223 lines
17 KiB
HTML
223 lines
17 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::mem_fn</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_mem_fn skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>mem_fn</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></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx11 t-until-cxx20">
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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> M, <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="coMULTI">/*unspecified*/</span> mem_fn<span class="br0">(</span>M T<span class="sy4">::</span><span class="sy2">*</span> pm<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-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</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> M, <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> mem_fn<span class="br0">(</span>M T<span class="sy4">::</span><span class="sy2">*</span> pm<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>函数模板 <code>std::mem_fn</code> 生成指向成员指针的包装对象，它可以存储、复制及调用指向成员指针。到对象的引用和指针（含智能指针）可在调用 <code>std::mem_fn</code> 时使用。</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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<td>pm</td>
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<td>-</td>
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<td>指向被包装成员的指针</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><code>std::mem_fn</code> 返回未指定类型的调用包装，该类型拥有下列成员：</p>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::mem_fn_.E8.BF.94.E5.9B.9E.E7.B1.BB.E5.9E.8B"><span style="font-size:0.7em; line-height:130%">std::mem_fn</span> <i>返回类型</i></span></h2>
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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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<table class="t-dsc-begin">
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<td>类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>result_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pm</span></span> 为指向成员函数的指针，则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">pm</span></span> 的返回类型，对指向成员对象的指针不定义</td>
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<tr class="t-dsc">
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<td><code>argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pm</span></span> 是指向无参数成员函数的指针则为 <code>T*</code> ，可有 cv 限定</td>
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<tr class="t-dsc">
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<td><code>first_argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pm</span></span> 是指向接收一个参数的成员函数的指针则为 <code>T*</code></td>
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<td><code>second_argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">pm</span></span> 是指向接收一个 <code>T1</code> 类型参数的成员函数的指针则为 <code>T1</code></td>
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</table>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</table>
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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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<table class="t-dcl-begin">
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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>... <span class="me1">Args</span><span class="sy1">></span><br /></span>
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<p><span class="coMULTI">/* see below */</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span> <span class="coMULTI">/* cvref-qualifiers */</span><br /></p>
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<span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</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-until-cxx20">(C++20 前)</span></td>
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<tr class="t-dcl t-since-cxx20">
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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>... <span class="me1">Args</span><span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="coMULTI">/* see below */</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span> <span class="coMULTI">/* cvref-qualifiers */</span><br /></p>
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<span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>令 <code>fn</code> 为以指向成员的指针 <code>pm</code> 调用<code>std::mem_fn</code> 所返回的调用包装器。则表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">fn<span class="br0">(</span>t, a2, ..., aN<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">INVOKE<span class="br0">(</span>pm, t, a2, ..., aN<span class="br0">)</span></span></span> ，其中 <i>INVOKE</i> 是定义于<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> 的操作。</p>
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<p>从而 <code>operator()</code> 的返回类型是<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-result_of.html"><span class="kw639">std::<span class="me2">result_of</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>pm<span class="br0">)</span><span class="br0">(</span>Args<span class="sy3">&&</span>...<span class="br0">)</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span></span></span><span class="t-rev-inl t-since-cxx17"><span>，或等价地为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invoke_result_t</span><span class="sy1"><</span>decltype<span class="br0">(</span>pm<span class="br0">)</span>, Args<span class="sy3">&&</span>...<span class="sy1">></span></span></span> ，而 noexcept 说明符中的值等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">is_nothrow_invocable_v</span><span class="sy1"><</span>decltype<span class="br0">(</span>pm<span class="br0">)</span>, Args<span class="sy3">&&</span>...<span class="sy1">></span><span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。</p>
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<p>完美转发 <code>args</code> 中的每个参数，如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1116">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 。</p>
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</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>用 <code>mem_fn</code> 存储执行成员函数和成员对象：</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 <functional></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
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<span class="kw4">void</span> display_greeting<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Hello, world.<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">void</span> display_number<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"number: "</span> <span class="sy1"><<</span> i <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> data <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
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<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> <span class="br0">{</span>
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Foo f<span class="sy4">;</span>
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<span class="kw4">auto</span> greet <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">mem_fn</span><span class="br0">(</span><span class="sy3">&</span>Foo<span class="sy4">::</span><span class="me2">display_greeting</span><span class="br0">)</span><span class="sy4">;</span>
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greet<span class="br0">(</span><span class="sy3">&</span>f<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> print_num <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">mem_fn</span><span class="br0">(</span><span class="sy3">&</span>Foo<span class="sy4">::</span><span class="me2">display_number</span><span class="br0">)</span><span class="sy4">;</span>
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print_num<span class="br0">(</span><span class="sy3">&</span>f, <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> access_data <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">mem_fn</span><span class="br0">(</span><span class="sy3">&</span>Foo<span class="sy4">::</span><span class="me2">data</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"data: "</span> <span class="sy1"><<</span> access_data<span class="br0">(</span><span class="sy3">&</span>f<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>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">Hello, world.
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number: 42
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data: 7</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/issue2048">LWG 2048</a></td>
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<td>C++11</td>
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<td>提供了不需要的重载</td>
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<td>已移除</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/issue2489">LWG 2489</a></td>
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<td>C++11</td>
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<td>未要求 noexcept</td>
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<td>已要求</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-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-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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</body>
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</html>
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