details></section>
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    <h3><a href="../../index.html">Table of Contents</a></h3>
    <ul>
<li><a class="reference internal" href="#">Step 7: Adding System Introspection</a><ul>
<li><a class="reference internal" href="#exercise-1-assessing-dependency-availability">Exercise 1 - Assessing Dependency Availability</a><ul>
<li><a class="reference internal" href="#goal">Goal</a></li>
<li><a class="reference internal" href="#helpful-resources">Helpful Resources</a></li>
<li><a class="reference internal" href="#files-to-edit">Files to Edit</a></li>
<li><a class="reference internal" href="#getting-started">Getting Started</a></li>
<li><a class="reference internal" href="#build-and-run">Build and Run</a></li>
<li><a class="reference internal" href="#solution">Solution</a></li>
</ul>
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