194 lines
13 KiB
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194 lines
13 KiB
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<title>std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_atomic_atomic_is_lock_free skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span class="t-dsc-small">std::</span>atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE</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 />
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<span class="kw4">bool</span> atomic_is_lock_free<span class="br0">(</span> <span class="kw4">const</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 <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="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">bool</span> atomic_is_lock_free<span class="br0">(</span> <span class="kw4">const</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="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="co2">#define ATOMIC_BOOL_LOCK_FREE /* unspecified */</span><br /></span>
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<p><span class="co2">#define ATOMIC_CHAR_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_CHAR32_T_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_WCHAR_T_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_SHORT_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_INT_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_LONG_LOCK_FREE /* unspecified */</span><br />
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<span class="co2">#define ATOMIC_LLONG_LOCK_FREE /* unspecified */</span><br /></p>
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<span class="co2">#define ATOMIC_POINTER_LOCK_FREE /* unspecified */</span></div>
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</td>
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<td>(2)</td>
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<td><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="co2">#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified */</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 如同通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">obj<span class="sy2">-</span><span class="sy1">></span>is_lock_free<span class="br0">(</span><span class="br0">)</span></span></span> ，确定 <code>obj</code> 所指向的原对象是否免锁地实现。任何给定的程序执行中，免锁查询的结果对于所有同类型的原子对象相同。</div>
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<div class="t-li1"><span class="t-li">2)</span> 展开成整数常量表达式，其值
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<ul>
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<li>对于决不免锁的内建原子类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> ；</li>
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<li>对于<i>有时</i>免锁的内建原子类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> ；</li>
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<li>对于始终免锁的内建原子类型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 。</li>
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</ul>
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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>指向要检验的原子对象的指针</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>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>obj</span></span> 为免锁原子对象则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><span class="t-lc"><a href="cpp-atomic-atomic_flag.html">std::atomic_flag</a></span> 以外的所有原子类型可用互斥或其他锁定操作实现，而不一定用免锁的原子 CPU 指令。亦允许原子类型<i>有时</i>免锁，例如若给定架构上仅对齐的内存访问是天然原子的，则同类型的错误对齐对象必须用锁。</p>
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<p>C++ 标准推荐（但不要求）免锁操作亦为免地址，即适用于使用共享内存的进程间交流。</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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<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 <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="co2">#include <atomic></span>
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<span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">100</span><span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw4">int</span> x, y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</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-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span>A<span class="sy1">></span> a<span class="sy4">;</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>B<span class="sy1">></span> b<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> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a>
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<span class="sy1"><<</span> <span class="st0">"std::atomic<A> is lock free? "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">atomic_is_lock_free</span><span class="br0">(</span><span class="sy3">&</span>a<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"std::atomic<B> is lock free? "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">atomic_is_lock_free</span><span class="br0">(</span><span class="sy3">&</span>b<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">std::atomic<A> is lock free? false
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std::atomic<B> is lock free? true</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://cplusplus.github.io/LWG/issue3249">LWG 3249</a></td>
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<td>C++11</td>
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<td><code>atomic_is_lock_free</code> 通过指针说明，这有歧义并可能接受非法指针值</td>
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<td>通过原子对象说明</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-is_lock_free.html"><span class="t-lines"><span>is_lock_free</span></span></a></div>
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</div>
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</td>
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<td>检查原子对象是否免锁<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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<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_is_lock_free<span class="t-dsc-small">(std::shared_ptr)</span></span></span></a></div>
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</div>
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</td>
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<td>对 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 特化的原子操作<br />
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<span class="t-mark">(函数模板)</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_flag.html"><span class="t-lines"><span>atomic_flag</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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</div>
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</td>
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<td>免锁的布尔原子类型<br />
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<span class="t-mark">(类)</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-is_always_lock_free.html"><span class="t-lines"><span>is_always_lock_free</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span> <span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>指示该类型是否始终免锁<br />
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<span class="t-mark">(<code>std::atomic<T></code> 的公开静态成员常量)</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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