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# and ## [C++ Reference]
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<h2><a name="and" id="and"># and ##</a></h2>
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<p>
The # and ## preprocessor operators are used with the <a href="../preprocessor/define.html" class="wikilink1" title="preprocessor:define">#define</a> preprocessor directive.
</p>
<ul>
<li class="level1"><div class="li"> Using # causes the first argument after the # to be returned as a string in quotes. </div>
</li>
<li class="level1"><div class="li"> Using ## concatenates what's before the ## with what's after it.</div>
</li>
</ul>
<p>
For example, the command
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#define to_string( s ) # s</span></pre>
<p>
will make the compiler turn this command
</p>
<pre class="c code c++" style="font-family:monospace;"> <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> to_string<span class="br0">(</span> Hello World<span class="sy0">!</span> <span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
into
</p>
<pre class="c code c++" style="font-family:monospace;"> <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> <span class="st0">"Hello World!"</span> <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
Here is an example of the ## command:
</p>
<pre class="c code c++" style="font-family:monospace;"> <span class="co2">#define concatenate( a, b ) a ## b</span>
...
<span class="kw4">int</span> xy <span class="sy0">=</span> <span class="nu0">10</span><span class="sy0">;</span>
...</pre>
<p>
This code will make the compiler turn
</p>
<pre class="c code c++" style="font-family:monospace;"> <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> concatenate<span class="br0">(</span> x<span class="sy0">,</span> y <span class="br0">)</span> <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
into
</p>
<pre class="c code c++" style="font-family:monospace;"> <a href="http://www.opengroup.org/onlinepubs/009695399/functions/cout.html"><span class="kw3">cout</span></a> <span class="sy0"><<</span> xy <span class="sy0"><<</span> endl<span class="sy0">;</span></pre>
<p>
which will, of course, display '10' to standard output.
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Related topics: <a href="../preprocessor/define.html" class="wikilink1" title="preprocessor:define">#define</a>
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