146 lines
12 KiB
HTML
146 lines
12 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_functional_placeholders skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::placeholders::</span>_1, <span style="font-size:0.7em; line-height:130%">std::placeholders::</span>_2, ..., <span style="font-size:0.7em; line-height:130%">std::placeholders::</span>_N</h1>
|
|
<div id="bodyContent">
|
|
<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>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl">
|
|
<td class="t-dcl-nopad">
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*see below*/</span> _1<span class="sy4">;</span><br /></span>
|
|
<p><span class="coMULTI">/*see below*/</span> _2<span class="sy4">;</span><br />
|
|
.<br />
|
|
.<br /></p>
|
|
<span class="coMULTI">/*see below*/</span> _N<span class="sy4">;</span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td class="t-dcl-nopad"></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><span class="t-lc"><strong class="selflink">std::placeholders</strong></span> 命名空间含有占位对象 <code>[_1, . . . _N]</code> ，其中 <code>N</code> 是实现定义的最大数字。</p>
|
|
<p>于 <span class="t-lc"><a href="cpp-utility-functional-bind.html">std::bind</a></span> 表达式用作参数时，占位符对象被存储于生成的函数对象，而以未绑定参数调用函数对象时，每个占位符 <code>_N</code> 被对应的第 N 个未绑定参数替换。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx17">
|
|
<td>
|
|
<p>每个占位符如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span> <span class="coMULTI">/*unspecified*/</span> _1<span class="sy4">;</span></span></span> 声明。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
|
|
</tr>
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<p>鼓励实现如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> _1<span class="sy4">;</span></span></span> 声明占位符，尽管标准仍然允许以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span> <span class="coMULTI">/*unspecified*/</span> _1<span class="sy4">;</span></span></span> 声明它们。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>占位符对象的类型<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 且<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> ，其默认复制/移动构造函数不抛异常，且对于任何占位符 <code>_N</code> ，类型 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-is_placeholder.html"><span class="kw1054">std::<span class="me2">is_placeholder</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>_N<span class="br0">)</span><span class="sy1">></span></span></span> 有定义且从 <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> 导出。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<p>下列代码展示以占位符参数创建函数对象。</p>
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <functional></span>
|
|
<span class="co2">#include <string></span>
|
|
<span class="co2">#include <iostream></span>
|
|
|
|
<span class="kw4">void</span> goodbye<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy3">&</span> s<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Goodbye "</span> <span class="sy1"><<</span> s <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">class</span> Object <span class="br0">{</span>
|
|
<span class="kw1">public</span><span class="sy4">:</span>
|
|
<span class="kw4">void</span> hello<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy3">&</span> s<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<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 "</span> <span class="sy1"><<</span> s <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> argc, <span class="kw4">char</span><span class="sy2">*</span> argv<span class="br0">[</span><span class="br0">]</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <a href="cpp-utility-functional-function.html"><span class="kw1059">std::<span class="me2">function</span></span></a><span class="sy1"><</span><span class="kw4">void</span><span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy3">&</span><span class="br0">)</span><span class="sy1">></span> ExampleFunction<span class="sy4">;</span>
|
|
Object instance<span class="sy4">;</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> str<span class="br0">(</span><span class="st0">"World"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
ExampleFunction f <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><span class="sy3">&</span>Object<span class="sy4">::</span><span class="me2">hello</span>, <span class="sy3">&</span>instance,
|
|
std<span class="sy4">::</span><span class="me2">placeholders</span><span class="sy4">::</span>_1<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 等价于 instance.hello(str)</span>
|
|
f<span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
|
|
f <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><span class="sy3">&</span>goodbye, std<span class="sy4">::</span><span class="me2">placeholders</span><span class="sy4">::</span>_1<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 等价于 goodbye(str)</span>
|
|
f<span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">Hello World
|
|
Goodbye World</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-functional-bind.html"><span class="t-lines"><span>bind</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>绑定一或多个实参到函数对象<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-functional-is_placeholder.html"><span class="t-lines"><span>is_placeholder</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>表明一个对象是标准占位符，或者可以用作标准占位符<br />
|
|
<span class="t-mark">(类模板)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-tuple-ignore.html"><span class="t-lines"><span>ignore</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>用 <a href="cpp-utility-tuple-tie.html"><tt>tie</tt></a> 解包 <code>tuple</code> 时用来跳过元素的占位符<br />
|
|
<span class="t-mark">(常量)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|