164 lines
19 KiB
HTML
164 lines
19 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>inline 说明符</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_inline skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><code>inline</code> 说明符</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><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 说明符，在用于函数的 <a href="cpp-language-declarations.html#.E8.AF.B4.E6.98.8E.E7.AC.A6">声明说明符序列</a> 时，将函数声明为一个 <i>内联（inline）函数</i>。</p>
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<p>完全在 <a href="cpp-language-classes.html">class/struct/union 的定义</a>之内定义<span class="t-rev-inl t-since-cxx20"><span>，<b>且被附着到全局模块</b></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>的函数是隐式的内联函数，无论它是成员函数还是非成员 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">friend</span></span></span> 函数。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>声明有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 的函数是隐式的内联函数。</p>
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<p>弃置的函数是隐式的内联函数：其（弃置）定义可出现在多于一个翻译单元中。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<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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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 说明符，在用于具有静态存储期的变量（静态类成员或命名空间作用域变量）的 <a href="cpp-language-declarations.html#.E8.AF.B4.E6.98.8E.E7.AC.A6">声明说明符序列</a> 时，将变量声明为<i>内联变量</i>。</p>
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<p>声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 的静态成员变量（但不是命名空间作用域变量）是隐式的内联变量。</p>
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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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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p><i>内联函数</i><span class="t-rev-inl t-since-cxx17"><span>或<i>内联变量</i></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>具有下列性质：</p>
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<ol>
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<li>内联函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>的定义必须在访问它的翻译单元中可达（不一定要在访问点前）。</li>
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<li>带<a href="cpp-language-storage_duration.html#.E5.A4.96.E9.83.A8.E8.BF.9E.E6.8E.A5">外部连接</a>的 inline 函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>（例如不声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span></span></span> ）拥有下列额外属性：</li>
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</ol>
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<dl>
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<dd>
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<ol>
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<li>内联函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>在程序中可有<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">多于一次定义</a>，只要每个定义都出现在不同翻译单元中（对于非静态的内联函数<span class="t-rev-inl t-since-cxx17"><span>和变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>）且所有定义等同即可。例如，内联函数<span class="t-rev-inl t-since-cxx17"><span>或内联变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>可定义于被多个源文件所 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="co2">#include</span></span></span> 的头文件中。</li>
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<li>它必须在每个翻译单元中都被声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 。</li>
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<li>它在每个翻译单元中都拥有相同的地址。</li>
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</ol>
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</dd>
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</dl>
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<p>在内联函数中，</p>
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<ul>
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<li>所有函数定义中的函数局部静态对象在所有翻译单元间共享（它们都指代相同的定义于某一个翻译单元中的对象）</li>
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<li>所有函数定义中所定义的类型同样在所有翻译单元中相同。</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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<p>命名空间作用域的内联 const 变量默认具有<a href="cpp-language-storage_duration.html#.E5.A4.96.E9.83.A8.E8.BF.9E.E6.8E.A5">外部连接</a>（这点不同于非内联非 volatile 的有 const 限定的变量）</p>
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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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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 关键词的本意是作为给优化器的指示器，以指示优先采用<a href="https://en.wikipedia.org/wiki/inline_expansion" class="extiw" title="enwiki:inline expansion">函数的内联替换</a>而非进行函数调用，即并不执行将控制转移到函数体内的函数调用 CPU 指令，而是代之以执行函数体的一份副本而无需生成调用。这会避免函数调用的开销（传递实参及返回结果），但它可能导致更大的可执行文件，因为函数体必须被复制多次。</p>
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<p>因为关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 的含义是非强制的，编译器拥有对任何未标记为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 的函数使用内联替换的自由，和对任何标记为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 的函数生成函数调用的自由。这些优化选择不改变上述关于多个定义和共享静态变量的规则。</p>
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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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<p>由于关键词 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 对于函数的含义已经变为“容许多次定义”而不是“优先内联”，因此这个含义也扩展到了变量。</p>
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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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<h4><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h4>
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<p>如果具有外部连接的内联函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>在不同翻译单元中的定义不同，则行为未定义。</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 说明符不能用于块作用域内（函数内部）的函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>声明</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 说明符不能重声明在翻译单元中已定义为非内联的函数<span class="t-rev-inl t-since-cxx17"><span>或变量</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。</p>
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<p>隐式生成的成员函数和任何在其首条声明中声明为预置的成员函数，与任何其他在类定义内定义的函数一样是内联的。</p>
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<p>如果一个内联函数在不同翻译单元中被声明，则其<a href="cpp-language-default_arguments.html">默认实参</a>的积累集合必须在每个翻译单元的末尾相同。</p>
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<p>在 C 中，内联函数不必在每个翻译单元声明为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span>（最多一个可以是非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span> <span class="kw1">inline</span></span></span>），函数定义不必相同（但如果程序依赖于调用的是哪个函数则行为未指明），且函数局部的静态变量在同一函数的不同定义间不同。</p>
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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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<p>关于内联静态成员的额外规则见<a href="cpp-language-static.html">静态数据成员</a>。</p>
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<p>内联变量消除了将 C++ 代码打包为唯头文件的库的主要障碍。</p>
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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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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<table class="t-rev-begin">
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<tr class="t-rev">
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<td>
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<p>头文件 "example.h"：</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="co2">#ifndef EXAMPLE_H</span>
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<span class="co2">#define EXAMPLE_H</span>
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<span class="co2">#include <atomic></span>
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<span class="co1">// 定义将被多个源文件包含的函数必须为 inline</span>
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<span class="kw1">inline</span> <span class="kw4">int</span> sum<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> a <span class="sy2">+</span> b<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 拥有外部链接的包含于多个源文件的变量必须为 inline</span>
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<span class="kw1">inline</span> <a href="cpp-atomic-atomic.html"><span class="kw2066">std::<span class="me2">atomic</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> counter<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co2">#endif</span></pre></div>
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</div>
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<p>源文件 #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"><span class="co2">#include "example.h"</span>
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<span class="kw4">int</span> a<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="sy2">++</span>counter<span class="sy4">;</span>
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<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</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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<p>源文件 #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"><span class="co2">#include "example.h"</span>
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<span class="kw4">int</span> a<span class="br0">(</span><span class="br0">)</span> <span class="co1">//这个函数与源文件#1中的不是同一个函数</span>
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<span class="br0">{</span>
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<span class="sy2">++</span>counter<span class="sy4">;</span>
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<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> b<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="sy2">++</span>counter<span class="sy4">;</span>
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<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">5</span>, <span class="nu0">6</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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</td>
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<td></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<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>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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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/issue1823">CWG 1823</a></td>
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<td>C++98</td>
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<td>内联函数的所有定义中的字符串字面量在所有翻译单元间共享</td>
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<td>由于一致性与实现移除该要求</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin"></table>
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<div class="visualClear"></div>
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</body>
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