234 lines
21 KiB
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234 lines
21 KiB
HTML
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<title>C++ 具名要求：可调用 (Callable)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-named_req.html">named req</a></span></div>
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<p><strong class="selflink"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></strong> 类型是可应用 INVOKE 操作（例如用于 <span class="t-lc"><a href="cpp-utility-functional-function.html">std::function</a></span>、<span class="t-lc"><a href="cpp-utility-functional-bind.html">std::bind</a></span> 和 <span class="t-lc"><a href="cpp-thread-thread-thread.html">std::thread::thread</a></span>）的类型。<span class="t-rev-inl t-since-cxx17"><span>可以用库函数 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> 显式进行此操作。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span></p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>若满足下列条件，则类型 <code>T</code> 满足<span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></p>
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<p>给定</p>
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<ul>
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<li><code>T</code> 类型的对象 <code>f</code></li>
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<li>适合的实参类型列表 <code>ArgTypes</code></li>
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<li>适合的返回类型 <code>R</code></li>
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</ul>
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<p>则下列表达式必须合法：</p>
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<table class="wikitable">
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<tr>
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<th>表达式</th>
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<th>要求</th>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="sy1"><</span>R<span class="sy1">></span><span class="br0">(</span>f, <a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>ArgTypes<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...<span class="br0">)</span></span></span></td>
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<td>该表达式在不求值语境中良构</td>
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</tr>
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</table>
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<p>其中 <i>INVOKE<R>(f, t1, t2, ..., tN)</i>，若 <code>R</code> 是可有 cv 限定的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>，则定义为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span><span class="br0">(</span>INVOKE<span class="br0">(</span>f, t1, t2, ..., tN<span class="br0">)</span><span class="br0">)</span></span></span>，否则为 <i>INVOKE(f, t1, t2, ..., tN)</i>，<a href="cpp-language-implicit_conversion.html">隐式转换</a>成 <code>R</code></p>
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<p>其中 <i>INVOKE(f, t1, t2, ..., tN)</i> 定义如下：</p>
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<ul>
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<li>若 <code>f</code> 是类 <code>T</code> 的<a href="cpp-language-pointer.html#.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0.E6.8C.87.E9.92.88">成员函数指针</a>：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_base_of.html"><span class="kw590">std::<span class="me2">is_base_of</span></span></a><span class="sy1"><</span>T, <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>t1<span class="br0">)</span><span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1, t2, ..., tN<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t1.<span class="sy2">*</span>f<span class="br0">)</span><span class="br0">(</span>t2, ..., tN<span class="br0">)</span></span></span></li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>decltype<span class="br0">(</span>t1<span class="br0">)</span><span class="sy1">></span></span></span> 为 <span class="t-lc"><a href="cpp-utility-functional-reference_wrapper.html">std::reference_wrapper</a></span> 的特化，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1, t2, ..., tN<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>t1.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span>.<span class="sy2">*</span>f<span class="br0">)</span><span class="br0">(</span>t2, ..., tN<span class="br0">)</span></span></span></li>
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<li>否则，若 <code>t1</code> 不满足前述条件，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1, t2, ..., tN<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">(</span><span class="sy2">*</span>t1<span class="br0">)</span>.<span class="sy2">*</span>f<span class="br0">)</span><span class="br0">(</span>t2, ..., tN<span class="br0">)</span></span></span>。</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>否则，若 N == 1 且 <code>f</code> 是类 <code>T</code> 的<a href="cpp-language-pointer.html#.E6.95.B0.E6.8D.AE.E6.88.90.E5.91.98.E6.8C.87.E9.92.88">数据成员指针</a>：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_base_of.html"><span class="kw590">std::<span class="me2">is_base_of</span></span></a><span class="sy1"><</span>T, <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>t1<span class="br0">)</span><span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">t1.<span class="sy2">*</span>f</span></span></li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>decltype<span class="br0">(</span>t1<span class="br0">)</span><span class="sy1">></span></span></span> 是 <span class="t-lc"><a href="cpp-utility-functional-reference_wrapper.html">std::reference_wrapper</a></span> 的特化，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">t1.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span>.<span class="sy2">*</span>f</span></span></li>
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<li>否则，若 <code>t1</code> 不满足前述条件，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="sy2">*</span>t1<span class="br0">)</span>.<span class="sy2">*</span>f</span></span></li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>否则，<span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>f, t1, t2, ..., tN<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">f<span class="br0">(</span>t1, t2, ..., tN<span class="br0">)</span></span></span>（即 <code>f</code> 是一个<a href="cpp-named_req-FunctionObject.html"><span style="font-style:italic">函数对象</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(FunctionObject)</span></a> ）</li>
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</ul>
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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>对于成员函数指针和数据成员指针，<code>t1</code> 可以是一个常规指针或一个重载了 <code>operator*</code> 的类的对象，例如 <span class="t-lc"><a href="cpp-memory-unique_ptr.html">std::unique_ptr</a></span> 或 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span>。</p>
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<p><a href="cpp-language-pointer.html#.E6.95.B0.E6.8D.AE.E6.88.90.E5.91.98.E6.8C.87.E9.92.88">数据成员指针</a>为<span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span> ，尽管并不发生函数调用。</p>
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<h3><span class="mw-headline" id=".E6.A0.87.E5.87.86.E5.BA.93">标准库</span></h3>
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<p>此外，下列标准库设施接受任何<span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span> 类型（不仅是<a href="cpp-named_req-FunctionObject.html"><span style="font-style:italic">函数对象</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(FunctionObject)</span></a> ）</p>
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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-utility-functional-function.html"><span class="t-lines"><span>function</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>包装具有指定函数调用签名的任意可复制构造类型的可调用对象<br />
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<span class="t-mark">(类模板)</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-utility-functional-move_only_function.html"><span class="t-lines"><span>move_only_function</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>包装具有指定函数调用签名的任意类型的可调用对象<br />
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<span class="t-mark">(类模板)</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-utility-functional-bind.html"><span class="t-lines"><span>bind</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>绑定一或多个实参到函数对象<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-utility-functional-bind_front.html"><span class="t-lines"><span>bind_front</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>按顺序绑定一定数量的参数到函数对象<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-utility-functional-reference_wrapper.html"><span class="t-lines"><span>reference_wrapper</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><a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 且<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 的引用包装器<br />
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<span class="t-mark">(类模板)</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-types-result_of.html"><span class="t-lines"><span>result_of</span><span>invoke_result</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 class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></span><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>推导以一组实参调用一个可调用对象的结果类型<br />
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<span class="t-mark">(类模板)</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-thread-thread.html"><span class="t-lines"><span>thread</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>管理单独的线程<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-thread-jthread.html"><span class="t-lines"><span>jthread</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>有自动合并和取消支持的 <span class="t-lc"><a href="cpp-thread-thread.html">std::thread</a></span><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-thread-call_once.html"><span class="t-lines"><span>call_once</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>仅调用函数一次，即使从多个线程调用<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-thread-async.html"><span class="t-lines"><span>async</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>异步运行一个函数（有可能在新线程中执行），并返回保有其结果的 <span class="t-lc"><a href="cpp-thread-future.html">std::future</a></span><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-thread-packaged_task.html"><span class="t-lines"><span>packaged_task</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>打包一个函数，存储其返回值以进行异步获取<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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</table>
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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/issue2219">LWG 2219</a></td>
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<td>C++11</td>
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<td>不可能以 <code>reference_wrapper</code> 为对象表达式 <code>INVOKE</code> 成员指针</td>
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<td>检测并处理 <code>reference_wrapper</code></td>
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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/issue2420">LWG 2420</a></td>
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<td>C++11</td>
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<td>当 <code>R</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 时，要求从结果到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的不可能的隐式转换</td>
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<td>当 <code>R</code> 为 cv <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 时结果可显式转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span></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-types-is_invocable.html"><span class="t-lines"><span>is_invocable</span><span>is_invocable_r</span><span>is_nothrow_invocable</span><span>is_nothrow_invocable_r</span></span></a></div>
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<div><span class="t-lines"><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>检查类型能否以给定的实参类型调用（如同以 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span>）<br />
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<span class="t-mark">(类模板)</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-concepts-invocable.html"><span class="t-lines"><span>invocable</span><span>regular_invocable</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>指定能以给定的一组实参类型调用的可调用类型<br />
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<span class="t-mark">(概念)</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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</html>
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