209 lines
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209 lines
16 KiB
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<title>std::atomic_exchange, std::atomic_exchange_explicit</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>atomic_exchange, <span style="font-size:0.7em; line-height:130%">std::</span>atomic_exchange_explicit</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-atomic.html">atomic</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-atomic.html"><atomic></a></code></div>
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<td></td>
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<td rowspan="142">(1)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<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 /></span>
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<p>T atomic_exchange<span class="br0">(</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br /></p>
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<span class="kw1">typename</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> desr <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<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 /></span>
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<p>T atomic_exchange<span class="br0">(</span> <span class="kw4">volatile</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br /></p>
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<span class="kw1">typename</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> desr <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<td rowspan="142">(2)</td>
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<td rowspan="142"><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<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 /></span>
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<p>T atomic_exchange_explicit<span class="br0">(</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
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<span class="kw1">typename</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> desr,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2090">std::<span class="me2">memory_order</span></span></a> order <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<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 /></span>
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<p>T atomic_exchange_explicit<span class="br0">(</span> <span class="kw4">volatile</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> obj,<br />
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<span class="kw1">typename</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> desr,<br /></p>
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<a href="cpp-atomic-memory_order.html"><span class="kw2090">std::<span class="me2">memory_order</span></span></a> order <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<div class="t-li1"><span class="t-li">1)</span> 原子地以 <code>desr</code> 的值替换 <code>obj</code> 所指向的值，并返回 <code>obj</code> 先前保有的值，如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>exchange<span class="br0">(</span>desr<span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 原子地以 <code>desr</code> 的值替换 <code>obj</code> 所指向的值，并返回 <code>obj</code> 先前保有的值，如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>exchange<span class="br0">(</span>desr, order<span class="br0">)</span></span></span> 。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>obj</td>
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<td>-</td>
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<td>指向要修改的原子对象的指针</td>
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<td>desr</td>
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<td>-</td>
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<td>要存储于原子对象的值</td>
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<td>order</td>
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<td>-</td>
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<td>此操作所用的内存同步顺序：容许所有值。</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p><code>obj</code> 所指向的原子对象先前保有的值。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<p>能以原子替换操作在用户空间实现自旋锁互斥，类似 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">atomic_flag_test_and_set</span></span></span> ：</p>
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <thread></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <atomic></span>
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<a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span> lock<span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 锁定时保有 true</span>
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<span class="co1">// 解锁时保有 false</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> cnt <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> cnt <span class="sy1"><</span> <span class="nu0">100</span><span class="sy4">;</span> <span class="sy2">++</span>cnt<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">while</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">atomic_exchange_explicit</span><span class="br0">(</span><span class="sy3">&</span>lock, <span class="kw2">true</span>, <a href="cpp-atomic-memory_order.html"><span class="kw2093">std::<span class="me2">memory_order_acquire</span></span></a><span class="br0">)</span><span class="br0">)</span>
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<span class="sy4">;</span> <span class="co1">// 自旋直至获得</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Output from thread "</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-atomic-atomic_store.html"><span class="kw2069">std::<span class="me2">atomic_store_explicit</span></span></a><span class="br0">(</span><span class="sy3">&</span>lock, <span class="kw2">false</span>, <a href="cpp-atomic-memory_order.html"><span class="kw2094">std::<span class="me2">memory_order_release</span></span></a><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-thread-thread.html"><span class="kw2144">std::<span class="me2">thread</span></span></a><span class="sy1">></span> v<span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
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v.<span class="me1">emplace_back</span><span class="br0">(</span>f, n<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> t <span class="sy4">:</span> v<span class="br0">)</span> <span class="br0">{</span>
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t.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Output from thread 2
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Output from thread 6
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Output from thread 7
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...< 准确 1000 行 >...</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P0558R1">P0558R1</a></td>
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<td>C++11</td>
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<td>曾要求准确类型匹配，因为从多个参数推导 <code>T</code></td>
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<td>仅从 <code>atomic</code> 参数推导 <code>T</code></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic-exchange.html"><span class="t-lines"><span>exchange</span></span></a></div>
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</div>
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</td>
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<td>原子地替换原子对象的值并获得它先前持有的值<br />
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<span class="t-mark">(<code>std::atomic<T></code> 的公开成员函数)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-atomic-atomic_compare_exchange.html"><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></a></div>
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<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>
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</div>
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</td>
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<td>原子地比较原子对象和非原子实参的值，若相等则进行 atomic_exchange，若不相等则进行 atomic_load<br />
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<span class="t-mark">(函数模板)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-shared_ptr-atomic.html"><span class="t-lines"><span>std::atomic_exchange<span class="t-dsc-small">(std::shared_ptr)</span></span> <span>std::atomic_exchange_explicit<span class="t-dsc-small">(std::shared_ptr)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-deprecated-cxx20">(C++20 中弃用)</span></span></span></div>
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</div>
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</td>
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<td>为 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 特化原子运算<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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