103 lines
8.1 KiB
HTML
103 lines
8.1 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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<title>std::declval</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_declval 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>declval</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></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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<table class="t-dcl-begin">
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<tbody>
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<td>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></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">
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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">typename</span> <a href="cpp-types-add_reference.html"><span class="kw610">std::<span class="me2">add_rvalue_reference</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> declval<span class="br0">(</span><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-cxx11">(C++11 起)</span></td>
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<p>将任意类型 <code>T</code> 转换成引用类型，令在 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype</span></span> 表达式中不必经过构造函数就能使用成员函数。</p>
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<p>通常在模板中使用 <code>declval</code> ，模板接受的模板实参通常可能无构造函数，但有同一成员函数，均返回所需类型。</p>
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<p>注意， <code>declval</code> 只能用于<a href="cpp-language-expressions.html#.E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F">不求值语境</a>，且不要求有定义；求值包含此函数的表达式是错误。通常，若 <a href="cpp-language-definition.html#ODR_.E5.BC.8F.E4.BD.BF.E7.94.A8">odr 使用</a>此函数被则程序为病式。</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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<p>（无）</p>
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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>不能被调用，因此不会返回一个值。返回类型是 <code>T&&</code> ，除非 <code>T</code> 是（可有 cv 限定的） <code>void</code> ，此情况下返回类型是 <code>T</code> 。</p>
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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 <utility></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Default <span class="br0">{</span> <span class="kw4">int</span> foo<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> NonDefault
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<span class="br0">{</span>
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NonDefault<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
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<span class="kw4">int</span> foo<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</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>
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<span class="br0">{</span>
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decltype<span class="br0">(</span>Default<span class="br0">(</span><span class="br0">)</span>.<span class="me1">foo</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> n1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// n1 的类型是 int</span>
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<span class="co1">// decltype(NonDefault().foo()) n2 = n1; // 错误：无默认构造函数</span>
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decltype<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">declval</span><span class="sy1"><</span>NonDefault<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">foo</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> n2 <span class="sy1">=</span> n1<span class="sy4">;</span> <span class="co1">// n2 的类型是 int</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">"n1 = "</span> <span class="sy1"><<</span> n1 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"n2 = "</span> <span class="sy1"><<</span> n2 <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">n1 = 1
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n2 = 1</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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<tr class="t-dsc">
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<td><a href="cpp-language-decltype.html"><code>decltype</code> 说明符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>获得表达式或实体的类型</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-types-result_of.html"><span class="t-lines"><span>result_of</span><span>invoke_result</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 class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>推导以一组实参调用一个可调用对象的结果类型<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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</table>
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