114 lines
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114 lines
8.0 KiB
HTML
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<title>operator==(std::move_only_function)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_move_only_function_operator skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">operator==<small>(std::move_only_function)</small></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-utility.html">utility</a>‎ | <a href="cpp-utility-functional.html">functional</a>‎ | <a href="cpp-utility-functional-move_only_function.html">move only function</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">friend</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span> <span class="kw4">const</span> move_only_function<span class="sy3">&</span> f, <a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>将 <code>std::move_only_function</code> 与 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 比较。空包装（即无可调用目标的包装）比较相等，非空包装比较不相等。</p>
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<p>此函数对通常<a href="cpp-language-unqualified_lookup.html">无限定</a>或<a href="cpp-language-qualified_lookup.html">有限定查找</a>不可见，而只能在 <code>std::move_only_function<FunctionType></code> 为参数的关联类时由<a href="cpp-language-adl.html">实参依赖查找</a>找到。</p>
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<p><code>!=</code> 运算符从 <code>operator==</code> <a href="cpp-language-operators.html#.E5.85.B3.E7.B3.BB.E8.BF.90.E7.AE.97.E7.AC.A6">合成</a>。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>f</td>
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<td>-</td>
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<td>要比较的 <code>std::move_only_function</code></td>
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</tr>
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</table>
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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><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span>f</span></span> 。</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 <functional></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">using</span> SomeVoidFunc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move_only_function</span><span class="sy1"><</span><span class="kw4">void</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">class</span> C <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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C<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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C<span class="br0">(</span>SomeVoidFunc func<span class="br0">)</span> <span class="sy4">:</span> void_func_<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>func<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw4">void</span> default_func<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>void_func_ <span class="sy1">==</span> nullptr<span class="br0">)</span> <span class="co1">// 与 nullptr 的特殊比较</span>
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default_func<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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void_func_<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">private</span><span class="sy4">:</span>
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SomeVoidFunc void_func_<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">void</span> user_func<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
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<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>i <span class="sy2">+</span> <span class="nu0">1</span><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>
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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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C c1<span class="sy4">;</span>
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C c2<span class="br0">(</span>user_func<span class="br0">)</span><span class="sy4">;</span>
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c1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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c2<span class="br0">(</span><span class="br0">)</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>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">7
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8</pre></div>
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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 class="t-dsc-member-div">
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<div><a href="cpp-utility-functional-function-operator_cmp.html"><span class="t-lines"><span>operator==</span><span>operator!=</span></span></a></div>
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<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></span></span></div>
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</div>
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</td>
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<td>比较 <span class="t-lc"><a href="cpp-utility-functional-function.html">std::function</a></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span><br />
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<span class="t-mark">(函数模板)</span></td>
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</div>
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<div class="visualClear"></div>
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