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<p>Declares an <a href="http://en.wikipedia.com/wiki/inline_function" class="extiw" title="enwiki:inline function">inline function</a>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
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<li class="toclevel-1 tocsection-1"><a href="inline.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="inline.html#Description"><span class="tocnumber">2</span> <span class="toctext">Description</span></a>
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<li class="toclevel-2 tocsection-3"><a href="inline.html#Notes"><span class="tocnumber">2.1</span> <span class="toctext">Notes</span></a></li>
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<li class="toclevel-1 tocsection-4"><a href="inline.html#Example"><span class="tocnumber">3</span> <span class="toctext">Example</span></a></li>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/inline&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> <span class="t-spar">function_declaration</span>
</td>
<td> (1)
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<td> <span class="t-spar">class</span> <code><b>{</b></code> <span class="t-spar">function_definition</span> <code><b>};</b></code>
</td>
<td> (2)
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<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> <span class="t-spar">function_declaration</span>
</td>
<td> (3)
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<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> A function at global scope can be declared inline using the keyword <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span></div>
<div class="t-li1"><span class="t-li">2)</span> A function defined entirely inside a <a href="classes.html" title="cpp/language/classes">class/struct/union definition</a>, whether it's a member function or a non-member <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">friend</span></span></span> function, is always inline.</div>
<div class="t-li1"><span class="t-li">3)</span> A function declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> is always inline.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/inline&action=edit&section=2" title="Edit section: Description">edit</a>]</span> <span class="mw-headline" id="Description">Description</span></h3>
<p>An <i>inline function</i> is a function with the following properties:
</p>
<div class="t-li1"><span class="t-li">1)</span> There may be <a href="definition.html#One_Definition_Rule" title="cpp/language/definition">more than one definition</a> of an inline function in the program as long as each definition appears in a different translation unit. For example, an inline function may be defined in a header file that is #include'd in multiple source files.</div>
<div class="t-li1"><span class="t-li">2)</span> The definition of an inline function must be present in the translation unit where it is called (not necessarily before the point of call).</div>
<div class="t-li1"><span class="t-li">3)</span> An inline function with <a href="storage_duration.html" title="cpp/language/storage duration">external linkage</a> (e.g. not declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span>) has the following additional properties:</div>
<div class="t-li2"><span class="t-li">1)</span> It must be declared <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> in every translation unit.</div>
<div class="t-li2"><span class="t-li">2)</span> It has the same address in every translation unit.</div>
<div class="t-li2"><span class="t-li">3)</span> Function-local static objects in all function definitions are shared across all translation units (they all refer to the same object defined in one translation unit)</div>
<div class="t-li2"><span class="t-li">4)</span> Types defined in all function definitions are also the same in all translation units.</div>
<table class="t-rev-begin">
<tr class="t-rev t-until-cxx17"><td>
<div class="t-li2"><span class="t-li">5)</span> <a href="string_literal.html" title="cpp/language/string literal">String literals</a> in all function definitions are shared (they are all the same string literal defined in just one translation unit)
</div></td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
</table>
<p>The intent of the <code>inline</code> keyword is to serve as an indicator to the optimizer that inline substitution of the function is preferred over function call, that is, instead of executing the call CPU instruction to transfer control to the function body, a copy of the function body is executed without generating the call. This avoids extra overhead created by the function call (copying the arguments and retrieving the result) but it may result in a larger executable as the code for the function has to be repeated multiple times.
</p><p>Since this meaning of the keyword <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> is non-binding, compilers are free to use inline substitution for any function that's not marked inline, and are free to generate function calls to any function marked inline. Those choices do not change the rules regarding multiple definitions and shared statics listed above.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/inline&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h4>
<p>If an inline function is defined differently in different translation units, the behavior is undefined.
</p><p>The inline specifier cannot be used with a function declaration at block scope (inside another function)
</p><p>The inline specifier cannot re-declare a function that was already defined in the translation unit as non-inline.
</p><p>The implicitly-generated member functions and any member function declared as deleted are inline just like any other function defined inside a class definition.
</p><p>If an inline function is declared in different translation units, the accumulated sets of <a href="default_arguments.html" title="cpp/language/default arguments">default arguments</a> must be the same at the end of each translation unit.
</p><p>In C, inline functions do not have to be declared inline in every translation unit (at most one may be non-inline or extern inline), the function definitions do not have to be identical (but the behavior of the program must not depend on which one is called), and the function-local statics are distinct between different definitions of the same function.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/inline&action=edit&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="co1">// header file</span>
<span class="co2">#ifndef EXAMPLE_H</span>
<span class="co2">#define EXAMPLE_H</span>
<span class="co1">// function included in multiple source files must be inline</span>
<span class="kw1">inline</span> <span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> a <span class="sy2">+</span> b<span class="sy4">;</span>
<span class="br0">}</span>
<span class="co2">#endif</span>
 
<span class="co1">// source file #2</span>
<span class="co2">#include "example.h"</span>
<span class="kw4">int</span> a<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// source file #1</span>
<span class="co2">#include "example.h"</span>
<span class="kw4">int</span> b<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/inline&action=edit&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 colspan="2"> <div class="t-dsc-see"><span><a href="../../c/language/inline.html" title="c/language/inline">C documentation</a></span> for <span class="t-dsc-see-tt"><span>inline</span></span></div>
</td></tr>
</table>
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