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</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><a href="conditional.html" title="cpp/preprocessor/conditional"><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></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>&#35;include</span><span>__has_include<span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></strong></div></div></td></tr>
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<p>Includes other source file into current source file at the line immediately after the directive .
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="include.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="include.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="include.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="include.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="include.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/include&amp;action=edit&amp;section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td> <code><b>#include &lt;</b></code><span class="t-spar">filename</span><code><b>&gt;</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>#include "</b></code><span class="t-spar">filename</span><code><b>"</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
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<td> <code><b>__has_include</b></code> <code><b>(</b></code> <code><b>"</b></code> <span class="t-spar">filename</span> <code><b>"</b></code> <code><b>)</b></code><br /><code><b>__has_include</b></code> <code><b>(</b></code> <code><b>&lt;</b></code> <span class="t-spar">filename</span> <code><b>&gt;</b></code> <code><b>)</b></code>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx17">(since C++17)</span>
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<p>Any preprocessing tokens (macro constants or expressions) are permitted as arguments to <code>#include</code> <span class="t-rev-inl t-since-cxx17"><span>and <code>__has_include</code> </span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> as long as they expand to a sequence of characters surrounded by <code><b>&lt;</b></code> <code><b>&gt;</b></code> or <code><b>"</b></code> <code><b>"</b></code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/include&amp;action=edit&amp;section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<div class="t-li1"><span class="t-li">1,2)</span> Includes source file, identified by <span class="t-spar">filename</span> into the current source file at the line immediately after the directive. In the case the file is not found, program is ill-formed.</div>
<div class="t-li2"><span class="t-li">1)</span> Searches for the file in implementation-defined mannner. The intent of this syntax is to search for the files under control of the implementation. Typical implementations search only standard include directories. The standard C++ library and the standard C library are implicitly included in these standard include directories. The standard include directories usually can be controlled by the user through compiler options. </div>
<div class="t-li2"><span class="t-li">2)</span> Searches for the file in implementation-defined manner. The intent of this syntax is to search for the files that are not controlled by the implementation. Typical implementations first search the directory where the current file resides and, only if the file is not found, search the standard include directories as with <span class="t-v">(1)</span>.</div>
<div class="t-li1"><span class="t-li">3)</span> Preprocessor constant expression that evaluates to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> if the file name is found and <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> if not. The program is ill-formed if the argument would not be a valid argument to the <code>#include</code> directive.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/include&amp;action=edit&amp;section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>When a file is included, it is processed by <a href="../language/translation_phases.html" title="cpp/language/translation phases">translation phases</a> 1-4, which may include, recursively, expansion of the nested <code>#include</code> directives. To avoid repeated inclusion of the same file and endless recursion when a file includes itself, perhaps transitively, <i>header guards</i> are commonly used: the entire header is wrapped in 
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#ifndef FOO_H_INCLUDED /* any name uniquely mapped to file name */</span>
<span class="co2">#define FOO_H_INCLUDED</span>
<span class="co1">// contents of the file are here</span>
<span class="co2">#endif</span></pre></div></div>
<p>Many compilers also implement the non-standard <a href="impl.html" title="cpp/preprocessor/impl"><tt>pragma</tt></a> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="co2">#pragma once</span></span></span> with similar effects: it disables processing of a file if the same file (where file identity is determined in OS-specific way) has already been included.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/include&amp;action=edit&amp;section=4" 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">#if __has_include(&lt;optional&gt;)</span>
<span class="co2">#  include &lt;optional&gt;</span>
<span class="co2">#  define have_optional 1</span>
<span class="co2">#elif __has_include(&lt;experimental/optional&gt;)</span>
<span class="co2">#  include &lt;experimental/optional&gt;</span>
<span class="co2">#  define have_optional 1</span>
<span class="co2">#  define experimental_optional 1</span>
<span class="co2">#else</span>
<span class="co2">#  define have_optional 0</span>
<span class="co2">#endif</span>
&#160;
<span class="co2">#ifndef TEXT</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>
    <span class="co2">#define TEXT &quot;Hello, world!&quot;</span>
    <span class="co2">#include __FILE__</span>
    <span class="co2">#define TEXT &quot;Hello again!&quot;</span>
    <span class="co2">#include __FILE__</span>
&#160;
    <span class="br0">&#125;</span>
<span class="co2">#else</span>
    <a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> TEXT <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="co2">#undef TEXT</span>
<span class="co2">#endif</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">Hello, world!
Hello again!</pre></div></div> 
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/preprocessor/include&amp;action=edit&amp;section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td> <a href="../header.html" title="cpp/header">cpp/header</a>
</td>
<td> a list of C++ Standard Library header files
</td></tr>


<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../c/preprocessor/include.html" title="c/preprocessor/include">C documentation</a></span> for <span class="t-dsc-see-tt"><span>Source file inclusion</span></span></div>
</td></tr>

</table>

<!-- 
NewPP limit report
Preprocessor visited node count: 2148/1000000
Preprocessor generated node count: 6525/1000000
Post‐expand include size: 114121/2097152 bytes
Template argument size: 13314/2097152 bytes
Highest expansion depth: 16/40
Expensive parser function count: 0/100
-->

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