143 lines
14 KiB
HTML
143 lines
14 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 函数说明符</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-c_language_inline 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"><code>inline</code> 函数说明符</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-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>声明<a href="https://en.wikipedia.org/wiki/inline_function" class="extiw" title="enwiki:inline function">内联函数</a>。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">inline</span></span></span> <span class="t-spar">函数声明</span></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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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><code><b>inline</b></code> 说明符的目的是提示编译器做优化，譬如<a href="https://en.wikipedia.org/wiki/inline_expansion" class="extiw" title="enwiki:inline expansion">函数内联</a>，这通常要求编译方能见到函数的定义。编译器能（并且经常）就优化的目的忽略 <code>inline</code> 说明符的存在与否。</p>
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<p>若编译器进行函数内联，则它会以函数体取代所有对它的调用，以避免函数调用的开销（将数据置于栈上和取得结果），这可能会生成更大的可执行文件，因为函数可能会被重复多次。结果同<a href="c-preprocessor-replace.html">仿函数宏</a>，只是用于该函数的标识符和宏指代可见于定义点的定义，而不指代调用点的定义。</p>
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<p>不管是否进行内联，内联函数都保证下列语义：</p>
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<p>任何拥有内部链接的函数都可以声明成 <code>static inline</code> ，没有其他限制。</p>
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<p>一个非 static 的内联函数不能定义一个非 const 的函数局部 static 对象，并且不能使用文件作用域的 static 对象。</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="kw4">static</span> <span class="kw4">int</span> x<span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">void</span><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> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// 错误：非 const 的 static 对象在非 static 的 inline 函数中</span>
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<span class="kw4">int</span> k <span class="sy1">=</span> x<span class="sy4">;</span> <span class="co1">// 错误：非 static 的 inline 函数访问 static 变量</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>若非 static 函数声明为 <code>inline</code> ，则必须在同一翻译单元中定义它。不使用 <code>extern</code> 的内联定义不会对外部可见，而且不会阻止其他翻译单元定义同一函数。这使得 <code><b>inline</b></code> 关键词成了 <code>static</code> 外另一种在头文件定义函数的方式，可以由同一程序的多个翻译单元包含该头文件。</p>
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<p>若函数在一些翻译单元中声明为 <code>inline</code> ，它就不需要在处处皆声明为 <code>inline</code> ：至多一个单元会提供常规的非 inline 非 static 函数，或是声明为 <code>extern inline</code> 的函数。称此翻译单元提供<i>外部定义</i>。为避免未定义行为，若在表达式中使用拥有外部链接的函数名，则程序中必须存在一个外部定义，见<a href="c-language-extern.html#.E4.B8.80.E4.B8.AA.E5.AE.9A.E4.B9.89.E8.A7.84.E5.88.99">一个定义规则</a>。</p>
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<p>内联函数的地址始终是外部定义的地址，但当以此地址进行函数调用时，调用<i>内联定义</i>（若存在于翻译单元中）还是<i>外部定义</i>是未指定的。定义于内联定义中的 static 对象与定义于外部定义中的 static 对象有别：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="kw1">inline</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>saddr<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="co1">// 用于此文件内的内联定义</span>
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<span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> name<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"saddr"</span><span class="sy4">;</span>
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<span class="kw1">return</span> name<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> compare_name<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> saddr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> saddr<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 未指定行为，调用可能是外部的</span>
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<span class="br0">}</span>
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<span class="kw2">extern</span> <span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>saddr<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 外部定义也会生成</span></pre></div>
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</div>
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<p>C 程序不应依赖于调用函数的内联版本还是外部版本，否则行为未指定。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="c-keyword-inline.html"><tt>inline</tt></a></p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>inline</code> 关键词是从 C++ 吸收的，但在 C++ 中，若函数声明为内联，则它必须在每一个翻译单元声明为内联，而且每一个内联函数都必须有准确相同的定义（ C 中，定义可以相异，而依赖这些差异仅导致未指定行为）。另一方面， C++ 允许非 const 的函数局域 static 对象，而且所有来自一个内联函数不同定义版本的函数局部 static 对象都相同，但它们在 C 中不同。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="co1">// file test.h</span>
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<span class="co2">#ifndef TEST_H_INCLUDED</span>
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<span class="co2">#define TEST_H_INCLUDED</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="co2">#endif</span>
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<span class="co1">// 文件 sum.c</span>
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<span class="co2">#include "test.h"</span>
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<span class="kw2">extern</span> <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><span class="sy4">;</span> <span class="co1">// 提供外部定义</span>
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<span class="co1">// 文件 test1.c</span>
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<span class="co2">#include <stdio.h></span>
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<span class="co2">#include "test.h"</span>
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<span class="kw2">extern</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%d<span class="es1">\n</span>"</span>, sum<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span><span class="br0">)</span> <span class="sy2">+</span> f<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 文件 test2.c</span>
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<span class="co2">#include "test.h"</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> sum<span class="br0">(</span><span class="nu0">2</span>, <span class="nu0">3</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>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">8</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.7.4 Function specifiers (p: 125-127)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>6.7.4 Function specifiers (p: 112-113)</li>
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</ul>
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</dd>
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</dl>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin"></table>
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</div>
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<div class="visualClear"></div>
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</body>
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</html>
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