820 lines
89 KiB
HTML
820 lines
89 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>待决名</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_dependent_name 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">待决名</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>在<a href="cpp-language-templates.html">模板</a>（<a href="cpp-language-class_template.html">类模板</a>和<a href="cpp-language-function_template.html">函数模板</a>）定义中，某些构造的含义可以在不同的实例化间有所不同。特别是，类型和表达式可能会取决于类型模板形参的类型和非类型模板形参的值。</p>
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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">struct</span> X <span class="sy4">:</span> B<span class="sy1"><</span>T<span class="sy1">></span> <span class="co1">// "B<T>" 取决于 T</span>
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<span class="br0">{</span>
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<span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">A</span><span class="sy2">*</span> pa<span class="sy4">;</span> <span class="co1">// "T::A" 取决于 T</span>
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<span class="co1">// （此 "typename" 的使用的目的见下文）</span>
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<span class="kw4">void</span> f<span class="br0">(</span>B<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> pb<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">int</span> i <span class="sy1">=</span> B<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">i</span><span class="sy4">;</span> <span class="co1">// "B<T>::i" 取决于 T</span>
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pb<span class="sy2">-</span><span class="sy1">></span>j<span class="sy2">++</span><span class="sy4">;</span> <span class="co1">// "pb->j" 取决于 T</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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<p>对<i>待决名</i>和非<i>待决名</i>的名字查找和绑定有所不同。</p>
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<h3><span class="mw-headline" id=".E7.BB.91.E5.AE.9A.E8.A7.84.E5.88.99">绑定规则</span></h3>
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<p>非待决名在模板定义点查找并绑定。即使在模板实例化点有更好的匹配，也保持此绑定：</p>
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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 <iostream></span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">double</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">"g(double)<span class="es1">\n</span>"</span><span class="sy4">;</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="kw1">struct</span> S
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<span class="br0">{</span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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g<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// "g" 是非待决名，现在绑定</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">int</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">"g(int)<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</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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g<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 g(int)</span>
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S<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> s<span class="sy4">;</span>
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s.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 g(double)</span>
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<span class="br0">}</span></pre></div>
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</div>
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</div>
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<p>如果<i>非待决名</i>的含义在定义点和模板特化的实例化点间有所改变，那么程序非良构，不要求诊断。在下列情形可能会出现这种情况：</p>
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<ul>
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<li>非待决名用到的类型在定义点<a href="cpp-language-type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B">不完整</a>但在实例化点完整</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-cxx17">
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<td>
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<ul>
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<li>模板定义中对名字的查找找到 <a href="cpp-language-using_declaration.html">using 声明</a>，但实例化中对应作用域找不到任何声明，因为该 using 声明是包展开而对应的包为空</li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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<ul>
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<li>实例化使用了在定义点尚未定义的默认实参或默认模板实参</li>
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<li>实例化点的某个<a href="cpp-language-constant_expression.html">常量表达式</a>使用了整型或无作用域枚举类型的 const 对象的值<span class="t-rev-inl t-since-cxx11"><span>、constexpr 对象的值、引用的值或者 constexpr 函数的定义</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>，而该对象<span class="t-rev-inl t-since-cxx11"><span>/引用/函数</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>在模板的定义点还没有定义</li>
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<li>该模板在实例化点使用了非待决的类模板特化<span class="t-rev-inl t-since-cxx14"><span>或变量模板特化</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>，而它所用的这个模板，或者从某个在定义点处还没有定义的部分特化所实例化，或者指名了某个在定义点处还没有声明的显式特化。</li>
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</ul>
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<p><i>待决名</i>的绑定则延迟到查找发生时。</p>
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<h3><span class="mw-headline" id=".E6.9F.A5.E6.89.BE.E8.A7.84.E5.88.99">查找规则</span></h3>
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<p>如<a href="cpp-language-lookup.html">名字查找</a>中所讨论的，对于模板中所使用的待决名的查找延迟到模板实参已知时，届时</p>
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<ul>
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<li>非 ADL 查找检验从<i>模板定义</i>的语境可见的具有外部连接的函数声明</li>
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<li><a href="cpp-language-adl.html">ADL</a> 一并检验从<i>模板定义</i>的语境或从<i>模板实例化</i>的语境可见的具有外部连接的函数声明</li>
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</ul>
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<p>（换言之，在模板定义后添加新函数声明不会令其可见，除非通过 ADL）。</p>
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<p>此规则的目的是帮助模板实例化抵御 <a href="cpp-language-definition.html#.E5.8D.95.E4.B8.80.E5.AE.9A.E4.B9.89.E8.A7.84.E5.88.99.EF.BC.88ODR.EF.BC.89">ODR</a> 违规：</p>
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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="co1">// 某个外部库</span>
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<span class="kw1">namespace</span> E
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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="kw4">void</span> writeObject<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> t<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Value = "</span> <span class="sy1"><<</span> t <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>
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<span class="br0">}</span>
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<span class="co1">// 翻译单元 1：</span>
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<span class="co1">// 程序员 1 希望允许 E::writeObject 与 vector<int> 一同使用</span>
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<span class="kw1">namespace</span> P1
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<span class="br0">{</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy3">&</span> v<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy4">:</span> v<span class="br0">)</span>
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os <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<span class="kw1">return</span> os<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> doSomething<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
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E<span class="sy4">::</span><span class="me2">writeObject</span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：找不到 P1::operator<<</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="co1">// 翻译单元 2：</span>
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<span class="co1">// 程序员 2 希望允许 E::writeObject 与 vector<int> 一同使用</span>
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<span class="kw1">namespace</span> P2
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<span class="br0">{</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy3">&</span> v<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy4">:</span> v<span class="br0">)</span>
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os <span class="sy1"><<</span> n <span class="sy1"><<</span><span class="st0">':'</span><span class="sy4">;</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> <span class="st0">"[]"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> doSomethingElse<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
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E<span class="sy4">::</span><span class="me2">writeObject</span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：找不到 P2::operator<<</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>在上例中，假如允许从实例化语境对 <code>operator<<</code> 进行非 ADL 查找，那么 <span class="t-c"><span class="mw-geshi cpp source-cpp">E<span class="sy4">::</span><span class="me2">writeObject</span><span class="sy1"><</span>vector<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span></span></span> 的实例化会拥有两个不同定义：一个使用 <span class="t-c"><span class="mw-geshi cpp source-cpp">P1<span class="sy4">::</span><span class="me2">operator</span><span class="sy1"><<</span></span></span>，一个使用 <span class="t-c"><span class="mw-geshi cpp source-cpp">P2<span class="sy4">::</span><span class="me2">operator</span><span class="sy1"><<</span></span></span>。连接器可能无法检测这种 ODR 违规，导致两个实例中都使用其中一个或另一个。</p>
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<p>为使得 ADL 检测用户定义命名空间，要么应当将 <code>std::vector</code> 替换成用户定义类，要么它的元素类型需要是用户定义类：</p>
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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">namespace</span> P1
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<span class="br0">{</span>
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<span class="co1">// 如果 C 是在 P1 命名空间中定义的类</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>C<span class="sy1">></span><span class="sy3">&</span> v<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span><span class="br0">(</span>C n <span class="sy4">:</span> v<span class="br0">)</span>
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os <span class="sy1"><<</span> n<span class="sy4">;</span>
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<span class="kw1">return</span> os<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> doSomething<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>C<span class="sy1">></span> v<span class="sy4">;</span>
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E<span class="sy4">::</span><span class="me2">writeObject</span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：实例化 writeObject(std::vector<P1::C>)</span>
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<span class="co1">// 通过 ADL 找到 P1::operator<<</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>注意：由于此规则，实践中不适合为标准库类型重载运算符：</p>
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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 <iostream></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <utility></span>
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<span class="co1">// 坏主意：全局命名空间中的运算符，但它的实参处于 std:: 中</span>
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<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">double</span><span class="sy1">></span> p<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> p.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">','</span> <span class="sy1"><<</span> p.<span class="me1">second</span><span class="sy4">;</span>
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<span class="br0">}</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="kw1">typedef</span> <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">double</span><span class="sy1">></span> elem_t<span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>elem_t<span class="sy1">></span> v<span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> v<span class="br0">[</span><span class="nu0">0</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">// OK，普通查找找到 ::operator<<</span>
|
|
<a href="cpp-algorithm-copy.html"><span class="kw1582">std::<span class="me2">copy</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>,
|
|
<a href="cpp-iterator-ostream_iterator.html"><span class="kw670">std::<span class="me2">ostream_iterator</span></span></a><span class="sy1"><</span>elem_t<span class="sy1">></span><span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>, <span class="st0">" "</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="co1">// 错误：从 std::ostream_iterator 定义点的普通查找和 ADL 只会考虑 std</span>
|
|
<span class="co1">// 命名空间，而且将找到 std::operator<< 的多个重载，因此这种查找会完成。</span>
|
|
<span class="co1">// 之后在查找所找到的集合中，重载决议为 elem_t 寻找 operator<< 会失败。</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</div>
|
|
<p>注意：在模板定义时，也会发生待决名的有限制的查找（但非绑定），以便将它们与非待决名进行区分，也用于确定它们是当前实例化的成员还是未知特化的成员。此查找所获得的信息可用于检测错误，见下文。</p>
|
|
<h3><span class="mw-headline" id=".E5.BE.85.E5.86.B3.E7.B1.BB.E5.9E.8B">待决类型</span></h3>
|
|
<p>下列类型是待决类型：</p>
|
|
<ul>
|
|
<li>模板形参</li>
|
|
<li><i>未知特化</i>（见下文）的成员</li>
|
|
<li>作为<i>未知特化</i>（见下文）的待决成员的嵌套类/枚举</li>
|
|
<li>待决类型的 cv 限定版本</li>
|
|
<li>从待决类型构成的复合类型</li>
|
|
<li>元素类型待决或边界（如果存在）是值待决的数组类型</li>
|
|
</ul>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<ul>
|
|
<li>形参包含一个或多个函数<a href="cpp-language-parameter_pack.html">形参包</a>的函数类型</li>
|
|
</ul>
|
|
</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>
|
|
<ul>
|
|
<li>异常说明是值待决的函数类型</li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<ul>
|
|
<li><a href="cpp-language-templates.html#.E6.A8.A1.E6.9D.BF.E6.A0.87.E8.AF.86">模板标识</a>，其中</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>模板名是某个模板形参，或者</li>
|
|
<li>有任何类型待决，或值待决<span class="t-rev-inl t-since-cxx11"><span>，或者是包展开</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>的模板实参（即使用到的模板标识不带实参列表，如<a href="cpp-language-injected-class-name.html">注入类名</a>）</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<ul>
|
|
<li>应用到类型待决表达式的 <a href="cpp-language-decltype.html"><code>decltype</code></a> 的结果</li>
|
|
</ul>
|
|
<p>应用到类型待决表达式的 <code>decltype</code> 的结果是唯一的待决类型。两个这样的结果只有在它们的表达式<a href="cpp-language-function_template.html#.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF.E9.87.8D.E8.BD.BD">等价</a>时才指代同一类型。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>注意；当前实例化的 typedef 成员只有在它指代的类型待决时才会是待决的。</p>
|
|
<h3><span class="mw-headline" id=".E7.B1.BB.E5.9E.8B.E5.BE.85.E5.86.B3.E8.A1.A8.E8.BE.BE.E5.BC.8F">类型待决表达式</span></h3>
|
|
<p>下列表达式是类型待决的：</p>
|
|
<ul>
|
|
<li>含有类型待决的子表达式的表达式</li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">this</span></span>，如果它的类是待决类型</li>
|
|
<li><span class="t-rev-inl t-since-cxx20"><span>不是<a href="cpp-language-constraints.html">概念标识</a>的</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span><a href="cpp-language-identifiers.html">标识表达式</a>，且</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>包含某个能被自身的名字查找找到至少一个待决声明的标识符</li>
|
|
<li>包含待决的<a href="cpp-language-templates.html#.E6.A8.A1.E6.9D.BF.E6.A0.87.E8.AF.86">模板标识</a></li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx11">
|
|
<td>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>包含特殊标识符 <code>__func__</code>（如果某个外围函数是模板，类模板的非模板成员<span class="t-rev-inl t-since-cxx14"><span>，或泛型 lambda</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>）</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>包含到某个待决类型的<a href="cpp-language-cast_operator.html">转换函数</a>名</li>
|
|
<li>包含作为<i>未知特化</i>成员的嵌套名说明符或<a href="cpp-language-identifiers.html">有限定标识</a></li>
|
|
<li>指名<i>当前实例化</i>的某个待决成员，且该成员是“未知边界的数组”类型的静态数据成员</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx14">
|
|
<td>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>包含某个能被自身的名字查找找到一个或更多的声明为具有<a href="cpp-language-function.html#.E8.BF.94.E5.9B.9E.E7.B1.BB.E5.9E.8B.E6.8E.A8.E5.AF.BC">返回类型推导</a>的当前实例化的成员函数声明的标识符</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
|
|
</tr>
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>包含能被自身的名字查找找到一个初始化器为类型待决的<a href="cpp-language-structured_binding.html">结构化绑定声明</a>的标识符</li>
|
|
<li>包含能被自身的名字查找找到类型含占位符 <code>auto</code> 的非类型模板形参的标识符</li>
|
|
<li>包含能被自身的名字查找找到以包含占位符类型的类型声明且初始化器是类型待决的变量（例如 <code>auto</code> 静态数据成员）的标识符，</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<ul>
|
|
<li>任何到待决类型的转型表达式</li>
|
|
<li>创建待决类型对象的 <a href="cpp-language-new.html">new 表达式</a></li>
|
|
<li>指代<i>当前实例化</i>的某个类型待决的成员的成员访问表达式</li>
|
|
<li>指代<i>未知特化</i>的某个成员的成员访问表达式</li>
|
|
</ul>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<ul>
|
|
<li><a href="cpp-language-fold.html">折叠表达式</a></li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>注意：字面量、伪析构函数调用<span class="t-rev-inl t-since-cxx11"><span>、<a href="cpp-language-alignof.html"><code>alignof</code></a>、<a href="cpp-language-noexcept.html">noexcept</a></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>、<a href="cpp-language-sizeof.html"><code>sizeof</code></a>、<a href="cpp-language-typeid.html"><code>typeid</code></a>、<a href="cpp-language-delete.html">delete</a>、和 <a href="cpp-language-throw.html">throw</a> 表达式始终不是类型待决的，因为这些表达式的类型不可能待决。</p>
|
|
<h3><span class="mw-headline" id=".E5.80.BC.E5.BE.85.E5.86.B3.E8.A1.A8.E8.BE.BE.E5.BC.8F">值待决表达式</span></h3>
|
|
<ul>
|
|
<li>在要求<a href="cpp-language-constant_expression.html">常量表达式</a>的语境中使用的表达式，且其有任何子表达式是值待决的</li>
|
|
<li><a href="cpp-language-identifiers.html">标识表达式</a>，其</li>
|
|
</ul>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx20">
|
|
<td>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>为<a href="cpp-language-constraints.html">概念标识</a>，而其实参有任何待决的</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>为类型待决的</li>
|
|
<li>为某个非类型模板形参的名字</li>
|
|
<li>指名某个作为<i>当前实例化</i>的待决成员的静态数据成员，且未被初始化。</li>
|
|
<li>指名某个作为<i>当前实例化</i>的待决成员的静态成员函数</li>
|
|
<li>为具有<span class="t-rev-inl t-until-cxx11"><span>整数或枚举</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span><span class="t-rev-inl t-since-cxx11"><span>字面</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>类型的常量，并从值待决表达式初始化</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<ul>
|
|
<li><span class="t-rev-inl t-since-cxx11"><span><a href="cpp-language-alignof.html"><code>alignof</code></a>、<a href="cpp-language-noexcept.html"><code>noexcept</code></a>、</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><a href="cpp-language-sizeof.html"><code>sizeof</code></a>、<a href="cpp-language-typeid.html"><code>typeid</code></a> 表达式，其实参是类型待决表达式或待决的类型标识</li>
|
|
<li>任何向待决类型转换或从值待决表达式转换的转型表达式</li>
|
|
<li>取址表达式，其实参是指名某个<i>当前实例化</i>的待决成员的<a href="cpp-language-identifiers.html">有限定标识</a></li>
|
|
<li>取址表达式，其实参是求值为核心<a href="cpp-language-constant_expression.html">常量表达式</a>的，指代某个作为具有静态<span class="t-rev-inl t-since-cxx11"><span>或线程</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>存储期的对象或成员函数的<a href="cpp-language-templates.html#.E6.A8.A1.E6.9D.BF.E5.8C.96.E5.AE.9E.E4.BD.93">模板化实体</a>。</li>
|
|
</ul>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<ul>
|
|
<li><a href="cpp-language-fold.html">折叠表达式</a></li>
|
|
</ul>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.BE.85.E5.86.B3.E5.90.8D">待决名</span></h3>
|
|
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
|
|
<tr>
|
|
<td class="mbox-empty-cell"></td>
|
|
<td class="mbox-text" style="">本节未完成<br />
|
|
原因：来自 [temp.dep] 的领句，现在缺失（后随有括号列表的 id 表达式……）</td>
|
|
</tr>
|
|
</table>
|
|
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
|
|
<tr>
|
|
<td class="mbox-empty-cell"></td>
|
|
<td class="mbox-text" style="">本节未完成<br />
|
|
原因：重新遣词以令它更清晰（或至少不要那么吓人），并在 ait 时应用 CWG 问题 591</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.BD.93.E5.89.8D.E5.AE.9E.E4.BE.8B.E5.8C.96">当前实例化</span></h3>
|
|
<p>在类模板定义内（包含它的成员函数和嵌套类），一些名字可以推导为指代<i>当前实例化</i>。这样就可以在定义点，而非在实例化点检测某些错误，并移除对待决名上的 <code>typename</code> 及 <code>template</code> 消歧义符的要求，见下文。</p>
|
|
<p>只有下列名字可以指代当前实例化：</p>
|
|
<ul>
|
|
<li>在类模板特化中：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>嵌套类、类模板成员、嵌套类成员、模板的注入类名、嵌套类的注入类名</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<ul>
|
|
<li>在主类模板的定义或在它的成员的定义中：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>类模板的名字后随主模板的模板实参列表（或等价的别名模板特化），其中每个实参和它对应的形参等价。注意如果表达式用作非类型模板实参（例如，N+0，其中 N 是形参），那么它即使在值匹配时也不指名当前实例化。</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<ul>
|
|
<li>在嵌套的类或类模板的定义中：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>被用作当前实例化成员的嵌套类的名字</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<ul>
|
|
<li>在部分特化或部分特化的成员的定义中：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>类模板的名字后随部分特化的模板实参列表，其中每个实参和它对应的形参等价</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<span class="sy1">></span>
|
|
<span class="kw1">class</span> A
|
|
<span class="br0">{</span>
|
|
A<span class="sy2">*</span> p1<span class="sy4">;</span> <span class="co1">// A 是当前实例化</span>
|
|
A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// A<T> 是当前实例化</span>
|
|
<span class="sy4">::</span><span class="me2">A</span><span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> p4<span class="sy4">;</span> <span class="co1">// ::A<T> 是当前实例化</span>
|
|
A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span> p3<span class="sy4">;</span> <span class="co1">// A<T*> 不是当前实例化</span>
|
|
|
|
<span class="kw1">class</span> B
|
|
<span class="br0">{</span>
|
|
B<span class="sy2">*</span> p1<span class="sy4">;</span> <span class="co1">// B 是当前实例化</span>
|
|
A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">B</span><span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// A<T>::B 是当前实例化</span>
|
|
<span class="kw1">typename</span> A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">B</span><span class="sy2">*</span> p3<span class="sy4">;</span> <span class="co1">// A<T*>::B 不是当前实例化</span>
|
|
<span class="br0">}</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">class</span> A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span>
|
|
<span class="br0">{</span>
|
|
A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="sy2">*</span> p1<span class="sy4">;</span> <span class="co1">// A<T*> 是当前实例化</span>
|
|
A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// A<T> 不是当前实例化</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw4">int</span> I<span class="sy1">></span>
|
|
<span class="kw1">struct</span> B
|
|
<span class="br0">{</span>
|
|
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> my_I <span class="sy1">=</span> I<span class="sy4">;</span>
|
|
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> my_I2 <span class="sy1">=</span> I <span class="sy2">+</span> <span class="nu0">0</span><span class="sy4">;</span>
|
|
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> my_I3 <span class="sy1">=</span> my_I<span class="sy4">;</span>
|
|
|
|
B<span class="sy1"><</span>my_I<span class="sy1">></span><span class="sy2">*</span> b3<span class="sy4">;</span> <span class="co1">// B<my_I> 是当前实例化</span>
|
|
B<span class="sy1"><</span>my_I2<span class="sy1">></span><span class="sy2">*</span> b4<span class="sy4">;</span> <span class="co1">// B<my_I2> 不是当前实例化</span>
|
|
B<span class="sy1"><</span>my_I3<span class="sy1">></span><span class="sy2">*</span> b5<span class="sy4">;</span> <span class="co1">// B<my_I3> 是当前实例化</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<p>注意，如果嵌套类派生自它的外围类模板，那么基类也可以是当前实例化。是待决类型但不是当前实例化的基类是<i>待决基类</i>：</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="kw1">struct</span> A
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw4">int</span> M<span class="sy4">;</span>
|
|
<span class="kw1">struct</span> B
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw4">void</span> M<span class="sy4">;</span>
|
|
<span class="kw1">struct</span> C<span class="sy4">;</span>
|
|
<span class="br0">}</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">struct</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">B</span><span class="sy4">::</span><span class="me2">C</span> <span class="sy4">:</span> A<span class="sy1"><</span>T<span class="sy1">></span>
|
|
<span class="br0">{</span>
|
|
M m<span class="sy4">;</span> <span class="co1">// OK, A<T>::M</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<p>符合以下情况的名字被归类为当前实例化的成员：</p>
|
|
<ul>
|
|
<li>由<a href="cpp-language-unqualified_lookup.html">无限定查找</a>在当前实例化或它的非待决基类中所找到的未限定名</li>
|
|
<li>限定符（<code><b>::</b></code> 左侧的名字）指名了当前实例化，且查找在当前实例化或其非待决基类中找到的<a href="cpp-language-qualified_lookup.html">限定名</a></li>
|
|
<li>用于类成员访问表达式的名字（<code>x.y</code> 或 <code>xp->y</code> 中的 <code>y</code>），其中对象表达式（<code>x</code> 或 <code>*xp</code>）是当前实例化，且查找在当前实例化或它的非待决基类中找到了该名字</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<span class="sy1">></span>
|
|
<span class="kw1">class</span> A
|
|
<span class="br0">{</span>
|
|
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> n1<span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span> <span class="co1">// i 指代当前实例化的成员</span>
|
|
<span class="kw4">int</span> n2<span class="br0">[</span>A<span class="sy4">::</span><span class="me2">i</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// A::i 指代当前实例化的成员</span>
|
|
<span class="kw4">int</span> n3<span class="br0">[</span>A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">i</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// A<T>::i 指代当前实例化的成员</span>
|
|
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</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="kw4">int</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">return</span> i<span class="sy4">;</span> <span class="co1">// i 指代当前实例化的成员</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>当前实例化的成员可以同时是待决和非待决的。</p>
|
|
<p>如果当前实例化的某个成员的查找在实例化点和定义点给出不同结果，那么查找有歧义。但要注意，在成员名被使用时，它不会自动转换成类成员访问表达式，只有显式成员访问表达式才会指示当前实例化的成员：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> m<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw4">int</span> m<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> C <span class="sy4">:</span> A, T
|
|
<span class="br0">{</span>
|
|
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> this<span class="sy2">-</span><span class="sy1">></span>m<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 在模板定义语境找到 A::m</span>
|
|
<span class="kw4">int</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> m<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 在模板定义语境找到 A::m</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span> <span class="kw4">int</span> C<span class="sy1"><</span>B<span class="sy1">></span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：找到 A::m 和 B::m</span>
|
|
|
|
<span class="kw1">template</span> <span class="kw4">int</span> C<span class="sy1"><</span>B<span class="sy1">></span><span class="sy4">::</span><span class="me2">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：没有在模板定义语境中进行向类成员访问语法的变换</span></pre></div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E6.9C.AA.E7.9F.A5.E7.89.B9.E5.8C.96">未知特化</span></h3>
|
|
<p>在模板定义内，某些名字被推导为属于某个<i>未知特化</i>，特别是，</p>
|
|
<ul>
|
|
<li>在 <code><b>::</b></code> 左侧出现了并非<i>当前实例化</i>成员的<i>待决类型</i>的名字的<a href="cpp-language-qualified_lookup.html">限定名</a></li>
|
|
<li>限定符是<i>当前实例化</i>，且无法在当前实例化或它的任何非待决基类中找到，并存在待决基类的<a href="cpp-language-qualified_lookup.html">限定名</a></li>
|
|
<li>类成员访问表达式中的成员名（<code>x.y</code> 或 <code>xp->y</code> 中的 <code>y</code>），如果对象表达式（<code>x</code> 或 <code>*xp</code>）的类型是<i>待决类型</i>且非<i>当前实例化</i></li>
|
|
<li>类成员访问表达式中的成员名（<code>x.y</code> 或 <code>xp->y</code> 中的 <code>y</code>），如果对象表达式（<code>x</code> 或 <code>*xp</code>）的类型是<i>当前实例化</i>，且在当前实例化或任何其非待决基类中找不到该名字，并存在待决基类</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">typename</span> T<span class="sy1">></span> <span class="kw1">struct</span> Base <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base<span class="sy1"><</span>T<span class="sy1">></span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// Derived<T> 指代当前实例化</span>
|
|
<span class="co1">// 当前实例化没有 'unknown_type' 但有一个待决基类（Base<T>）</span>
|
|
<span class="co1">// 因此 unknown_type 是未知特化的成员</span>
|
|
<span class="kw1">typename</span> Derived<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">unknown_type</span> z<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> Base<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> <span class="co1">// 它在此特化提供</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw4">int</span> unknown_type<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<p>此分类允许在模板定义（而非实例化）点检测下列错误：</p>
|
|
<ul>
|
|
<li>如果有模板定义拥有某个<a href="cpp-language-qualified_lookup.html">限定名</a>，其中的限定符指代<i>当前实例化</i>，且该名字既不是<i>当前实例化</i>的成员也不是<i>未知特化</i>的成员，那么程序非良构（不要求诊断），即使模板永远不会被实例化。</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<span class="sy1">></span>
|
|
<span class="kw1">class</span> A
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw4">int</span> type<span class="sy4">;</span>
|
|
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> i<span class="sy4">;</span> <span class="co1">// OK：'type' 是当前实例化的成员</span>
|
|
<span class="kw1">typename</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">other</span> j<span class="sy4">;</span> <span class="co1">// 错误：</span>
|
|
|
|
<span class="co1">// 'other' 不是当前实例化的成员，也不是未知特化的成员，</span>
|
|
<span class="co1">// 因为 A<T>（指名当前实例化）没有暗藏有 'other' 的待决基类。</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<ul>
|
|
<li>如果有模板定义拥有某个成员访问表达式，其对象表达式是<i>当前实例化</i>，但其名字既不是<i>当前实例化</i>的成员也不是<i>未知特化</i>的成员，那么程序非良构，即使模板永远不会被实例化。</li>
|
|
</ul>
|
|
<p>未知特化的成员始终是待决的，而且它们与所有待决名一样在实例化点进行查找和绑定（见上文）。</p>
|
|
<h3><span class="mw-headline" id=".E5.BE.85.E5.86.B3.E5.90.8D.E7.9A.84_typename_.E6.B6.88.E6.AD.A7.E4.B9.89.E7.AC.A6">待决名的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 消歧义符</span></h3>
|
|
<p>在模板（包括别名模版）的声明或定义中，不是<i>当前实例化</i>的成员且取决于某个模板形参的名字不会被认为是类型，除非使用关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 或它已经被设立为类型名（例如用 typedef 声明或通过用作基类名）。</p>
|
|
<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 <iostream></span>
|
|
<span class="co2">#include <vector></span>
|
|
|
|
<span class="kw4">int</span> p <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw4">void</span> foo<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&</span>v<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// std::vector<T>::const_iterator 是待决名，</span>
|
|
<span class="kw1">typename</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">const_iterator</span> it <span class="sy1">=</span> v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// 下列内容因为没有 'typename' 而会被解析成</span>
|
|
<span class="co1">// 类型待决的成员变量 'const_iterator' 和某变量 'p' 的乘法。</span>
|
|
<span class="co1">// 因为在此处有一个可见的全局 'p'，所以此模板定义能编译。</span>
|
|
<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">const_iterator</span><span class="sy2">*</span> p<span class="sy4">;</span>
|
|
|
|
<span class="kw1">typedef</span> <span class="kw1">typename</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">const_iterator</span> iter_t<span class="sy4">;</span>
|
|
iter_t <span class="sy2">*</span> p2<span class="sy4">;</span> <span class="co1">// iter_t 是待决名，但已知它是类型名</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> S
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw4">int</span> value_t<span class="sy4">;</span> <span class="co1">// 当前实例化的成员</span>
|
|
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
S<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_t</span> n<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// S<T> 待决，但不需要 'typename'</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
|
|
foo<span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 模板实例化失败：类型 std::vector<int> 中没有</span>
|
|
<span class="co1">// 名字是 'const_iterator' 的成员变量</span>
|
|
S<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</div>
|
|
<p>关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 只能以这种方式用于限定名（例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="me2">x</span></span></span>）之前，但这些名字不必待决。</p>
|
|
<p>对前附 <code>typename</code> 的标识符使用通常的<a href="cpp-language-qualified_lookup.html">有限定名字查找</a>。这与用<a href="cpp-language-elaborated_type_specifier.html">详述类型说明符</a>的情况不同，不管限定符如何都不改变查找规则：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">struct</span> A <span class="co1">// A 拥有嵌套变量 X 和嵌套类型 struct X</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> X<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">struct</span> B
|
|
<span class="br0">{</span>
|
|
<span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// B 拥有嵌套类型 struct X</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">void</span> f<span class="br0">(</span>T t<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">X</span> x<span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
A a<span class="sy4">;</span>
|
|
B b<span class="sy4">;</span>
|
|
f<span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：实例化 f<B>，T::X 指代 B::X</span>
|
|
f<span class="br0">(</span>a<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：不能实例化 f<A>：因为 A::X 的有限定名字查找找到了数据成员</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx11">
|
|
<td>
|
|
<p>关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 只能在模板声明和定义中使用，而且只能用于可以使用待决名的语境中。这排除了显式特化声明和显式实例化声明。</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>关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 即便在模板外也可以使用。</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <vector></span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> <span class="kw1">typename</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">const_iterator</span> iter_t<span class="sy4">;</span> <span class="co1">// C++11 中 OK</span>
|
|
<span class="kw1">typename</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span> <span class="co1">// C++11 中也 OK</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</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-cxx20">
|
|
<td>
|
|
<p>某些语境中，只有类型名能合法地出现。在这些语境中，假定待决的限定名指名的就是类型而不必使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span>：</p>
|
|
<ul>
|
|
<li>用作以下各项的（顶层）<span class="t-spar">声明说明符序列</span> 中的<a href="cpp-language-declarations.html#.E8.AF.B4.E6.98.8E.E7.AC.A6">声明说明符</a>的限定名：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>位于命名空间作用域的<a href="cpp-language-declarations.html#.E7.AE.80.E5.8D.95.E5.A3.B0.E6.98.8E">简单声明</a>或<a href="cpp-language-function.html#.E5.87.BD.E6.95.B0.E5.A3.B0.E6.98.8E">函数声明</a>；</li>
|
|
<li><a href="cpp-language-class.html#.E6.88.90.E5.91.98.E8.AF.B4.E6.98.8E">类成员声明</a>；</li>
|
|
<li><a href="cpp-language-class.html#.E6.88.90.E5.91.98.E8.AF.B4.E6.98.8E">类成员声明</a>（包括友元函数声明）中的<a href="cpp-language-function.html#.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">形参声明</a>，不包括默认实参；</li>
|
|
<li>名字有限定的<a href="cpp-language-function.html">函数或函数模板的声明符</a>的<a href="cpp-language-function.html#.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">形参声明</a>，不包括默认实参；</li>
|
|
<li><a href="cpp-language-lambda.html">lambda 表达式</a>的<a href="cpp-language-function.html#.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">形参声明</a>，不包括默认实参；</li>
|
|
<li><a href="cpp-language-constraints.html#requires_.E8.A1.A8.E8.BE.BE.E5.BC.8F">requires 表达式</a>的形参声明；</li>
|
|
<li><a href="cpp-language-template_parameters.html#.E9.9D.9E.E7.B1.BB.E5.9E.8B.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">非类型模板形参</a>声明中的类型；</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<ul>
|
|
<li>在<i><a href="cpp-language-type.html#.E7.B1.BB.E5.9E.8B.E7.9A.84.E5.91.BD.E5.90.8D">类型标识</a></i>中出现的限定名，其中最小的外围<i>类型标识</i>是：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>不对其类型加括号的 <a href="cpp-language-new.html">new 表达式</a>中的 <span class="t-spar">类型</span>；</li>
|
|
<li><a href="cpp-language-type_alias.html">别名声明</a>中的 <span class="t-spar">类型标识</span>；</li>
|
|
<li><a href="cpp-language-function.html">尾随返回类型</a>，</li>
|
|
<li><a href="cpp-language-template_parameters.html#.E7.B1.BB.E5.9E.8B.E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">类型模板形参的默认实参</a>，或</li>
|
|
<li><a href="cpp-language-static_cast.html"><code>static_cast</code></a>、<a href="cpp-language-dynamic_cast.html"><code>dynamic_cast</code></a>、<a href="cpp-language-const_cast.html"><code>const_cast</code></a>，或 <a href="cpp-language-reinterpret_cast.html"><code>reinterpret_cast</code></a> 的 <span class="t-spar">类型标识</span>。</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.BE.85.E5.86.B3.E5.90.8D.E7.9A.84_template_.E6.B6.88.E6.AD.A7.E4.B9.89.E7.AC.A6">待决名的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span> 消歧义符</span></h3>
|
|
<p>与此相似，模板定义中不是<i>当前实例化</i>的成员的待决名同样不被认为是模板名，除非使用消歧义关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span>，或它已被设立为模板名：</p>
|
|
<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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw1">struct</span> S
|
|
<span class="br0">{</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> U<span class="sy1">></span> <span class="kw4">void</span> foo<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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
|
|
<span class="kw4">void</span> bar<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
S<span class="sy1"><</span>T<span class="sy1">></span> s<span class="sy4">;</span>
|
|
s.<span class="me1">foo</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：< 被解析为小于运算符</span>
|
|
s.<span class="kw1">template</span> foo<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</div>
|
|
<p>关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span> 只能以这种方式用于运算符 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span></span></span>（作用域解析）、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">></span></span></span>（通过指针的成员访问）和 <span class="t-c"><span class="mw-geshi cpp source-cpp">.</span></span>（成员访问）之后，下列表达式都是合法示例：</p>
|
|
<ul>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy4">::</span><span class="kw1">template</span> foo<span class="sy1"><</span>X<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">s.<span class="kw1">template</span> foo<span class="sy1"><</span>X<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span><span class="kw1">template</span> foo<span class="sy1"><</span>X<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span></li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> T<span class="sy4">::</span><span class="kw1">template</span> iterator<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span> v<span class="sy4">;</span></span></span></li>
|
|
</ul>
|
|
<p>与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span></span></span> 的情况一样，即使名字并非待决<span class="t-rev-inl t-since-cxx11"><span>或它的使用并未在模板的作用域中出现</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>，也允许使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span> 前缀。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<p>即使 <code><b>::</b></code> 左侧的名字指代命名空间，也允许使用 template 消歧义符：</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">typename</span><span class="sy1">></span> <span class="kw1">struct</span> s <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="sy4">::</span><span class="kw1">template</span> s<span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span> q<span class="sy4">;</span> <span class="co1">// 允许，但不需要</span></pre></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx23">
|
|
<td>
|
|
<p>根据<a href="cpp-language-unqualified_lookup.html">无限定名字查找</a>针对成员访问表达式中的模板名的特殊规则，当非待决的模板名在成员访问表达式中出现时（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">></span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">.</span></span> 后），如果通过表达式语境中的常规名字查找找到了的具有相同名字的<span class="t-rev-inl t-since-cxx11"><span>类或别名</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>模板，那么就不需要消歧义符。<span class="t-rev-inl t-until-cxx11"><span>然而，如果表达式语境中的查找所找到的模板与类语境中所找到的不同，那么程序非良构。</span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span></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="kw4">int</span><span class="sy1">></span> <span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> value<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="kw4">void</span> f<span class="br0">(</span>T t<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
t.<span class="me1">A</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span> <span class="co1">// A 的常规查找找到类模板。A<0>::value 指名类 A<0> 的成员</span>
|
|
<span class="co1">// t.A < 0; // 错误：'<' 被当做模板实参列表的起始</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx23">(C++23 前)</span></td>
|
|
</tr>
|
|
</table>
|
|
<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/issue1413">CWG 1413</a></td>
|
|
<td>C++98</td>
|
|
<td>未初始化静态数据成员、静态成员函数和类模板的成员的地址未列为值待决</td>
|
|
<td>已列入</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue1850">CWG 1850</a></td>
|
|
<td>C++98</td>
|
|
<td>含义在定义语境和实例化点之间变更的情况列表不完整</td>
|
|
<td>使之完整</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2100">CWG 2100</a></td>
|
|
<td>C++98</td>
|
|
<td>类模板的静态成员常量地址未列为值待决</td>
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<td>已列入</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2457">CWG 2457</a></td>
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<td>C++11</td>
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<td>带函数形参包的函数类型未列为待决类型</td>
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<td>已列入</td>
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</tr>
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</table>
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