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<section id="step-8-adding-a-custom-command-and-generated-file">
<span id="guide:tutorial/Adding a Custom Command and Generated File"></span><h1>Step 8: Adding a Custom Command and Generated File<a class="headerlink" href="#step-8-adding-a-custom-command-and-generated-file" title="Permalink to this heading">¶</a></h1>
<p>Suppose, for the purpose of this tutorial, we decide that we never want to use
the platform <code class="docutils literal notranslate"><span class="pre">log</span></code> and <code class="docutils literal notranslate"><span class="pre">exp</span></code> functions and instead would like to
generate a table of precomputed values to use in the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function.
In this section, we will create the table as part of the build process,
and then compile that table into our application.</p>
<p>First, let's remove the check for 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 in
<code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>. Then remove the check for <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> from <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code>. At the same time, we can remove
<code class="code docutils literal notranslate"><span class="pre">#include</span> <span class="pre"><cmath></span></code>.</p>
<p>In the <code class="docutils literal notranslate"><span class="pre">MathFunctions</span></code> subdirectory, a new source file named
<code class="docutils literal notranslate"><span class="pre">MakeTable.cxx</span></code> has been provided to generate the table.</p>
<p>After reviewing the file, we can see that the table is produced as valid C++
code and that the output filename is passed in as an argument.</p>
<p>The next step is to add the appropriate commands to the
<code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code> file to build the MakeTable executable and
then run it as part of the build process. A few commands are needed to
accomplish this.</p>
<p>First, at the top of <code class="docutils literal notranslate"><span class="pre">MathFunctions/CMakeLists.txt</span></code>, the executable for
<code class="docutils literal notranslate"><span class="pre">MakeTable</span></code> is added as any other executable would be added.</p>
<div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-add-executable-maketable">
<div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-add-executable-maketable" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">add_executable(</span><span class="nb">MakeTable</span><span class="w"> </span><span class="nb">MakeTable.cxx</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>Then we add a custom command that specifies how to produce <code class="docutils literal notranslate"><span class="pre">Table.h</span></code>
by running MakeTable.</p>
<div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-add-custom-command-table-h">
<div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-add-custom-command-table-h" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">add_custom_command(</span><span class="w"></span>
<span class="w"> </span><span class="no">OUTPUT</span><span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_BINARY_DIR</span><span class="o">}</span><span class="na">/Table.h</span><span class="w"></span>
<span class="w"> </span><span class="no">COMMAND</span><span class="w"> </span><span class="nb">MakeTable</span><span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_BINARY_DIR</span><span class="o">}</span><span class="na">/Table.h</span><span class="w"></span>
<span class="w"> </span><span class="no">DEPENDS</span><span class="w"> </span><span class="nb">MakeTable</span><span class="w"></span>
<span class="w"> </span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>Next we have to let CMake know that <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code> depends on the generated
file <code class="docutils literal notranslate"><span class="pre">Table.h</span></code>. This is done by adding the generated <code class="docutils literal notranslate"><span class="pre">Table.h</span></code> to the list
of sources for the library MathFunctions.</p>
<div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-add-library-table-h">
<div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-add-library-table-h" 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">MathFunctions</span><span class="w"></span>
<span class="w"> </span><span class="nb">mysqrt.cxx</span><span class="w"></span>
<span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_BINARY_DIR</span><span class="o">}</span><span class="na">/Table.h</span><span class="w"></span>
<span class="w"> </span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>We also have to add the current binary directory to the list of include
directories so that <code class="docutils literal notranslate"><span class="pre">Table.h</span></code> can be found and included by <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code>.</p>
<div class="literal-block-wrapper docutils container" id="mathfunctions-cmakelists-txt-target-include-directories-table-h">
<div class="code-block-caption"><span class="caption-text">MathFunctions/CMakeLists.txt</span><a class="headerlink" href="#mathfunctions-cmakelists-txt-target-include-directories-table-h" title="Permalink to this code">¶</a></div>
<div class="highlight-cmake notranslate"><div class="highlight"><pre><span></span><span class="nf">target_include_directories(</span><span class="nb">MathFunctions</span><span class="w"></span>
<span class="w"> </span><span class="no">INTERFACE</span><span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_SOURCE_DIR</span><span class="o">}</span><span class="w"></span>
<span class="w"> </span><span class="no">PRIVATE</span><span class="w"> </span><span class="o">${</span><span class="nt">CMAKE_CURRENT_BINARY_DIR</span><span class="o">}</span><span class="w"></span>
<span class="w"> </span><span class="nf">)</span><span class="w"></span>
<span class="c"># link our compiler flags interface library</span>
<span class="nf">target_link_libraries(</span><span class="nb">MathFunctions</span><span class="w"> </span><span class="nb">tutorial_compiler_flags</span><span class="nf">)</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>Now let's use the generated table. First, modify <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code> to include
<code class="docutils literal notranslate"><span class="pre">Table.h</span></code>. Next, we can rewrite the <code class="docutils literal notranslate"><span class="pre">mysqrt</span></code> function to use the table:</p>
<div class="literal-block-wrapper docutils container" id="mathfunctions-mysqrt-cxx">
<div class="code-block-caption"><span class="caption-text">MathFunctions/mysqrt.cxx</span><a class="headerlink" href="#mathfunctions-mysqrt-cxx" title="Permalink to this code">¶</a></div>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">double</span><span class="w"> </span><span class="nf">mysqrt</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">x</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="c1">// use the table to help find an initial value</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>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">x</span><span class="w"> </span><span class="o">>=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">)</span><span class="w"> </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"><<</span><span class="w"> </span><span class="s">"Use the table to help find an initial value "</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">endl</span><span class="p">;</span><span class="w"></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">sqrtTable</span><span class="p">[</span><span class="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">x</span><span class="p">)];</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="c1">// do ten iterations</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">result</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0.1</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">delta</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="p">(</span><span class="n">result</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">result</span><span class="p">);</span><span class="w"></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">result</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mf">0.5</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">delta</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">result</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"><<</span><span class="w"> </span><span class="s">"Computing sqrt of "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" to be "</span><span class="w"> </span><span class="o"><<</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">endl</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">result</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
</div>
<p>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.</p>
<p>When this project is built it will first build the <code class="docutils literal notranslate"><span class="pre">MakeTable</span></code> executable.
It will then run <code class="docutils literal notranslate"><span class="pre">MakeTable</span></code> to produce <code class="docutils literal notranslate"><span class="pre">Table.h</span></code>. Finally, it will
compile <code class="docutils literal notranslate"><span class="pre">mysqrt.cxx</span></code> which includes <code class="docutils literal notranslate"><span class="pre">Table.h</span></code> to produce the
<code class="docutils literal notranslate"><span class="pre">MathFunctions</span></code> library.</p>
<p>Run the Tutorial executable and verify that it is using the table.</p>
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