186 lines
12 KiB
HTML
186 lines
12 KiB
HTML
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<title>std::is_placeholder</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_is_placeholder 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>is_placeholder</h1>
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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>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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<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="kw1">struct</span> is_placeholder<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>若 <code>T</code> 是标准占位符（ _1 、 _2 、 _3 ……）的类型，则此模板分别派生自 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>,<span class="nu0">1</span><span class="sy1">></span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>,<span class="nu0">2</span><span class="sy1">></span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>,<span class="nu0">3</span><span class="sy1">></span></span></span> 。</p>
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<p>若 <code>T</code> 不是标准占位符类型，则此模板派生自 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>,<span class="nu0">0</span><span class="sy1">></span></span></span> 。</p>
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<p>可以为任何用户定义 <code>T</code> 类型特化模板：特化必须满足<a href="cpp-named_req-UnaryTypeTrait.html"><span style="font-style:italic">一元类型特征</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(UnaryTypeTrait)</span></a> ，拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, N<span class="sy1">></span></span></span> 而 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">></span> <span class="nu0">0</span></span></span> 指示 <code>T</code> 应被处理成第 N 个占位符类型的<i>基特征</i>。</p>
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<p><span class="t-lc"><a href="cpp-utility-functional-bind.html">std::bind</a></span> 用 <code>std::is_placeholder</code> 检测未绑定参数的占位符。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.8F.98.E9.87.8F.E6.A8.A1.E6.9D.BF">辅助变量模板</span></h3>
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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="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">int</span> is_placeholder_v <span class="sy1">=</span> is_placeholder<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</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-cxx17">(C++17 起)</span></td>
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<div class="t-inherited">
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<h2><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::integral_constant">继承自 <a href="cpp-types-integral_constant.html"><span style="font-size:0.7em; line-height:130%">std::</span>integral_constant</a></span></h2>
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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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<table class="t-dsc-begin">
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<div><span class="t-lines"><span>value</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>占位符值，或对于非占位符类型为 0<br />
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<span class="t-mark">(公开静态成员常量)</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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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>operator int</span></span></div>
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<td>转换对象为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> ，返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>operator()</span></span></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div>
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<td>返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</table>
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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>value_type</code></td>
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<td><code>int</code></td>
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<tr class="t-dsc">
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<td><code>type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, value<span class="sy1">></span></span></span></td>
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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 <type_traits></span>
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<span class="co2">#include <functional></span>
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<span class="kw1">struct</span> My_2 <span class="br0">{</span>
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<span class="br0">}</span> my_2<span class="sy4">;</span>
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<span class="kw1">namespace</span> std <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw1">struct</span> is_placeholder<span class="sy1"><</span>My_2<span class="sy1">></span> <span class="sy4">:</span> <span class="kw1">public</span> integral_constant<span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">2</span><span class="sy1">></span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> n1, <span class="kw4">int</span> n2<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> n1<span class="sy2">+</span>n2<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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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Standard placeholder _5 is for the argument number "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">is_placeholder</span><span class="sy1"><</span>decltype<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">placeholders</span><span class="sy4">::</span>_5<span class="br0">)</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>
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<span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">auto</span> b <span class="sy1">=</span> <a href="cpp-utility-functional-bind.html"><span class="kw1052">std::<span class="me2">bind</span></span></a><span class="br0">(</span>f, my_2, <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Adding 2 to 11 selected with a custom placeholder gives "</span>
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<span class="sy1"><<</span> b<span class="br0">(</span><span class="nu0">10</span>, <span class="nu0">11</span><span class="br0">)</span> <span class="co1">// 忽略首参数，即 10</span>
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<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">Standard placeholder _5 is for the argument number 5
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Adding 2 to 11 selected with a custom placeholder gives 13</pre></div>
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</div>
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</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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<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-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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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-functional-placeholders.html"><span class="t-lines"><span>_1, _2, _3, _4, ...</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>用作 <code>std::bind</code> 表达式中的未绑定实参的占位符<br />
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<span class="t-mark">(常量)</span></td>
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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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