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<tr class="t-nv"><td colspan="5"> <a href="../language/alignas.html" title="cpp/language/alignas"> alignas specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Initialization</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/default_initialization.html" title="cpp/language/default initialization"> default initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/value_initialization.html" title="cpp/language/value initialization"> value initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/zero_initialization.html" title="cpp/language/zero initialization"> zero initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/copy_initialization" title="cpp/language/copy initialization"> copy initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/direct_initialization.html" title="cpp/language/direct initialization"> direct initialization</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="../language/aggregate_initialization.html" title="cpp/language/aggregate initialization"> aggregate initialization</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/list_initialization.html" title="cpp/language/list initialization"> list initialization</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/constant_initialization.html" title="cpp/language/constant initialization">constant initialization</a></td></tr>
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<div style="display:inline-block">
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<tr class="t-nv"><td colspan="5"> <a href="../language/integer_literal.html" title="cpp/language/integer literal"> integer literals</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="../language/nullptr.html" title="cpp/language/nullptr"> nullptr</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/user_literal.html" title="cpp/language/user literal"> user-defined</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> Expressions</td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="../language/operator_alternative.html" title="cpp/language/operator alternative"> alternative representations</a></td></tr>
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<tr class="t-nv-h2"><td colspan="5"> Casts</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/implicit_cast.html" title="cpp/language/implicit cast"> implicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/const_cast.html" title="cpp/language/const cast"> <code>const_cast</code> conversion</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="../language/explicit_cast.html" title="cpp/language/explicit cast"> C-style and functional cast</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="../language/delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> Classes</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/class.html" title="cpp/language/class"> class declaration</a></td></tr>
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</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/virtual.html" title="cpp/language/virtual"> virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/override.html" title="cpp/language/override"> override specifier</a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/final.html" title="cpp/language/final"> final specifier</a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/explicit.html" title="cpp/language/explicit"> explicit</a>  <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/static.html" title="cpp/language/static"> static</a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/default_constructor.html" title="cpp/language/default constructor"> default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/copy_constructor.html" title="cpp/language/copy constructor"> copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/move_constructor.html" title="cpp/language/move constructor"> move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../language/as_operator.html" title="cpp/language/as operator"> copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/move_operator.html" title="cpp/language/move operator"> move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/destructor.html" title="cpp/language/destructor"> destructor</a></td></tr>
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<tr class="t-nv-h1"><td colspan="5">Templates </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/class_template.html" title="cpp/language/class template"> class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/function_template.html" title="cpp/language/function template"> function template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/template_specialization.html" title="cpp/language/template specialization"> template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="../language/parameter_pack.html" title="cpp/language/parameter pack"> parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../language/asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div>
</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/navbar_content&amp;action=edit">&#91;edit&#93;</a></span></div></div></div></div><div class="t-navbar-sep">&#160;</div><div class="t-navbar-head"><a href="../preprocessor.html" title="cpp/preprocessor"> Preprocessor</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>&#35;if</span><span>&#35;ifdef</span><span>&#35;ifndef</span><span>&#35;else</span><span>&#35;elif</span><span>&#35;endif</span></span></strong></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="replace.html" title="cpp/preprocessor/replace"><span class="t-lines"><span>&#35;define</span><span>&#35;undef</span><span>&#35;,&#35;&#35; operators</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="include.html" title="cpp/preprocessor/include"><span class="t-lines"><span>&#35;include</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="warning" title="cpp/preprocessor/warning"><span class="t-lines"><span>&#35;warning</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="error.html" title="cpp/preprocessor/error"><span class="t-lines"><span>&#35;error</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="impl.html" title="cpp/preprocessor/impl"><span class="t-lines"><span>&#35;pragma</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="line.html" title="cpp/preprocessor/line"><span class="t-lines"><span>&#35;line</span></span></a></div></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/preprocessor/navbar_content&amp;action=edit">&#91;edit&#93;</a></span></div></div></div></div><div class="t-navbar-sep">&#160;</div></div>
<p>The preprocessor supports conditional compilation of parts of source file. This behavior is controlled by <code>#if</code>, <code>#else</code>, <code>#elif</code>, <code>#ifdef</code>, <code>#ifndef</code> and <code>#endif</code> directives.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="#Condition_evaluation"><span class="tocnumber">3</span> <span class="toctext">Condition evaluation</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="#.23if.2C_.23elif"><span class="tocnumber">3.1</span> <span class="toctext">#if, #elif</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="#.23ifdef.2C_.23ifndef"><span class="tocnumber">3.2</span> <span class="toctext">#ifdef, #ifndef</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-6"><a href="#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#if</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifdef</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifndef</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#elif</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#else</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#endif</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
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<td colspan="10" class="t-sdsc-sep">
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The conditional preprocessing block starts with <code>#if</code>, <code>#ifdef</code> or <code>#ifndef</code> directive, then optionally includes any number of <code>#elif</code> directives, then optionally includes at most one <code>#else</code> directive and is terminated with <code>#endif</code> directive. Any inner conditional preprocessing blocks are processed separately.
</p><p>Each of <code>#if</code>, <code>#elif</code>, <code>#else</code>, <code>#ifdef</code> and <code>#ifndef</code> directives control code block until first <code>#elif</code>, <code>#else</code>, <code>#endif</code> directive not belonging to any inner conditional preprocessing blocks. 
</p><p><code>#if</code>, <code>#ifdef</code> and <code>#ifndef</code>  directives test the specified condition (see below) and if it evaluates to true, compiles the controlled code block. In that case subsequent <code>#else</code> and <code>#elif</code> directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent <code>#else</code> or <code>#elif</code> directive (if any) is processed. In the former case, the code block controlled by the <code>#else</code> directive is unconditionally compiled. In the latter case, the <code>#elif</code> directive acts as if it was <code>#if</code> directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent <code>#elif</code> and <code>#else</code> directives. The conditional preprocessing block is terminated by <code>#endif</code> directive.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=3" title="Edit section: Condition evaluation">edit</a>]</span> <span class="mw-headline" id="Condition_evaluation">Condition evaluation</span></h3>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=4" title="Edit section: #if, #elif">edit</a>]</span> <span class="mw-headline" id=".23if.2C_.23elif"><code>#if, #elif</code></span></h4>
<p>The <span class="t-spar">expression</span> is a constant expression, using only <a href="../language/expressions.html#Literals" title="cpp/language/expressions">literals</a> and identifiers, defined using <a href="replace.html" title="cpp/preprocessor/replace"><tt>
#define</tt></a> directive. Any identifier, which is not literal, non defined using <code>#define</code> directive, evaluates to <i>0</i>. 
</p><p>The expression may contain unary operators in form <code>defined</code>&#160;<span class="t-spar">identifier</span> or  <code>defined (</code><span class="t-spar">identifier</span><code>)</code> which return <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> if the <span class="t-spar">identifier</span> was defined using <a href="replace.html" title="cpp/preprocessor/replace"><tt>
#define</tt></a> directive and <i>0</i> otherwise. If the <span class="t-spar">expression</span> evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span>.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=5" title="Edit section: #ifdef, #ifndef">edit</a>]</span> <span class="mw-headline" id=".23ifdef.2C_.23ifndef"><code>#ifdef, #ifndef</code></span></h4>
<p>Checks if the identifier was defined using <a href="replace.html" title="cpp/preprocessor/replace"><tt>
#define</tt></a> directive. 
</p><p><code>#ifdef</code>&#160;<span class="t-spar">identifier</span> is essentially equivalent to <code>#if defined(</code>&#160;<span class="t-spar">identifier</span><code>)</code>.
</p><p><code>#ifndef</code>&#160;<span class="t-spar">identifier</span> is essentially equivalent to <code>#if&#160;!defined(</code>&#160;<span class="t-spar">identifier</span><code>)</code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/conditional&amp;action=edit&amp;section=6" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#define ABCD 2</span>
<span class="co2">#include &lt;iostream&gt;</span>
&#160;
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
&#160;
<span class="co2">#ifdef ABCD</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;1: yes<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;1: no<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#endif</span>
&#160;
<span class="co2">#ifndef ABCD</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;2: no1<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#elif ABCD == 2</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;2: yes<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#else</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;2: no2<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#endif</span>
&#160;
<span class="co2">#if&#160;!defined(DCBA) &amp;&amp; (ABCD &lt; 2*4-3)</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;3: yes<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co2">#endif</span>
<span class="br0">&#125;</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1: yes
2: yes
3: yes</pre></div></div> 
</div>

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