256 lines
21 KiB
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256 lines
21 KiB
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<title>std::atomic_fetch_add, std::atomic_fetch_add_explicit</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>atomic_fetch_add, <span style="font-size:0.7em; line-height:130%">std::</span>atomic_fetch_add_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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<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_fetch_add<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">difference_type</span> arg <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_fetch_add<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">difference_type</span> arg <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></div>
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<tbody></tbody>
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<td rowspan="142">(2)</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_fetch_add_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">difference_type</span> arg,<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_fetch_add_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">difference_type</span> arg,<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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<p>进行原子加法。</p>
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<p>原子地加 <code>arg</code> 到 <code>obj</code> 所指向的值,并返回 <code>obj</code> 先前保有的值。如同执行下列内容一般进行运算:</p>
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<div class="t-li2"><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>fetch_add<span class="br0">(</span>arg<span class="br0">)</span></span></span></div>
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<div class="t-li2"><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>fetch_add<span class="br0">(</span>arg, 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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<tr class="t-par">
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<td>arg</td>
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<td>-</td>
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<td>要加到存储于原子对象的值的值</td>
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</tr>
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<tr class="t-par">
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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> 的<a href="cpp-atomic-memory_order.html#.E4.BF.AE.E6.94.B9.E9.A1.BA.E5.BA.8F">修改顺序</a>中,立即前趋此函数效应的值。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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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="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span>
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T atomic_fetch_add<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, <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">difference_type</span> arg <span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> obj<span class="sy2">-</span><span class="sy1">></span>fetch_add<span class="br0">(</span>arg<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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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>单写者/多读者锁能用 fetch_add 制作。注意此过分简化的实现不是免锁的。</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 <string></span>
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<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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<span class="co2">#include <chrono></span>
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<span class="co1">// cnt 的含义:</span>
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<span class="co1">// 10 :无活跃读者或写者</span>
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<span class="co1">// 1...9 :有 9...1 个读者活跃。写者被阻塞</span>
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<span class="co1">// 0 : fetch_sub 和 fetch_add 间的临时值在读者锁中</span>
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<span class="co1">// -1 :有活跃写者。读者被阻塞。</span>
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<span class="kw4">const</span> <span class="kw4">int</span> N <span class="sy1">=</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="co1">// 允许九个共时读者</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">int</span><span class="sy1">></span> cnt <span class="sy1">=</span> <a href="cpp-atomic-ATOMIC_VAR_INIT.html"><span class="kw2088">ATOMIC_VAR_INIT</span></a><span class="br0">(</span>N<span class="br0">)</span><span class="sy4">;</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><span class="kw4">int</span><span class="sy1">></span> data<span class="sy4">;</span>
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<span class="kw4">void</span> reader<span class="br0">(</span><span class="kw4">int</span> id<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="sy4">;;</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 锁定</span>
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<span class="kw1">while</span><span class="br0">(</span><a href="cpp-atomic-atomic_fetch_sub.html"><span class="kw2078">std::<span class="me2">atomic_fetch_sub</span></span></a><span class="br0">(</span><span class="sy3">&</span>cnt, <span class="nu0">1</span><span class="br0">)</span> <span class="sy1"><=</span> <span class="nu0">0</span><span class="br0">)</span>
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std<span class="sy4">::</span><span class="me2">atomic_fetch_add</span><span class="br0">(</span><span class="sy3">&</span>cnt, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 读取</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>data.<span class="me1">empty</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</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="br0">(</span> <span class="st0">"reader "</span> <span class="sy2">+</span> <a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span>id<span class="br0">)</span>
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<span class="sy2">+</span> <span class="st0">" sees "</span> <span class="sy2">+</span> <a href="cpp-string-basic_string-to_string.html"><span class="kw1265">std::<span class="me2">to_string</span></span></a><span class="br0">(</span><span class="sy2">*</span>data.<span class="me1">rbegin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">+</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>data.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">100</span><span class="br0">)</span>
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<span class="kw1">break</span><span class="sy4">;</span>
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<span class="co1">// 解锁</span>
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std<span class="sy4">::</span><span class="me2">atomic_fetch_add</span><span class="br0">(</span><span class="sy3">&</span>cnt, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 暂停</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2147">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span><a href="cpp-chrono-duration.html"><span class="kw996">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="br0">(</span><span class="nu0">1</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>
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<span class="kw4">void</span> writer<span class="br0">(</span><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> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">100</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 锁定</span>
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<span class="kw1">while</span><span class="br0">(</span><a href="cpp-atomic-atomic_fetch_sub.html"><span class="kw2078">std::<span class="me2">atomic_fetch_sub</span></span></a><span class="br0">(</span><span class="sy3">&</span>cnt, N<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> N<span class="br0">)</span>
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std<span class="sy4">::</span><span class="me2">atomic_fetch_add</span><span class="br0">(</span><span class="sy3">&</span>cnt, N<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 写入</span>
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data.<span class="me1">push_back</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</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">"writer pushed back "</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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<span class="co1">// 解锁</span>
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std<span class="sy4">::</span><span class="me2">atomic_fetch_add</span><span class="br0">(</span><span class="sy3">&</span>cnt, N<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 暂停</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2147">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span><a href="cpp-chrono-duration.html"><span class="kw996">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="br0">(</span><span class="nu0">1</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>
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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> N<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>reader, n<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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v.<span class="me1">emplace_back</span><span class="br0">(</span>writer<span class="br0">)</span><span class="sy4">;</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">writer pushed back 0
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reader 8 sees 0
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reader 3 sees 0
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reader 1 sees 0
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<...>
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reader 2 sees 99
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reader 6 sees 99
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reader 1 sees 99</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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</tr>
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<tr>
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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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<tr class="t-dsc">
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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-fetch_add.html"><span class="t-lines"><span>fetch_add</span></span></a></div>
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</div>
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</td>
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<td>原子地将参数加到存储于原子对象的值,并返回先前保有的值<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>
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<tr class="t-dsc">
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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_fetch_sub.html"><span class="t-lines"><span>atomic_fetch_sub</span><span>atomic_fetch_sub_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></div>
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</div>
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</td>
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<td>从原子对象减去非原子值,并获得原子对象的先前值<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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</div>
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</div>
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</div>
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</body>
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