175 lines
22 KiB
HTML
175 lines
22 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::bind_front</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_bind_front 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::</span>bind_front</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 t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br />
|
|
<span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> bind_front<span class="br0">(</span> F<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>函数模板 <code>bind_front</code> 为 <code>f</code> 生成转发调用包装。调用此包装等价于绑定首 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>Args<span class="br0">)</span></span></span> 个参数到 <code>args</code> 再调用 <code>f</code> 。</p>
|
|
<p>换言之 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">bind_front</span><span class="br0">(</span>f, bound_args...<span class="br0">)</span><span class="br0">(</span>call_args...<span class="br0">)</span></span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>f, bound_args..., call_args....<span class="br0">)</span></span></span> 。</p>
|
|
<p>若下列任一为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则程序为非良构：</p>
|
|
<ul>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>F<span class="sy1">></span>, F<span class="sy1">></span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw539">std::<span class="me2">is_move_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>F<span class="sy1">>></span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>, Args<span class="sy1">></span> <span class="sy3">&&</span> ...<span class="br0">)</span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><a href="cpp-types-is_move_constructible.html"><span class="kw539">std::<span class="me2">is_move_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">>></span> <span class="sy3">&&</span> ...<span class="br0">)</span></span></span></li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>f</td>
|
|
<td>-</td>
|
|
<td>将绑定某些参数到的<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> 对象（函数对象、指向函数指针、到函数的引用、指向成员函数指针或指向数据成员指针）</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>args</td>
|
|
<td>-</td>
|
|
<td>参数列表，绑定首 <code>sizeof...(Args)</code> 个参数到 <code>f</code></td>
|
|
</tr>
|
|
<tr class="t-par-hitem">
|
|
<td colspan="3">类型要求</td>
|
|
</tr>
|
|
<tr class="t-par-req">
|
|
<td colspan="3">-
|
|
<div class="spacer"></div>
|
|
<code><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>F<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...</span></span> 中的每个类型</code> 必须满足<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 的要求。</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p>未指定的 <code>T</code> 类型函数对象，除了二次以相同参数调用 <code>std::bind_front</code> 返回的类型相同。</p>
|
|
<p>返回的对象（调用包装）有下列属性：</p>
|
|
<div class="t-member">
|
|
<h2><span class="mw-headline" id="std::bind_front_.E8.BF.94.E5.9B.9E.E7.B1.BB.E5.9E.8B"><span style="font-size:0.7em; line-height:130%">std::bind_front</span> <i>返回类型</i></span></h2>
|
|
<h4><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.AF.B9.E8.B1.A1">成员对象</span></h4>
|
|
<p>返回的对象表现为如同它保有从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>F<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span></span></span> 直接非列表初始化的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>F<span class="sy1">></span></span></span> 类型成员对象 <code>fd</code> ，以及用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span>...<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">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 class="br0">)</span></span></span> 构造的 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 对象 <code>tup</code> （这会解包任何 <span class="t-lc"><a href="cpp-utility-functional-reference_wrapper.html">std::reference_wrapper</a></span> ），除了返回对象的赋值行为未指定，且名称只为说明。</p>
|
|
<h4><span class="mw-headline" id=".E6.9E.84.E9.80.A0.E5.87.BD.E6.95.B0">构造函数</span></h4>
|
|
<p><code>std::bind_front</code> 的构造函数表现为其赋值/移动构造函数进行逐成员复制/移动。若其所有成员对象（指定如上）为<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> )则它为 <span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span> ，否则为<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 。</p>
|
|
<h4><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0_operator.28.29">成员函数 <code>operator()</code></span></h4>
|
|
<p>给定从先前调用 <code>bind_front(f, args...)</code> 获得的对象 <code>G</code> ，在函数调用表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">g<span class="br0">(</span>call_args...<span class="br0">)</span></span></span> 中调用指代 <code>G</code> 的泛左值 <code>g</code> 时，发生存储对象的调用，如同用</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><a href="cpp-utility-functional-invoke.html"><span class="kw1058">std::<span class="me2">invoke</span></span></a><span class="br0">(</span>g.<span class="me1">fd</span>, std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>Ns<span class="sy1">></span><span class="br0">(</span>g.<span class="me1">tup</span><span class="br0">)</span>..., call_args...<span class="br0">)</span></pre></div>
|
|
</div>
|
|
<p>，其中</p>
|
|
<ul>
|
|
<li><code>Ns</code> 是整数包 <code>0, 1, ..., (sizeof...(Args) - 1)</code></li>
|
|
<li><code>g</code> 若在调用表达式中是左值，则在 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> 表达式中是左值，否则是右值。从而 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>g<span class="br0">)</span><span class="br0">(</span>call_args...<span class="br0">)</span></span></span> 能移动绑定的参数到调用中，而 <span class="t-c"><span class="mw-geshi cpp source-cpp">g<span class="br0">(</span>call_args...<span class="br0">)</span></span></span> 会复制。</li>
|
|
</ul>
|
|
<p>若 <code>g</code> 拥有 volatile 限定类型则行为未定义。</p>
|
|
<p>若其所调用的 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> 表达式为 noexcept 则成员 <code>operator()</code> 为 noexcept （换言之，它保持底层调用运算符的异常说明）。</p>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
|
<p>仅若构造存储的函数对象或任何绑定的参数时抛出才会抛出。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>有意令此函数取代 <span class="t-lc"><a href="cpp-utility-functional-bind.html">std::bind</a></span> 。不同于 <code>std::bind</code> ，它不支持任意参数重排，而且不特别处理嵌套的 bind 表达式或 <span class="t-lc"><a href="cpp-utility-functional-reference_wrapper.html">std::reference_wrapper</a></span> 。另一方面，它注重调用包装对象的值类别，并传播底层调用运算符的异常规定。</p>
|
|
<p>如 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> 中描述，调用指向非静态成员函数指针或指向非静态数据成员指针时，首参数必须是到要访问其成员的对象的引用或指针（可以包含智能指针，如 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 与 <span class="t-lc"><a href="cpp-memory-unique_ptr.html">std::unique_ptr</a></span> ）。</p>
|
|
<p>复制或移动给 <code>std::bind_front</code> 的参数，而决不按引用传递，除非用 <span class="t-lc"><a href="cpp-utility-functional-ref.html">std::ref</a></span> 或 <span class="t-lc"><a href="cpp-utility-functional-ref.html">std::cref</a></span> 包装它们。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<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 <iostream></span>
|
|
|
|
<span class="kw4">int</span> minus<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span><span class="br0">{</span>
|
|
<span class="kw1">return</span> a<span class="sy2">-</span>b<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">struct</span> S
|
|
<span class="br0">{</span>
|
|
<span class="kw4">int</span> val<span class="sy4">;</span>
|
|
<span class="kw4">int</span> minus<span class="br0">(</span><span class="kw4">int</span> arg<span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span> <span class="br0">{</span> <span class="kw1">return</span> val <span class="sy2">-</span> arg<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="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">auto</span> fifty_minus <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bind_front</span><span class="br0">(</span>minus, <span class="nu0">50</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> fifty_minus <span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">auto</span> member_minus <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bind_front</span><span class="br0">(</span><span class="sy3">&</span>S<span class="sy4">::</span><span class="me2">minus</span>, S<span class="br0">{</span><span class="nu0">50</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> member_minus <span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 保持 noexcept 说明！</span>
|
|
static_assert<span class="br0">(</span><span class="sy3">!</span> <span class="kw1">noexcept</span><span class="br0">(</span>fifty_minus <span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
static_assert<span class="br0">(</span><span class="kw1">noexcept</span><span class="br0">(</span>member_minus <span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 绑定 lambda</span>
|
|
<span class="kw4">auto</span> plus <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> a<span class="sy2">+</span>b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">auto</span> forty_plus <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bind_front</span><span class="br0">(</span>plus, <span class="nu0">40</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> forty_plus<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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">47
|
|
47
|
|
47</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-mem_fn.html"><span class="t-lines"><span>mem_fn</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>从成员指针创建出函数对象<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|