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Preprocessor Conditionals [C++ Reference]
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<h2><a name="preprocessor_conditionals" id="preprocessor_conditionals">Preprocessor Conditionals</a></h2>
<div class="level2">
<pre class="c code c++" style="font-family:monospace;"><span class="co2">#if, #ifdef, #ifndef, #else, #elif, #endif</span></pre>
<p>
These six preprocessor 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="c code c++" style="font-family:monospace;"> <span class="co2">#if expression</span></pre>
<p>
If the value of expression is true, then the code that immediately follows the
command will be compiled.
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#ifdef macro</span></pre>
<p>
If the macro has been defined by a <a href="../preprocessor/define.html" class="wikilink1" title="preprocessor:define">#define</a> statement, then the code immediately
following the command will be compiled.
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#ifndef macro</span></pre>
<p>
If the macro has not been defined by a <a href="../preprocessor/define.html" class="wikilink1" title="preprocessor:define">#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
“elseif” and works like you think it would. You can also throw in a “defined”
or ”!defined” after an #if to get added functionality.
</p>
<p>
Here's an example of all these:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#ifdef DEBUG</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"This is the test version, i="</span> <span class="sy0"><<</span> i <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="co2">#else</span>
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"This is the production version!"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span>
<span class="co2">#endif</span></pre>
<p>
You might notice how that second example could make debugging a lot easier than
inserting and removing a million “cout”s in your code.
</p>
<p>
Related topics: <a href="../preprocessor/define.html" class="wikilink1" title="preprocessor:define">#define</a>
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