229 lines
20 KiB
HTML
229 lines
20 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::forward</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_forward 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>forward</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</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-utility.html"><utility></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(1)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11 t-until-cxx14">
|
|
<td>
|
|
<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 />
|
|
T<span class="sy3">&&</span> forward<span class="br0">(</span> <span class="kw1">typename</span> <a href="cpp-types-remove_reference.html"><span class="kw606">std::<span class="me2">remove_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy3">&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
|
|
<span class="t-mark-rev t-until-cxx14">(C++14 前)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx14">
|
|
<td>
|
|
<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 />
|
|
<span class="kw4">constexpr</span> T<span class="sy3">&&</span> forward<span class="br0">(</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody></tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(2)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11 t-until-cxx14">
|
|
<td>
|
|
<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 />
|
|
T<span class="sy3">&&</span> forward<span class="br0">(</span> <span class="kw1">typename</span> <a href="cpp-types-remove_reference.html"><span class="kw606">std::<span class="me2">remove_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy3">&&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
|
|
<span class="t-mark-rev t-until-cxx14">(C++14 前)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx14">
|
|
<td>
|
|
<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 />
|
|
<span class="kw4">constexpr</span> T<span class="sy3">&&</span> forward<span class="br0">(</span> <a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&&</span> t <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<div class="t-li1"><span class="t-li">1)</span> 转发左值为左值或右值，依赖于 T</div>
|
|
<p>当 <code>t</code> 是<a href="cpp-language-reference.html#.E8.BD.AC.E5.8F.91.E5.BC.95.E7.94.A8">转发引用</a>（作为到无 cv 限定函数模板形参的右值引用的函数实参），此重载将参数以在传递给调用方函数时的<a href="cpp-language-value_category.html">值类别</a>转发给另一个函数。</p>
|
|
<p>例如，若用于如下的包装器，则模板表现为下方所描述：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
|
<span class="kw4">void</span> wrapper<span class="br0">(</span>T<span class="sy3">&&</span> arg<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// arg 始终是左值</span>
|
|
foo<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>arg<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 转发为左值或右值，依赖于 T</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<ul>
|
|
<li>若对 <code>wrapper()</code> 的调用传递右值 <code>std::string</code> ，则推导 <code>T</code> 为 <code>std::string</code> （非 <code>std::string&</code> 或 <code>std::string&&</code> ，且 <code>std::forward</code> 确保将右值引用传递给 <code>foo</code> 。</li>
|
|
<li>若对 <code>wrapper()</code> 的调用传递 const 左值 <code>std::string</code> ，则推导 <code>T</code> 为 <code>const std::string&</code> ，且 <code>std::forward</code> 确保将 const 左值引用传递给 <code>foo</code> 。</li>
|
|
<li>若对 <code>wrapper()</code> 的调用传递非 const 左值 <code>std::string</code> ，则推导 <code>T</code> 为 <code>std::string&</code> ，且 <code>std::forward</code> 确保将非 const 左值引用传递给 <code>foo</code> 。</li>
|
|
</ul>
|
|
<div class="t-li1"><span class="t-li">2)</span> 转发右值为右值并禁止右值的转发为左值</div>
|
|
<p>此重载令转发表达式（如函数调用）的结果可行，结果可以是右值或左值，同转发引用参数的原始值类别。</p>
|
|
<p>例如，若包装器不仅转发其参数，还在参数上调用成员函数，并转发其结果：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co1">// 转换包装器 </span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
|
<span class="kw4">void</span> wrapper<span class="br0">(</span>T<span class="sy3">&&</span> arg<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
foo<span class="br0">(</span>forward<span class="sy1"><</span>decltype<span class="br0">(</span>forward<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>arg<span class="br0">)</span>.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">></span><span class="br0">(</span>forward<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>arg<span class="br0">)</span>.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>其中 arg 的类型可以是</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> Arg
|
|
<span class="br0">{</span>
|
|
<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> get<span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span> <span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 此重载的调用为右值</span>
|
|
<span class="kw4">int</span><span class="sy3">&</span> get<span class="br0">(</span><span class="br0">)</span> <span class="sy3">&</span> <span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 此重载的调用为左值</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<p>试图转发右值为左值，例如通过以左值引用类型 T 实例化形式 <span class="t-v">(2)</span> ，会产生编译时错误。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
|
|
<p>转发引用背后的特殊规则（ <code>T&&</code> 用作函数参数）见<a href="cpp-language-template_argument_deduction.html">模板实参推导</a>，其他细节见<a href="cpp-language-reference.html#.E8.BD.AC.E5.8F.91.E5.BC.95.E7.94.A8">转发引用</a>。</p>
|
|
<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>t</td>
|
|
<td>-</td>
|
|
<td>要转发的对象</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span>T<span class="sy3">&&</span><span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span></span></span></p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<p>此示例演示参数到类 <code>T</code> 构造函数实参的完美转发。并展示参数包的完美转发。</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 <iostream></span>
|
|
<span class="co2">#include <memory></span>
|
|
<span class="co2">#include <utility></span>
|
|
|
|
<span class="kw1">struct</span> A <span class="br0">{</span>
|
|
A<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&&</span> n<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">"rvalue overload, n="</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
|
|
A<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&</span> n<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">"lvalue overload, n="</span> <span class="sy1"><<</span> n <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="kw1">class</span> B <span class="br0">{</span>
|
|
<span class="kw1">public</span><span class="sy4">:</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T1, <span class="kw1">class</span> T2, <span class="kw1">class</span> T3<span class="sy1">></span>
|
|
B<span class="br0">(</span>T1<span class="sy3">&&</span> t1, T2<span class="sy3">&&</span> t2, T3<span class="sy3">&&</span> t3<span class="br0">)</span> <span class="sy4">:</span>
|
|
a1_<span class="br0">{</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>T1<span class="sy1">></span><span class="br0">(</span>t1<span class="br0">)</span><span class="br0">}</span>,
|
|
a2_<span class="br0">{</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>T2<span class="sy1">></span><span class="br0">(</span>t2<span class="br0">)</span><span class="br0">}</span>,
|
|
a3_<span class="br0">{</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>T3<span class="sy1">></span><span class="br0">(</span>t3<span class="br0">)</span><span class="br0">}</span>
|
|
<span class="br0">{</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">private</span><span class="sy4">:</span>
|
|
A a1_, a2_, a3_<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
|
|
<a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> make_unique1<span class="br0">(</span>U<span class="sy3">&&</span> u<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">return</span> <a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>new T<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">U</span><span class="sy1">></span>
|
|
<a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> make_unique2<span class="br0">(</span>U<span class="sy3">&&</span>... <span class="me1">u</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">return</span> <a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>new T<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">forward</span><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>u<span class="br0">)</span>...<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">auto</span> p1 <span class="sy1">=</span> make_unique1<span class="sy1"><</span>A<span class="sy1">></span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 右值</span>
|
|
<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
|
|
<span class="kw4">auto</span> p2 <span class="sy1">=</span> make_unique1<span class="sy1"><</span>A<span class="sy1">></span><span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 左值</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">"B<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="kw4">auto</span> t <span class="sy1">=</span> make_unique2<span class="sy1"><</span>B<span class="sy1">></span><span class="br0">(</span><span class="nu0">2</span>, i, <span class="nu0">3</span><span class="br0">)</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">rvalue overload, n=2
|
|
lvalue overload, n=1
|
|
B
|
|
rvalue overload, n=2
|
|
lvalue overload, n=1
|
|
rvalue overload, n=3</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
|
|
<p>常数</p>
|
|
<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-move.html"><span class="t-lines"><span>move</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>获得右值引用<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-move_if_noexcept.html"><span class="t-lines"><span>move_if_noexcept</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>若移动构造函数不抛出则获得右值引用<br />
|
|
<span class="t-mark">(函数模板)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|