132 lines
13 KiB
HTML
132 lines
13 KiB
HTML
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<title>常量初始化</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_constant_initialization skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">常量初始化</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>设置<a href="cpp-language-storage_duration.html">静态</a>变量的初值为编译时常量。</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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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>static</b></code> <span class="t-spar">T</span> <code><b>&</b></code> <span class="t-spar">引用</span> <code><b>= <span class="t-spar">常量表达式</span>;</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><code><b>static</b></code> <span class="t-spar">T</span> <span class="t-spar">对象</span> <code><b>= <span class="t-spar">常量表达式</span>;</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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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>静态<span class="t-rev-inl t-since-cxx11"><span>及线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对象的常量取代零初始化，并早于所有其他初始化进行。只有下列变量会进行常量初始化：</p>
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<div class="t-li1"><span class="t-li">1)</span> 静态<span class="t-rev-inl t-since-cxx11"><span>或线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>引用，如果它绑定到了静态泛左值<span class="t-rev-inl t-since-cxx11"><span>，临时对象（或其子对象）</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>，或函数，且如果引用的<a href="cpp-language-reference_initialization.html">初始化器</a>中每个表达式（包含隐式转换）都是<a href="cpp-language-constant_expression.html">常量表达式</a>。</div>
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<div class="t-li1"><span class="t-li">2)</span> 不由构造函数的调用所初始化的静态<span class="t-rev-inl t-since-cxx11"><span>或线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><span class="t-rev-inl t-until-cxx11"><span><a href="cpp-named_req-PODType.html"><span style="font-style:italic">简旧数据类型</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(PODType)</span></a></span> <span><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></span></span>对象（不必为类类型），如果对象被<a href="cpp-language-value_initialization.html">值初始化</a>，或者如果它的初始化器中的每个表达式均为常量表达式。</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<div class="t-li1"><span class="t-li">3)</span> 由构造函数的调用所初始化的类类型的静态或线程局部对象，如果构造函数是 <a href="cpp-language-constexpr.html">constexpr</a> 且所有构造函数实参（包含隐式转换）都是<a href="cpp-language-constant_expression.html">常量表达式</a>，且如果构造函数的初始化器列表中的各初始化器，以及各类成员的花括号或等号初始化器都仅含有常量表达式。</div>
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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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<p>常量初始化的效果与其所对应的初始化的效果相同，但保证它在任何其他静态<span class="t-rev-inl t-since-cxx11"><span>或线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对象的初始化前完成，并可能在编译时进行。</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>如果编译器可以保证其值与当遵循初始化的标准顺序时的结果相同，那么允许它用常量初始化对其他的静态<span class="t-rev-inl t-since-cxx11"><span>及线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>对象进行初始化。</p>
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<p>实践中，常量初始化在编译时进行，并将预先计算的对象表示作为程序映像的一部分（如 <code>.data</code> 段）存储。如果变量既为 <code>const</code> 又被常量初始化，那么它的对象表示可存储于程序映像的只读段（如 <code>.rodata</code> 段）。</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <array></span>
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<span class="kw1">struct</span> S <span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> c<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">int</span> d <span class="sy1">=</span> <span class="nu0">10</span> <span class="sy2">*</span> S<span class="sy4">::</span><span class="me2">c</span><span class="sy4">;</span> <span class="co1">// 非常量表达式：此 S::c 前没有初始化器，此初始化发生晚于常量初始化</span>
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<span class="kw4">const</span> <span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">c</span> <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span> <span class="co1">// 常量初始化，保证首先发生</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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"d = "</span> <span class="sy1"><<</span> d <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, S<span class="sy4">::</span><span class="me2">c</span><span class="sy1">></span> a1<span class="sy4">;</span> <span class="co1">// OK：S::c 是常量表达式</span>
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<span class="co1">// std::array<int, d> a2; // 错误：d 不是常量表达式</span>
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<span class="br0">}</span></pre></div>
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</div>
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<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">d = 50</pre></div>
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</div>
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</div>
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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/issue441">CWG 441</a></td>
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<td>C++98</td>
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<td>不能常量初始化引用</td>
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<td>使得可常量初始化</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/issue2026">CWG 2026</a></td>
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<td>C++98</td>
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<td>曾指定始终首先进行零初始化，甚至先于常量初始化</td>
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<td>如果适用常量初始化则无零初始化</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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<ul>
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<li><a href="cpp-language-constexpr.html"><code>constexpr</code></a></li>
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<li><a href="cpp-language-constructor.html">构造函数</a></li>
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<li><a href="cpp-language-converting_constructor.html">转换构造函数</a></li>
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<li><a href="cpp-language-copy_constructor.html">复制构造函数</a></li>
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<li><a href="cpp-language-default_constructor.html">默认构造函数</a></li>
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<li><a href="cpp-language-explicit.html">explicit</a></li>
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<li><a href="cpp-language-initialization.html">初始化</a>
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<ul>
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<li><a href="cpp-language-aggregate_initialization.html">聚合初始化</a></li>
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<li><a href="cpp-language-copy_initialization.html">复制初始化</a></li>
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<li><a href="cpp-language-default_initialization.html">默认初始化</a></li>
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<li><a href="cpp-language-direct_initialization.html">直接初始化</a></li>
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<li><a href="cpp-language-list_initialization.html">列表初始化</a></li>
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<li><a href="cpp-language-reference_initialization.html">引用初始化</a></li>
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<li><a href="cpp-language-value_initialization.html">值初始化</a></li>
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<li><a href="cpp-language-zero_initialization.html">零初始化</a></li>
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</ul>
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</li>
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<li><a href="cpp-language-move_constructor.html">移动构造函数</a></li>
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</ul>
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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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