425 lines
42 KiB
HTML
425 lines
42 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>SFINAE</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_sfinae 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">SFINAE</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-language.html">language</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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<p>“替换失败不是错误” (Substitution Failure Is Not An Error)</p>
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<p><br />
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在函数模板的重载决议中会应用此规则：当模板形参在<a href="cpp-language-function_template.html#.E6.A8.A1.E6.9D.BF.E5.AE.9E.E5.8F.82.E6.9B.BF.E6.8D.A2">替换</a>成显式指定的类型或<a href="cpp-language-template_argument_deduction.html">推导出的类型</a>失败时，从<a href="cpp-language-overload_resolution.html">重载集</a>中丢弃这个特化，而非导致编译失败。</p>
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<p>此特性被用于模板元编程。</p>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>对函数模板形参进行两次替换（由模板实参所替代）：</p>
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<ul>
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<li>显式指定的模板实参在模板实参推导前替换</li>
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<li>推导出的实参和从默认项获得的实参在模板实参推导后替换</li>
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</ul>
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<p>替换发生于</p>
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<ul>
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<li>函数类型中使用的所有类型（包括返回类型和所有形参的类型）</li>
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<li>各个模板形参声明中使用的所有类型</li>
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</ul>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li>函数类型中使用的所有表达式</li>
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<li>各个模板形参声明中使用的所有表达式</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<td>
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<ul>
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<li><a href="cpp-language-explicit.html">explicit 说明符</a>中使用的所有表达式</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<p>以上类型或表达式在以用来替换的实参写出时谬构（并带有必要的诊断）的场合是<i>替换失败</i>。</p>
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<p>只有在函数类型或其模板形参类型<span class="t-rev-inl t-since-cxx20"><span>或其 <a href="cpp-language-explicit.html">explicit 说明符</a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>的<i>立即语境</i>中的类型与表达式中的失败是 SFINAE 错误。如果对替换后的类型/表达式的求值导致副作用，例如实例化某模板特化、生成某隐式定义的成员函数等，那么这些副作用中的错误被当做硬错误。<span class="t-rev-inl t-since-cxx20"><span><a href="cpp-language-lambda.html">lambda 表达式</a>不被当作是立即语境的一部分。</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span></p>
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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<tr>
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：含有此类影响的小示例</td>
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</tr>
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</table>
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<p>替换以词法序进行，并在遇到失败时终止。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>如果多个声明具有不同的词法顺序（例如，某个函数模板声明为具有尾随返回类型，它在某个形参之后替换，然后被重声明为具有常规返回类型，它则在该形参之前替换），而这会导致模板实例化以不同顺序出现或完全不出现，那么程序谬构；不要求诊断。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> A<span class="sy1">></span>
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<span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw1">using</span> type <span class="sy1">=</span> <span class="kw1">typename</span> A<span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span>
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<span class="kw1">class</span> T,
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<span class="kw1">class</span> U <span class="sy1">=</span> <span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">type</span>, <span class="co1">// 如果 T 没有成员 type 那么就是 SFINAE 失败</span>
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<span class="kw1">class</span> V <span class="sy1">=</span> <span class="kw1">typename</span> B<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy1">></span> <span class="co1">// 如果 T 没有成员 type 那么就是硬错误</span>
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<span class="co1">// （经由 CWG 1227 保证不出现，</span>
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<span class="co1">// 因为到 U 的默认模板实参中的替换会首先失败）</span>
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<span class="kw4">void</span> foo <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">type</span> h<span class="br0">(</span><span class="kw1">typename</span> B<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">auto</span> h<span class="br0">(</span><span class="kw1">typename</span> B<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span> <span class="co1">// 重声明</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">void</span> h<span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">using</span> R <span class="sy1">=</span> decltype<span class="br0">(</span>h<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 谬构，不要求诊断</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E7.B1.BB.E5.9E.8B_SFINAE">类型 SFINAE</span></h3>
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<p>下列类型的错误是 SFINAE 错误：</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<ul>
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<li>试图实例化含有多个不同长度的包的包展开</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<ul>
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<li>试图创建 void 的数组，引用的数组，函数的数组，负大小的数组，非整型大小的数组，或者零大小的数组：</li>
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</ul>
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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="kw1">template</span><span class="sy1"><</span><span class="kw4">int</span> I<span class="sy1">></span>
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<span class="kw4">void</span> div<span class="br0">(</span><span class="kw4">char</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span>I <span class="sy2">%</span> <span class="nu0">2</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 当 I 是偶数时选择这个重载</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw4">int</span> I<span class="sy1">></span>
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<span class="kw4">void</span> div<span class="br0">(</span><span class="kw4">char</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span>I <span class="sy2">%</span> <span class="nu0">2</span> <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// 当 I 是奇数时选择这个重载</span>
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<span class="br0">}</span></pre></div>
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</div>
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<ul>
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<li>试图在作用域解析运算符 <code><b>::</b></code> 左侧使用类和枚举以外的类型：</li>
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</ul>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">B</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">int</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> f<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 使用第二个重载</span></pre></div>
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</div>
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<ul>
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<li>试图使用类型的成员，其中</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>类型不包含指定成员</li>
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<li>在要求类型的地方，指定成员不是类型</li>
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<li>在要求模板的地方，指定成员不是模板</li>
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<li>在要求非类型的地方，指定成员不是非类型</li>
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</ul>
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</dd>
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</dl>
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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">templat <span class="sy1"><</span><span class="kw4">int</span> I<span class="sy1">></span>
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<span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">class</span><span class="sy1">></span>
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<span class="kw1">struct</span> Z <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">Y</span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">void</span> g<span class="br0">(</span>X<span class="sy1"><</span>T<span class="sy4">::</span><span class="me2">N</span><span class="sy1">></span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">void</span> h<span class="br0">(</span>Z<span class="sy1"><</span>T<span class="sy4">::</span><span class="kw1">template</span> TT<span class="sy1">></span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw4">int</span> Y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> C <span class="br0">{</span> <span class="kw1">typedef</span> <span class="kw4">int</span> N<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> D <span class="br0">{</span> <span class="kw1">typedef</span> <span class="kw4">int</span> TT<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> B1 <span class="br0">{</span> <span class="kw1">typedef</span> <span class="kw4">int</span> Y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> C1 <span class="br0">{</span> <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> N <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> D1
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<span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> TT <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</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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<span class="co1">// 下列各个情况推导失败：</span>
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f<span class="sy1"><</span>A<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不含成员 Y</span>
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f<span class="sy1"><</span>B<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// B 的 Y 成员不是类型</span>
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g<span class="sy1"><</span>C<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C 的 N 成员不是非类型</span>
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h<span class="sy1"><</span>D<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// D 的 TT 成员不是模板</span>
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<span class="co1">// 下列各个情况推导成功：</span>
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f<span class="sy1"><</span>B1<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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g<span class="sy1"><</span>C1<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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h<span class="sy1"><</span>D1<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 未完成：需要演示重载决议，而不只是失败</span></pre></div>
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</div>
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<ul>
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<li>试图创建指向引用的指针</li>
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<li>试图创建到 void 的引用</li>
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<li>试图创建指向 T 的成员的指针，其中 T 不是类类型：</li>
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</ul>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">class</span> is_class
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<span class="br0">{</span>
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<span class="kw1">typedef</span> <span class="kw4">char</span> yes<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw1">typedef</span> <span class="kw4">char</span> no<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> C<span class="sy1">></span>
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<span class="kw4">static</span> yes<span class="sy3">&</span> test<span class="br0">(</span><span class="kw4">int</span> C<span class="sy4">::</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 如果 C 是类类型就选择它</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> C<span class="sy1">></span>
|
|
<span class="kw4">static</span> no<span class="sy3">&</span> test<span class="br0">(</span>...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 否则选择它</span>
|
|
<span class="kw1">public</span><span class="sy4">:</span>
|
|
<span class="kw4">static</span> <span class="kw4">bool</span> <span class="kw4">const</span> value <span class="sy1">=</span> sizeof<span class="br0">(</span>test<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>yes<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<ul>
|
|
<li>试图将非法类型给予非类型模板形参：</li>
|
|
</ul>
|
|
<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, T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> S <span class="br0">{</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="sy1">></span>
|
|
<span class="kw4">int</span> f<span class="br0">(</span>S<span class="sy1"><</span>T, T<span class="br0">(</span><span class="br0">)</span><span class="sy1">></span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> i0 <span class="sy1">=</span> f<span class="sy1"><</span>X<span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="co1">// 未完成：需要演示重载决议，而不只是失败</span></pre></div>
|
|
</div>
|
|
<ul>
|
|
<li>试图在以下语境中进行非法转换：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>模板实参表达式</li>
|
|
<li>函数声明中使用的表达式：</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<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, T<span class="sy2">*</span><span class="sy1">></span>
|
|
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">int</span> i2 <span class="sy1">=</span> f<span class="sy1"><</span><span class="kw4">int</span>,<span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不能将 1 转换为 int*</span>
|
|
<span class="co1">// 未完成：需要演示重载决议，而非仅是失败</span></pre></div>
|
|
</div>
|
|
<ul>
|
|
<li>试图创建形参类型为 void 的函数类型</li>
|
|
<li>试图创建返回数组类型或函数类型的函数类型</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F_SFINAE">表达式 SFINAE</span></h3>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx11">
|
|
<td>
|
|
<p>C++11 前，只有在类型中使用的常量表达式（例如数组边界）才要求被当做 SFINAE（而非硬错误）。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
|
|
</tr>
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<p>下列表达式错误是 SFINAE 错误：</p>
|
|
<ul>
|
|
<li>模板形参类型中使用的谬构表达式</li>
|
|
<li>函数类型中使用的谬构表达式</li>
|
|
</ul>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw1">struct</span> Y <span class="br0">{</span> Y<span class="br0">(</span>X<span class="br0">)</span><span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// X 可以转换到 Y</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
|
<span class="kw4">auto</span> f<span class="br0">(</span>T t1, T t2<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span>t1 <span class="sy2">+</span> t2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重载 #1</span>
|
|
|
|
X f<span class="br0">(</span>Y, Y<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重载 #2</span>
|
|
|
|
X x1, x2<span class="sy4">;</span>
|
|
X x3 <span class="sy1">=</span> f<span class="br0">(</span>x1, x2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 推导在 #1 上失败（表达式 x1+x2 为谬构）</span>
|
|
<span class="co1">// 重载集中只有 #2，调用它</span></pre></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E9.83.A8.E5.88.86.E7.89.B9.E5.8C.96.E4.B8.AD.E7.9A.84_SFINAE">部分特化中的 SFINAE</span></h3>
|
|
<p>在确定一个类<span class="t-rev-inl t-since-cxx14"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>模板的特化是由<a href="cpp-language-partial_specialization.html">部分特化</a>还是主模板生成的时候也会出现推导与替换。在这种确定期间，编译器不会把替换失败当作硬错误，反而像涉及函数模板的重载决议中一样忽略对应的部分特化声明。</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co1">// 主模板处理无法引用的类型：</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> <span class="sy1">=</span> <span class="kw4">void</span><span class="sy1">></span>
|
|
<span class="kw1">struct</span> reference_traits
|
|
<span class="br0">{</span>
|
|
<span class="kw1">using</span> add_lref <span class="sy1">=</span> T<span class="sy4">;</span>
|
|
<span class="kw1">using</span> add_rref <span class="sy1">=</span> T<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 特化识别可以引用的类型：</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> reference_traits<span class="sy1"><</span>T, <a href="cpp-types-void_t.html"><span class="kw632">std::<span class="me2">void_t</span></span></a><span class="sy1"><</span>T<span class="sy3">&</span><span class="sy1">>></span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">using</span> add_lref <span class="sy1">=</span> T<span class="sy3">&</span><span class="sy4">;</span>
|
|
<span class="kw1">using</span> add_rref <span class="sy1">=</span> T<span class="sy3">&&</span><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="sy1">></span>
|
|
<span class="kw1">using</span> add_lvalue_reference_t <span class="sy1">=</span> <span class="kw1">typename</span> reference_traits<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">add_lref</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
|
|
<span class="kw1">using</span> add_rvalue_reference_t <span class="sy1">=</span> <span class="kw1">typename</span> reference_traits<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">add_rref</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<p>注：当前部分特化 SFINAE 未被标准正式支持（参阅 <a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2054">CWG 问题 2054</a>），然而标准库扩展 TS 自版本 2 起要求它能起效（参阅<a href="cpp-experimental-is_detected.html">检测手法</a>）。</p>
|
|
<h3><span class="mw-headline" id=".E5.BA.93.E6.94.AF.E6.8C.81">库支持</span></h3>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<p>标准库组件 <span class="t-lc"><a href="cpp-types-enable_if.html">std::enable_if</a></span> 允许创建替换失败，以基于某个在编译时求值的条件来启用或禁用特定的重载。</p>
|
|
<p>另外，许多<a href="cpp-types.html">类型特性</a>在适合的编译器扩展不可用时必须以 SFINAE 实现。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<p>标准库组件 <span class="t-lc"><a href="cpp-types-void_t.html">std::void_t</a></span> 是另一个简化部分特化 SFINAE 的应用的工具元函数。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E6.9B.BF.E4.BB.A3.E6.96.B9.E6.A1.88">替代方案</span></h3>
|
|
<p>只要可用，<a href="cpp-iterator-iterator_tags.html#.E7.A4.BA.E4.BE.8B">标签派发</a><span class="t-rev-inl t-since-cxx17"><span>、<a href="cpp-language-if.html#constexpr_if"><code>if constexpr</code></a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><span class="t-rev-inl t-since-cxx20"><span>以及<a href="cpp-language-constraints.html">概念</a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>通常都比直接使用 SFINAE 更好。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<p>如果只想要条件性的编译时错误，<a href="cpp-language-static_assert.html"><code>static_assert</code></a> 通常比 SFINAE 更适合。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<p>一种常见手法，是在返回类型上使用表达式 SFINAE，其中表达式使用逗号运算符，其左子表达式是所检验的（转型到 void 以确保不会选择返回类型上的用户定义逗号运算符），而右子表达式具有期望函数返回的类型。</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="co1">// 此重载始终在重载集中</span>
|
|
<span class="co1">// 省略号形参对于重载决议具有最低等级</span>
|
|
<span class="kw4">void</span> test<span class="br0">(</span>...<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"调用了保底重载<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="co1">// 如果 C 是类的引用类型且 F 是指向 C 的成员函数的指针，</span>
|
|
<span class="co1">// 那么这个重载会被添加到重载集</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> C, <span class="kw1">class</span> F<span class="sy1">></span>
|
|
<span class="kw4">auto</span> test<span class="br0">(</span>C c, F f<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="br0">(</span>c.<span class="sy2">*</span>f<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span>, <span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"调用了引用重载<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="co1">// 如果 C 是类的指针类型且 F 是指向 C 的成员函数的指针，</span>
|
|
<span class="co1">// 那么这个重载会被添加到重载集</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> C, <span class="kw1">class</span> F<span class="sy1">></span>
|
|
<span class="kw4">auto</span> test<span class="br0">(</span>C c, F f<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="br0">(</span><span class="br0">(</span>c<span class="sy2">-</span><span class="sy1">></span><span class="sy2">*</span>f<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, <span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"调用了指针重载<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">struct</span> X <span class="br0">{</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</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>
|
|
X x<span class="sy4">;</span>
|
|
test<span class="br0">(</span>x, <span class="sy3">&</span>X<span class="sy4">::</span><span class="me2">f</span><span class="br0">)</span><span class="sy4">;</span>
|
|
test<span class="br0">(</span><span class="sy3">&</span>x, <span class="sy3">&</span>X<span class="sy4">::</span><span class="me2">f</span><span class="br0">)</span><span class="sy4">;</span>
|
|
test<span class="br0">(</span><span class="nu0">42</span>, <span class="nu0">1337</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">调用了引用重载
|
|
调用了指针重载
|
|
调用了保底重载</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue295">CWG 295</a></td>
|
|
<td>C++98</td>
|
|
<td>创建 cv 限定的函数类型可能会导致替换失败</td>
|
|
<td>使之不是失败，舍弃 cv 限定</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1227">CWG 1227</a></td>
|
|
<td>C++98</td>
|
|
<td>替换顺序未指明</td>
|
|
<td>与词法顺序相同</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2322">CWG 2322</a></td>
|
|
<td>C++11</td>
|
|
<td>不同词法顺序的声明会导致模板实例化以不同顺序或完全不出现</td>
|
|
<td>这种情况为谬构，不要求诊断</td>
|
|
</tr>
|
|
</table>
|
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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