222 lines
30 KiB
HTML
222 lines
30 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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<title>翻译单元局部实体 (C++20 起)</title>
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<link rel="stylesheet" href="ext.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_tu_local skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">翻译单元局部实体 <span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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>翻译单元局部（ TU 局部）实体是为阻止假定为局部（不用于任何其他翻译单元）的实体暴露且用于其他翻译单元而引入的。</p>
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<p>来自<a rel="nofollow" class="external text" href="https://vector-of-bool.github.io/2019/03/31/modules-2.html">了解 C++ 模块：部分 2</a> 的例子描绘了不约束暴露的问题：</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">// 无 TU 局部约束的模块单元</span>
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export module Foo<span class="sy4">;</span>
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import <span class="sy1"><</span>iostream<span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">namespace</span> <span class="br0">{</span>
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<span class="kw1">class</span> LolWatchThis <span class="br0">{</span> <span class="co1">// 内部链接，不能导出</span>
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<span class="kw4">static</span> <span class="kw4">void</span> say_hello<span class="br0">(</span><span class="br0">)</span> <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">"Hello, everyone!<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><span class="sy4">;</span>
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<span class="br0">}</span>
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export LolWatchThis lolwut<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// LolWatchThis 被暴露为返回类型</span>
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<span class="kw1">return</span> LolWatchThis<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</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="co1">// main.cpp</span>
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import Foo<span class="sy4">;</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">auto</span> evil <span class="sy1">=</span> lolwut<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 'evil' 拥有 'LolWatchThis' 类型</span>
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decltype<span class="br0">(</span>evil<span class="br0">)</span><span class="sy4">::</span><span class="me2">say_hello</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 'LolWatchThis' 的定义不再为内部</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h3><span class="mw-headline" id="TU_.E5.B1.80.E9.83.A8.E5.AE.9E.E4.BD.93">TU 局部实体</span></h3>
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<p>一个实体为 <i>TU 局部</i>，若它为</p>
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<ol>
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<li>类型、函数、变量或模板，且
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<ol>
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<li>拥有带<a href="cpp-language-storage_duration.html#.E5.86.85.E9.83.A8.E8.BF.9E.E6.8E.A5">内部链接</a>的名字，或</li>
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<li>无拥有内部链接的名字，且在 TU 局部实体之内声明或由 <a href="cpp-language-lambda.html">lambda 表达式</a>引入、</li>
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</ol>
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</li>
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<li>定义在<a href="cpp-language-class.html">类说明符</a>、函数体或初始化器外，或由用于声明仅 TU 局部实体的定义类型说明符（类型说明符、类说明符或枚举说明符）引入、</li>
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<li>TU 局部模板的特化、</li>
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<li>有任何 TU 局部模板实参的模板特化，或</li>
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<li>其声明为暴露（定义后述）的（可能实例化的）模板特化。</li>
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</ol>
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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">// 拥有内部链接的 TU 局部实体</span>
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<span class="kw1">namespace</span> <span class="br0">{</span> <span class="co1">// 声明于无名命名空间的所有名字均拥有内部链接</span>
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<span class="kw4">int</span> tul_var <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// TU 局部变量</span>
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<span class="kw4">int</span> tul_func<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// TU 局部函数</span>
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<span class="kw1">struct</span> tul_type <span class="br0">{</span> <span class="kw4">int</span> mem<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// TU 局部（类）类型</span>
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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">static</span> <span class="kw4">int</span> tul_func_temp<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// TU 局部模板</span>
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<span class="co1">// TU 局部模板特化</span>
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<span class="kw1">template</span><span class="sy1"><></span>
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<span class="kw4">static</span> <span class="kw4">int</span> tul_func_temp<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// TU 局部特化</span>
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<span class="co1">// 有 TU 局部模板实参的模板特化</span>
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<span class="kw1">template</span> <span class="sy1"><></span> <span class="kw1">struct</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>tul_type<span class="sy1">></span> <span class="br0">{</span> <span class="co1">// TU 局部特化</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> tul_type<span class="sy3">&</span> t<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> 4u<span class="sy4">;</span> <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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<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：缺少规则 #1.2、 #2 及 #5 的示例</td>
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</tr>
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</table>
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<p>值或对象为 <i>TU 局部</i>，若它</p>
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<ol>
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<li>是 TU 局部的函数或与 TU 局部变量关联的对象，或是指向它的指针，或者</li>
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<li>是类或数组类型对象，而其任一<a href="cpp-language-object.html#.E5.AD.90.E5.AF.B9.E8.B1.A1">子对象</a>，或其引用类型非静态数据成员所引用的对象或函数，为 TU 局部并且<a href="cpp-language-constant_expression.html#.E5.8F.AF.E7.94.A8.E4.BA.8E.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F">可用于常量表达式</a>。</li>
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</ol>
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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="kw4">static</span> <span class="kw4">int</span> tul_var <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// TU 局部变量</span>
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<span class="kw4">static</span> <span class="kw4">int</span> tul_func<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// TU 局部函数</span>
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<span class="kw4">int</span><span class="sy2">*</span> tul_var_ptr <span class="sy1">=</span> <span class="sy3">&</span>tul_var<span class="sy4">;</span> <span class="co1">// TU 局部：指向 TU 局部变量的指针</span>
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<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span> tul_func_ptr<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">&</span>tul_func<span class="sy4">;</span> <span class="co1">// TU 局部：指向 TU 局部函数的指针</span>
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<span class="kw4">constexpr</span> <span class="kw4">static</span> <span class="kw4">int</span> tul_const <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// TU 局部变量可用于常量表达式</span>
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<span class="kw4">int</span> tul_arr<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> tul_const <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// TU 局部： constexpr TU 局部对象的数组</span>
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<span class="kw1">struct</span> tul_class <span class="br0">{</span> <span class="kw4">int</span> mem<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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tul_class tul_obj<span class="br0">{</span>tul_const<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// TU 局部：拥有成员 constexpr TU 局部对象</span></pre></div>
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</div>
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<h3><span class="mw-headline" id=".E6.9A.B4.E9.9C.B2">暴露</span></h3>
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<p>声明 D <i>指名</i>实体 E ，若</p>
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<ol>
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<li>D 含有闭包类型为 E 的 lambda 表达式、</li>
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<li>E 不是函数或函数模板且 D 含有指代 E 的标识表达式、类型说明符、嵌套类型说明符、模板名或概念名，或</li>
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<li>E 是函数或函数模板且 D 含有指名 E 的表达式或指代含有 E 的重载集的标识表达式。</li>
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</ol>
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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">// lambda 指名</span>
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<span class="kw4">auto</span> x <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 指名 decltype(x)</span>
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<span class="co1">// 非函数（模板）指名</span>
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<span class="kw4">int</span> y1 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 指名 y1 （标识表达式）</span>
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<span class="kw1">struct</span> y2 <span class="br0">{</span> <span class="kw4">int</span> mem<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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y2 y2_obj<span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 指名 y2 （类型说明符）</span>
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<span class="kw1">struct</span> y3 <span class="br0">{</span> <span class="kw4">int</span> mem_func<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> y3<span class="sy4">::</span><span class="me2">mem_func</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 指名 y3 （嵌套类型说明符）</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> <span class="kw4">int</span> y4 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
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<span class="kw4">int</span> var <span class="sy1">=</span> y4<span class="sy1"><</span>y2<span class="sy1">></span><span class="sy4">;</span> <span class="co1">// 指名 y4 （模板名）</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> concept y5 <span class="sy1">=</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span>T<span class="sy3">&&</span><span class="br0">)</span> requires y5<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span> <span class="co1">// 指名 y5 （概念名）</span>
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<span class="co1">// 函数（模板）指名</span>
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<span class="kw4">int</span> z1<span class="br0">(</span><span class="kw4">int</span> arg<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">"no overload"</span><span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> z2<span class="br0">(</span><span class="kw4">int</span> arg<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">"overload 1"</span><span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> z2<span class="br0">(</span><span class="kw4">double</span> arg<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">"overload 2"</span><span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">int</span> val1 <span class="sy1">=</span> z1<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 指名 z1</span>
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<span class="kw4">int</span> val2 <span class="sy1">=</span> z2<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 指名 z2 （int z2(int)）</span></pre></div>
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</div>
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<p>一个声明是<i>暴露</i>，若它要么指名一个 TU 局部实体，不过忽略</p>
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<ol>
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<li>非 inline 函数或函数模板的函数体（但非用<a href="cpp-language-auto.html">占位符类型</a>声明返回类型的函数的（可能实例化的）定义的推导返回类型），</li>
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<li>变量或变量模板的初始化器（但非变量类型），</li>
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<li>类定义中的友元声明，及</li>
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<li>任何到拥有内部链接或无链接的、以非 <a href="cpp-language-definition.html#ODR_.E4.BD.BF.E7.94.A8">odr 使用</a>的常量表达式初始化的非 volatile 的 const 对象或引用的引用，</li>
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</ol>
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<p>这些情况，要么定义以 TU 局部值初始化的 constexpr 变量。</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：缺少暴露的示例</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id="TU_.E5.B1.80.E9.83.A8.E7.BA.A6.E6.9D.9F">TU 局部约束</span></h3>
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<p>若<a href="cpp-language-modules.html">模块接口单元</a>（在可能有的私有模块片段外）或模块划分中的非 TU 局部实体的（可能实例化的）<a href="cpp-language-declarations.html">声明</a>或<a href="cpp-language-class_template_argument_deduction.html#.E7.B1.BB.E6.A8.A1.E6.9D.BF.E7.9A.84.E6.8E.A8.E5.AF.BC">推导指引</a>为暴露，则程序非良构。这种声明在在任何其他语境中被弃用。</p>
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<p>若出现于一个翻译单元中的声明指名在另一非头文件单元的翻译单元中声明的 TU 局部实体，则程序非良构。为模板特化实例化的声明出现在特化的实例化点。</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：缺少约束的示例</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<p>翻译单元 #1：</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">export module A<span class="sy4">;</span>
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<span class="kw4">static</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">inline</span> <span class="kw4">void</span> it<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// 错误：为 f 的暴露</span>
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<span class="kw4">static</span> <span class="kw1">inline</span> <span class="kw4">void</span> its<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> its<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK</span>
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<span class="kw1">template</span> <span class="kw4">void</span> g<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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decltype<span class="br0">(</span>f<span class="br0">)</span> <span class="sy2">*</span>fp<span class="sy4">;</span> <span class="co1">// 错误： f （尽管不是其类型）为 TU 局部</span>
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<span class="kw4">auto</span> <span class="sy3">&</span>fr <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> <span class="sy3">&</span>fr2 <span class="sy1">=</span> fr<span class="sy4">;</span> <span class="co1">// 错误：为 f 的暴露</span>
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<span class="kw4">constexpr</span> <span class="kw4">static</span> <span class="kw4">auto</span> fp2 <span class="sy1">=</span> fr<span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">void</span> <span class="br0">(</span><span class="sy3">&</span>ref<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> s<span class="br0">{</span>f<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK： value 为 TU 局部</span>
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<span class="kw4">constexpr</span> <span class="kw4">extern</span> <span class="kw1">struct</span> W <span class="br0">{</span> S <span class="sy3">&</span>s<span class="sy4">;</span> <span class="br0">}</span> wrap<span class="br0">{</span>s<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK： value 非 TU 局部 TU-local</span>
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<span class="kw4">static</span> <span class="kw4">auto</span> x <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw4">auto</span> x2 <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// 错误：闭包类型为 TU 局部</span>
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<span class="kw4">int</span> y <span class="sy1">=</span> <span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">{</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span><span class="br0">(</span><span class="br0">)</span>,<span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：闭包类型非 TU 局部</span>
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<span class="kw4">int</span> y2 <span class="sy1">=</span> <span class="br0">(</span>x,<span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="kw1">namespace</span> N <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="kw4">void</span> adl<span class="br0">(</span>A<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">static</span> <span class="kw4">void</span> adl<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> adl<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">void</span> h<span class="br0">(</span><span class="kw4">auto</span> x<span class="br0">)</span> <span class="br0">{</span> adl<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK ，但特化可能为暴露</span></pre></div>
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</div>
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<p>翻译单元 #2：</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">module A<span class="sy4">;</span>
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<span class="kw4">void</span> other<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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g<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：显式实例化特化</span>
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g<span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：实例化使用其 TU 局部实体</span>
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h<span class="br0">(</span>N<span class="sy4">::</span><span class="me2">A</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：重载集含 TU 局部的 N::adl(int)</span>
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h<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：调用 adl(double)</span>
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adl<span class="br0">(</span>N<span class="sy4">::</span><span class="me2">A</span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：找不到 N::adl(int) ，调用 N::adl(N::A)</span>
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fr<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：调用 f</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> ptr <span class="sy1">=</span> fr<span class="sy4">;</span> <span class="co1">// 错误：此处 fr 不可用于常量表达式</span>
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<span class="br0">}</span></pre></div>
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</div>
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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：示例太复杂，需要更好的安排</td>
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</tr>
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</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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