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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>atomic_compare_exchange_weak, <span style="font-size:0.7em; line-height:130%">std::</span>atomic_compare_exchange_strong, <span style="font-size:0.7em; line-height:130%">std::</span>atomic_compare_exchange_weak_explicit, <span style="font-size:0.7em; line-height:130%">std::</span>atomic_compare_exchange_strong_explicit</h1>
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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>atomic_compare_exchange_weak</span><span>atomic_compare_exchange_weak_explicit</span><span>atomic_compare_exchange_strong</span><span>atomic_compare_exchange_strong_explicit</span></span></strong></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_fetch_add.html" title="cpp/atomic/atomic fetch add"><span class="t-lines"><span>atomic_fetch_add</span><span>atomic_fetch_add_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_fetch_sub.html" title="cpp/atomic/atomic fetch sub"><span class="t-lines"><span>atomic_fetch_sub</span><span>atomic_fetch_sub_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_fetch_and.html" title="cpp/atomic/atomic fetch and"><span class="t-lines"><span>atomic_fetch_and</span><span>atomic_fetch_and_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_fetch_or.html" title="cpp/atomic/atomic fetch or"><span class="t-lines"><span>atomic_fetch_or</span><span>atomic_fetch_or_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_fetch_xor.html" title="cpp/atomic/atomic fetch xor"><span class="t-lines"><span>atomic_fetch_xor</span><span>atomic_fetch_xor_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Atomic flags</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_flag.html" title="cpp/atomic/atomic flag"><span class="t-lines"><span>atomic_flag</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_flag_test_and_set.html" title="cpp/atomic/atomic flag test and set"><span class="t-lines"><span>atomic_flag_test_and_set</span><span>atomic_flag_test_and_set_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_flag_clear.html" title="cpp/atomic/atomic flag clear"><span class="t-lines"><span>atomic_flag_clear</span><span>atomic_flag_clear_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Initialization</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_init.html" title="cpp/atomic/atomic init"><span class="t-lines"><span>atomic_init</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="ATOMIC_VAR_INIT.html" title="cpp/atomic/ATOMIC VAR INIT"><span class="t-lines"><span>ATOMIC_VAR_INIT</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="ATOMIC_FLAG_INIT.html" title="cpp/atomic/ATOMIC FLAG INIT"><span class="t-lines"><span>ATOMIC_FLAG_INIT</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Memory ordering</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="memory_order.html" title="cpp/atomic/memory order"><span class="t-lines"><span>memory_order</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="kill_dependency.html" title="cpp/atomic/kill dependency"><span class="t-lines"><span>kill_dependency</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_thread_fence.html" title="cpp/atomic/atomic thread fence"><span class="t-lines"><span>atomic_thread_fence</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="atomic_signal_fence.html" title="cpp/atomic/atomic signal fence"><span class="t-lines"><span>atomic_signal_fence</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></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/atomic/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>Defined in header <code><a href="../header/atomic.html" title="cpp/header/atomic"><atomic></a></code>
</div></td>
<td></td>
<td></td>
</tr>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num t-dcl-rev-notes t-since-cxx11"><tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(1)</td>
<td rowspan="142"> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_weak<span class="br0">(</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj, <br />
</p>
                                   T<span class="sy2">*</span> expected, T desired <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_weak<span class="br0">(</span> <span class="kw4">volatile</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj, <br />
</p>
                                   T<span class="sy2">*</span> expected, T desired <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
</tbody><tbody>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num t-dcl-rev-notes t-since-cxx11"><tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(2)</td>
<td rowspan="142"> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_strong<span class="br0">(</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
</p>
                                     T<span class="sy2">*</span> expected, T desired <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_strong<span class="br0">(</span> <span class="kw4">volatile</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
</p>
                                     T<span class="sy2">*</span> expected, T desired <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
</tbody><tbody>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num t-dcl-rev-notes t-since-cxx11"><tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(3)</td>
<td rowspan="142"> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_weak_explicit<span class="br0">(</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
                                            T<span class="sy2">*</span> expected, T desired,<br />
                                            <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> succ, <br />
</p>
                                            <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> fail <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_weak_explicit<span class="br0">(</span> <span class="kw4">volatile</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
                                            T<span class="sy2">*</span> expected, T desired,<br />
                                            <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> succ, <br />
</p>
                                            <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> fail <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
</tbody><tbody>
</tbody>
<tbody class="t-dcl-rev t-dcl-rev-num t-dcl-rev-notes t-since-cxx11"><tr class="t-dcl-rev-aux">
<td></td>
<td rowspan="142">(4)</td>
<td rowspan="142"> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_strong_explicit<span class="br0">(</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
                                              T<span class="sy2">*</span> expected, T desired,<br />
                                              <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> succ, <br />
</p>
                                              <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> fail <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
<tr class="t-dcl">
<td class="t-dcl-nopad"> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
<p><span class="kw4">bool</span> atomic_compare_exchange_strong_explicit<span class="br0">(</span> <span class="kw4">volatile</span> <a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj, <br />
                                              T<span class="sy2">*</span> expected, T desired,<br />
                                              <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> succ, <br />
</p>
                                              <a href="memory_order.html"><span class="kw2085">std::<span class="me2">memory_order</span></span></a> fail <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad"> </td>
<td class="t-dcl-nopad"> </td>
</tr>
</tbody><tbody>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>Atomically compares the <a href="../language/objects.html" title="cpp/language/object">object representation</a> of the object pointed to by <code>obj</code> with the object representation of the object pointed to by <code>expected</code>, as if by <span class="t-lc"><a href="../string/byte/memcmp.html" title="cpp/string/byte/memcmp">std::memcmp</a></span>, and if those are bitwise-equal, replaces the former with <code>desired</code> (performs read-modify-write operation). Otherwise, loads the actual value pointed to by <code>obj</code> into <code>*expected</code> (performs load operation). Copying is performed as if by <span class="t-lc"><a href="../string/byte/memcpy.html" title="cpp/string/byte/memcpy">std::memcpy</a></span>.
</p><p>The memory models for the read-modify-write and load operations are <code>succ</code> and <code>fail</code> respectively. The (1-2) versions use <span class="t-lc"><a href="memory_order.html" title="cpp/atomic/memory order">std::memory_order_seq_cst</a></span> by default.
</p><p>These functions are defined in terms of member functions of <span class="t-lc"><a href="atomic.html" title="cpp/atomic/atomic">std::atomic</a></span>:
</p>
<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>compare_exchange_weak<span class="br0">(</span><span class="sy2">*</span>expected, desired<span class="br0">)</span></span></span></div>
<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>compare_exchange_strong<span class="br0">(</span><span class="sy2">*</span>expected, desired<span class="br0">)</span></span></span></div>
<div class="t-li1"><span class="t-li">3)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>compare_exchange_weak<span class="br0">(</span><span class="sy2">*</span>expected, desired, succ, fail<span class="br0">)</span></span></span></div>
<div class="t-li1"><span class="t-li">4)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>compare_exchange_strong<span class="br0">(</span><span class="sy2">*</span>expected, desired, succ, fail<span class="br0">)</span></span></span></div>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="atomic_compare_exchange.html#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="atomic_compare_exchange.html#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="atomic_compare_exchange.html#Exceptions"><span class="tocnumber">3</span> <span class="toctext">Exceptions</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="atomic_compare_exchange.html#Notes"><span class="tocnumber">4</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="atomic_compare_exchange.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="atomic_compare_exchange.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_compare_exchange&action=edit&section=1" title="Edit section: Parameters">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td> obj
</td>
<td> -
</td>
<td> pointer to the atomic object to test and modify
</td></tr>
<tr class="t-par">
<td> expected
</td>
<td> -
</td>
<td> pointer to the value expected to be found in the atomic object
</td></tr>
<tr class="t-par">
<td> desired
</td>
<td> -
</td>
<td> the value to store in the atomic object if it is as expected
</td></tr>
<tr class="t-par">
<td> succ
</td>
<td> -
</td>
<td> the memory synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.
</td></tr>
<tr class="t-par">
<td> fail
</td>
<td> -
</td>
<td> the memory synchronization ordering for the load operation if the comparison fails. Cannot be <span class="t-lc"><a href="memory_order.html" title="cpp/atomic/memory order">std::memory_order_release</a></span> or <span class="t-lc"><a href="memory_order.html" title="cpp/atomic/memory order">std::memory_order_acq_rel</a></span><span class="t-rev-inl t-until-cxx17"><span> and cannot specify stronger ordering than <code>succ</code></span> <span><span class="t-mark-rev t-until-cxx17">(until C++17)</span></span></span>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_compare_exchange&action=edit&section=2" title="Edit section: Return value">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span></h3>
<p>The result of the comparison: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> if <code>*obj</code> was equal to <code>*expected</code>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> otherwise.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_compare_exchange&action=edit&section=3" title="Edit section: Exceptions">edit</a>]</span> <span class="mw-headline" id="Exceptions">Exceptions</span></h3>
<div class="t-noexcept-inline"><a href="../language/noexcept_spec.html" title="cpp/language/noexcept spec"><tt>noexcept</tt></a> specification:  <div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span></span></span></div>  </div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_compare_exchange&action=edit&section=4" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>The weak forms ((1) and (3)) of the functions are allowed to fail spuriously, that is, act as if <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>obj <span class="sy3">!</span><span class="sy1">=</span> <span class="sy2">*</span>expected</span></span> even if they are equal. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms.
</p><p>When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable unless the object representation of <code>T</code> may include padding bits, trap bits, or offers multiple object representations for the same value (e.g. floating-point NaN). In those cases, weak compare-and-exchange typically works because it quickly converges on some stable object representation.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/atomic/atomic_compare_exchange&action=edit&section=5" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><p> compare and exchange operations are often used as basic building blocks of lockfree data structures
</p><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">#include <atomic></span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
<span class="kw1">struct</span> node
<span class="br0">{</span>
T data<span class="sy4">;</span>
node<span class="sy2">*</span> next<span class="sy4">;</span>
node<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> data<span class="br0">)</span> <span class="sy4">:</span> data<span class="br0">(</span>data<span class="br0">)</span>, next<span class="br0">(</span>nullptr<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
<span class="kw1">class</span> stack
<span class="br0">{</span>
<a href="atomic.html"><span class="kw2063">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>node<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span><span class="sy1">></span> head<span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw4">void</span> push<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> data<span class="br0">)</span>
<span class="br0">{</span>
node<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> new_node <span class="sy1">=</span> new node<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>data<span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// put the current value of head into new_node->next</span>
new_node<span class="sy2">-</span><span class="sy1">></span>next <span class="sy1">=</span> head.<span class="me1">load</span><span class="br0">(</span><a href="memory_order.html"><span class="kw2086">std::<span class="me2">memory_order_relaxed</span></span></a><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// now make new_node the new head, but if the head</span>
<span class="co1">// is no longer what's stored in new_node->next</span>
<span class="co1">// (some other thread must have inserted a node just now)</span>
<span class="co1">// then put that new head into new_node->next and try again</span>
<span class="kw1">while</span><span class="br0">(</span><span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">atomic_compare_exchange_weak_explicit</span><span class="br0">(</span>
<span class="sy3">&</span>head,
<span class="sy3">&</span>new_node<span class="sy2">-</span><span class="sy1">></span>next,
new_node,
<a href="memory_order.html"><span class="kw2089">std::<span class="me2">memory_order_release</span></span></a>,
<a href="memory_order.html"><span class="kw2086">std::<span class="me2">memory_order_relaxed</span></span></a><span class="br0">)</span><span class="br0">)</span>
<span class="sy4">;</span> <span class="co1">// the body of the loop is empty</span>
<span class="co1">// note: the above loop is not thread-safe in at least</span>
<span class="co1">// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)</span>
<span class="co1">// MSVC prior to 2014-03-17 (bug 819819). See member function version for workaround</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
stack<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> s<span class="sy4">;</span>
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
s.<span class="me1">push</span><span class="br0">(</span><span class="nu0">3</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/atomic/atomic_compare_exchange&action=edit&section=6" 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> <div class="t-dsc-member-div"><div><a href="atomic/compare_exchange.html" title="cpp/atomic/atomic/compare exchange"> <span class="t-lines"><span>compare_exchange_weak</span><span>compare_exchange_strong</span></span></a></div></div>
</td>
<td> atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not <br /> <span class="t-mark">(public member function of <code>std::atomic</code>)</span> <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/atomic/atomic/dsc_compare_exchange&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="atomic_exchange.html" title="cpp/atomic/atomic exchange"> <span class="t-lines"><span>atomic_exchange</span><span>atomic_exchange_explicit</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div>
</td>
<td> atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic <br /> <span class="t-mark">(function template)</span> <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/atomic/dsc_atomic_exchange&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <div class="t-dsc-member-div"><div><a href="../memory/shared_ptr/atomic.html" title="cpp/memory/shared ptr/atomic"> <span class="t-lines"><span>std::atomic_compare_exchange_weak<span class="t-dsc-small">(std::shared_ptr)</span>
</span><span>std::atomic_compare_exchange_strong<span class="t-dsc-small">(std::shared_ptr)</span></span></span></a></div></div>
</td>
<td> specializes atomic operations for std::shared_ptr <br /> <span class="t-mark">(function template)</span>
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../c/atomic/atomic_compare_exchange.html" title="c/atomic/atomic compare exchange">C documentation</a></span> for <span class="t-dsc-see-tt"><span>atomic_compare_exchange</span>, <span>atomic_compare_exchange_explicit</span></span></div>
</td></tr>
</table>
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