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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head>  <meta name="generator" content=  "HTML Tidy for Linux/x86 (vers 1 September 2005), see www.w3.org">  <title>#if, #ifdef, #ifndef, #else, #elif, #endif</title>  <link href="../cppreference.css" rel="stylesheet" type="text/css"></head><body><table>  <tr>  <td>  <div class="body-content">  <div class="header-box">    <a href="../index.html">cppreference.com</a> &gt; <a href=    "index.html">C/C++ Pre-processor Commands</a> &gt; <a href=    "preprocessor_if.html">#if, #ifdef, #ifndef, #else, #elif,    #endif</a>  </div>  <div class="name-format">    #if, #ifdef, #ifndef, #else, #elif, #endif  </div>  <p>These commands give simple logic control to the compiler. As a  file is being compiled, you can use these commands to cause certain  lines of code to be included or not included.</p>  <pre class="example-code">   #if expression               </pre>  <p>If the value of expression is true, then the code that immediately  follows the command will be compiled.</p>  <pre class="example-code">   #ifdef macro         </pre>  <p>If the <em>macro</em> has been defined by a <a href=  "define.html">#define</a> statement, then the code immediately  following the command will be compiled.</p>  <pre class="example-code">   #ifndef macro                </pre>  <p>If the <em>macro</em> has not been defined by a <a href=  "define.html">#define</a> statement, then the code immediately  following the command will be compiled.</p>  <p>A few side notes: The command #elif is simply a horribly truncated  way to say &quot;elseif&quot; and works like you think it would. You  can also throw in a &quot;defined&quot; or &quot;!defined&quot; after  an #if to get added functionality.</p>  <div class="related-examples-format">    Example code:  </div>  <div class="related-examples">    <p>Here&#39;s an example of all these:</p>    <pre class="example-code">   #ifdef DEBUG     cout &lt;&lt; &quot;This is the test version, i=&quot; &lt;&lt; i &lt;&lt; endl;   #else     cout &lt;&lt; &quot;This is the production version!&quot; &lt;&lt; endl;   #endif               </pre>    <p>You might notice how that second example could make debugging a    lot easier than inserting and removing a million &quot;cout&quot;s    in your code.</p>  </div>  <div class="related-name-format">    Related topics:  </div>  <div class="related-content">    <a href="define.html">#define</a>  </div>  </div>  </td>    </tr>  </table></body></html>
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