387 lines
34 KiB
HTML
387 lines
34 KiB
HTML
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<title>std::atomic(std::shared_ptr)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_shared_ptr_atomic2 skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>atomic<small>(std::shared_ptr)</small></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a>‎ | <a href="cpp-memory-shared_ptr.html">shared ptr</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></a></code></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> <span class="kw1">struct</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 对 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 的部分模板特化允许用户原子地操纵 shared_ptr 。</p>
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<p>若多个执行线程同时访问同一 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 对象而不同步，且任何这些访问使用了 <span class="t-lc">shared_ptr</span> 的非 const 成员函数，则将出现数据竞争，除非通过 <span class="t-lc">std::atomic<std::shared_ptr>></span> 的实例进行所有访问（或通过从 C++20 起弃用的<a href="cpp-memory-shared_ptr-atomic.html">孤立函数</a>对 std::shared_ptr 进行原子访问）。</p>
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<p>关联 use_count 的自增保证是原子操作的一部分。关联 use_count 的自减后序于原子操作，但不要求是原子操作的一部分，除非在覆写失败的 CAS 中的 <code>expected</code> 时， use_count 更改。任何关联删除和解分配后序于原子更新步骤，且不是原子操作的一部分。</p>
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<p>注意 shared_ptr 的控制块是线程安全的：多个线程能同时用可变操作，例如 operator= 或 reset ，访问不同的非原子 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 对象，即使这些实例互为副本，于内部共享同一控制块。</p>
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<p>类型 T 可为不完整类型。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<td>成员类型</td>
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<td><code>value_type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span></td>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<p>此特化提供所有非特化的 <span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 的成员函数，不提供附加成员函数。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::atomic"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>atomic</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> atomic<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp">atomic<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp">atomic<span class="br0">(</span><span class="kw4">const</span> atomic<span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<div class="t-li1"><span class="t-li">1)</span> 初始化底层 <code>shared_ptr<T></code> 为默认构造值</div>
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<div class="t-li1"><span class="t-li">2)</span> 初始化底层 <code>shared_ptr<T></code> 为 <code>desired</code> 的副本。同任何 <span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 类型，初始化不是原子操作。</div>
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<div class="t-li1"><span class="t-li">3)</span> 原子类型不可复制/移动构造。</div>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::operator.3D"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>operator=</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> atomic<span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> operator<span class="sy1">=</span><span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div class="t-li1"><span class="t-li">1)</span> 原子类型不可复制/移动构造</div>
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<div class="t-li1"><span class="t-li">2)</span> 值赋值，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">store<span class="br0">(</span>desired<span class="br0">)</span></span></span></div>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::is_lock_free"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>is_lock_free</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> is_lock_free<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>若此类型所有对象上的原子操作为免锁则返回 true ，否则返回 false 。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::store"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>store</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> store<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp">p.<span class="me1">swap</span><span class="br0">(</span>desired<span class="br0">)</span></span></span> ，原子地以 <code>desired</code> 的值替换 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 的值，其中 <span class="t-c"><span class="mw-geshi cpp source-cpp">p</span></span> 是底层的 <span class="t-lc">std::shared_ptr<T></span> 。按照 <code>order</code> 排序内存。若 <code>order</code> 非 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_consume</a></span> 、 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acquire</a></span> 或 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> 则行为未定义。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::load"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>load</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> load<span class="br0">(</span><a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>原子地返回底层共享指针的副本。按照 <code>order</code> 排序内存。若 <code>order</code> 不是 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 或 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> 则行为未定义。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::operator_std::shared_ptr.3CT.3E"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>operator std::shared_ptr<T></span></h2>
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<div><span class="mw-geshi cpp source-cpp">operator <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> load<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> 。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::exchange"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>exchange</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> exchange<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp">p.<span class="me1">swap</span><span class="br0">(</span>desired<span class="br0">)</span></span></span> ，原子地以 <code>desired</code> 替换底层 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> ，其中 <span class="t-c"><span class="mw-geshi cpp source-cpp">p</span></span> 为底层 <span class="t-lc">std::shared_ptr<T></span> ，并返回该 <span class="t-c"><span class="mw-geshi cpp source-cpp">p</span></span> 的值的副本。按照 <code>order</code> 排序内存。这是原子读修改写操作。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::compare_exchange_weak.2C_compare_exchange_strong"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>compare_exchange_weak, compare_exchange_strong</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> expected, <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success, <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> expected, <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> success, <a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> failure<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_strong<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> expected, <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> compare_exchange_weak<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> expected, <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> desired,<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<div class="t-li1"><span class="t-li">1)</span> 若底层 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 存储同 <code>expected</code> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span></span></span> 并与之共享所有权，或若底层指针和 <code>expected</code> 均为空，则从 <code>desired</code> 赋值给底层 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 并返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，按照 <code>success</code> 排序内存；否则从底层 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 赋值给 <code>expected</code> 并返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，按照 <code>failure</code> 排序内存。若 <code>failure</code> 为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 或 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> 则行为未定义。成功时，操作为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上的读修改写操作，而且在原子更新后不访问 <code>expected</code> 。失败时，操作为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上的原子加载操作，并更新 <code>expected</code> 为从原子对象读取的既存值。这个对 <code>expected</code> 的 use_count 的更新是原子操作的一部分，尽管不要求写入自身（以及任何后继的解分配/析构）是其一部分。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，但允许虚假地失败。</div>
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<div class="t-li1"><span class="t-li">3)</span> 等价于： <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> compare_exchange_strong<span class="br0">(</span>expected, desired, order, fail_order<span class="br0">)</span><span class="sy4">;</span></span></span> ，其中 <code>fail_order</code> 与 <code>order</code> 相同，除了 <span class="t-lc">std:memory_order_acq_rel</span> 被替换为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acquire</a></span> ，而 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 被替换为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_relaxed</a></span> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 等价于： <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> compare_exchange_weak<span class="br0">(</span>expected, desired, order, fail_order<span class="br0">)</span><span class="sy4">;</span></span></span> ，其中 <code>fail_order</code> 与 <code>order</code> 相同，除了 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> 被替换为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acquire</a></span> 而 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 被替换为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_relaxed</a></span> 。</div>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::wait"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>wait</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> wait<span class="br0">(</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> old<br />
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<a href="cpp-atomic-memory_order.html"><span class="kw2092">std::<span class="me2">memory_order</span></span></a> order <span class="sy1">=</span> <a href="cpp-atomic-memory_order.html"><span class="kw2098">std::<span class="me2">memory_order_seq_cst</span></span></a><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>进行原子等待操作。</p>
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<p>比较 <span class="t-c"><span class="mw-geshi cpp source-cpp">load<span class="br0">(</span>order<span class="br0">)</span></span></span> 与 <code>old</code> ，而若它们等价则阻塞直至 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 被 <code>notify_one()</code> 或 <code>notify_all()</code> 提醒。重复此操作直至 <span class="t-c"><span class="mw-geshi cpp source-cpp">load<span class="br0">(</span>order<span class="br0">)</span></span></span> 更改。此函数保证仅若值更改才返回，即使底层实现虚假地除阻。</p>
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<p>按照 <code>order</code> 排序内存。若 <code>order</code> 为 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_release</a></span> 或 <span class="t-lc"><a href="cpp-atomic-memory_order.html">std::memory_order_acq_rel</a></span> 则行为未定义。</p>
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<p>注解：若二个 <code>shared_ptr</code> 存储同一指针，且要么共享所有权要么都为空，则它们等价。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::notify_one"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>notify_one</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> notify_one<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>进行原子提醒操作。</p>
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<p>若有一个线程在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上的原子等待操作（即 <code>wait()</code> ）中阻塞，则除阻至少一个这种线程，否则不做任何事。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::notify_all"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>notify_all</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> notify_all<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>进行原子提醒操作。</p>
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<p>除阻所有在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 上的原子等待操作（即 <code>wait()</code> ）中阻塞的线程，若存在；否则不做任何事。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.B8.B8.E9.87.8F">成员常量</span></h3>
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<p>此特化亦提供标准 <span class="t-lc"><a href="cpp-atomic-atomic.html">std::atomic</a></span> 仅有的成员常量 <code>is_always_lock_free</code> 。</p>
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<h2><span class="mw-headline" id="atomic.3Cshared_ptr.3CT.3E.3E::is_always_lock_free"><span style="font-size:0.7em; line-height:130%">atomic<shared_ptr<T>>::</span>is_always_lock_free</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> is_always_lock_free <span class="sy1">=</span> <span class="coMULTI">/*implementation-defined*/</span><span class="sy4">;</span></span></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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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div><a href="cpp-atomic-atomic.html"><span class="t-lines"><span>atomic</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></div>
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<td>atomic 类模板及其针对布尔、整型和指针类型的特化<br />
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<span class="t-mark">(类模板)</span></td>
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