139 lines
13 KiB
HTML
139 lines
13 KiB
HTML
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<title>std::bit_cast</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_bit_cast skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>bit_cast</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-numeric.html">numeric</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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<table class="t-dcl-begin">
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-bit.html"><bit></a></code></div>
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</td>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> To, <span class="kw1">class</span> From <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> To bit_cast<span class="br0">(</span> <span class="kw4">const</span> From<span class="sy3">&</span> from <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>通过转译 <code>from</code> 的对象表示获得 <code>To</code> 类型值。返回的 <code>To</code> 对象值表示中的每位都等于 <code>from</code> 的对象表示中的对应位。返回的 <code>To</code> 对象中的填充位的值未指定。</p>
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<p>若无 <code>To</code> 类型值对应产生的值表示，则行为未定义。若有多个这种值，则产生哪个值是未指定的。</p>
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<p>结果的值表示中的一个位为<i>不确定</i>，若它</p>
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<ul>
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<li>不对应 <code>From</code> 的值表示中的位（即它对应填充位），或</li>
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<li>对应一个不在其<a href="cpp-language-lifetime.html">生存期</a>内的对象的位，或</li>
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<li>拥有<a href="cpp-language-default_initialization.html">不确定值</a>。</li>
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</ul>
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<p>对于每个结果的值表示中的不确定位，含该位的最小对象拥有不确定值；若该对象不拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 或 <span class="t-lc">std::byte</span> 类型则行为未定义。其他情况下结果不含不确定值。</p>
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<p>此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof<span class="br0">(</span>To<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>From<span class="br0">)</span></span></span> 且 <code>To</code> 和 <code>From</code> 两者都是<a href="cpp-named_req-TriviallyCopyable.html"><span style="font-style:italic">可平凡复制</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span></a> 类型才参与重载决议。</p>
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<p>此函数模板为 <code>constexpr</code> ，若且唯若 <code>To</code>、 <code>From</code> 和 <code>To</code> 及 <code>From</code> 的所有子对象的类型都：</p>
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<ul>
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<li>不是联合体类型；</li>
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<li>不是指针类型；</li>
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<li>不是指向成员指针类型；</li>
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<li>不是 volatile 限定类型；且</li>
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<li>无引用类型的非静态数据成员。</li>
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</ul>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>值表示描述如上的 <code>To</code> 类型对象。</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>大多数情况下不应该用指针或引用类型间的 <a href="cpp-language-reinterpret_cast.html"><tt>reinterpret_cast</tt></a> （或等价的<a href="cpp-language-explicit_cast.html">显式转型</a>）转译对象表示，因为<a href="cpp-language-reinterpret_cast.html#.E7.B1.BB.E5.9E.8B.E5.88.AB.E5.90.8D.E4.BD.BF.E7.94.A8">类型别名使用规则</a>。</p>
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<p>在 <code>std::bit_cast</code> 前，需要翻译对象表示为另一类型的对象表示时，能使用 <span class="t-lc"><a href="cpp-string-byte-memcpy.html">std::memcpy</a></span> ：</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">class</span> To, <span class="kw1">class</span> From<span class="sy1">></span>
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<span class="kw1">typename</span> <a href="cpp-types-enable_if.html"><span class="kw631">std::<span class="me2">enable_if_t</span></span></a><span class="sy1"><</span>
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sizeof<span class="br0">(</span>To<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>From<span class="br0">)</span> <span class="sy3">&&</span>
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<a href="cpp-types-is_trivially_copyable.html"><span class="kw501">std::<span class="me2">is_trivially_copyable_v</span></span></a><span class="sy1"><</span>From<span class="sy1">></span> <span class="sy3">&&</span>
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<a href="cpp-types-is_trivially_copyable.html"><span class="kw501">std::<span class="me2">is_trivially_copyable_v</span></span></a><span class="sy1"><</span>To<span class="sy1">></span>,
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To<span class="sy1">></span>
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<span class="co1">// constexpr 支持需要编译器魔法</span>
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bit_cast<span class="br0">(</span><span class="kw4">const</span> From<span class="sy3">&</span> src<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="co1">// 此实现要求 To 可平凡默认构造</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_constructible.html"><span class="kw523">std::<span class="me2">is_trivially_constructible_v</span></span></a><span class="sy1"><</span>To<span class="sy1">></span>,
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<span class="st0">"This implementation additionally requires destination type to be trivially constructible"</span><span class="br0">)</span><span class="sy4">;</span>
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To dst<span class="sy4">;</span>
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<a href="cpp-string-byte-memcpy.html"><span class="kw1159">std::<span class="me2">memcpy</span></span></a><span class="br0">(</span><span class="sy3">&</span>dst, <span class="sy3">&</span>src, sizeof<span class="br0">(</span>To<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> dst<span class="sy4">;</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=".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 <cstdint></span>
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<span class="co2">#include <bit></span>
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<span class="co2">#include <iostream></span>
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<span class="kw4">constexpr</span> <span class="kw4">double</span> f64v <span class="sy1">=</span> <span class="nu16">19880124.0</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> u64v <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bit_cast</span><span class="sy1"><</span><a href="cpp-types-integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a><span class="sy1">></span><span class="br0">(</span>f64v<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <a href="cpp-types-integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a> u64v2 <span class="sy1">=</span> 0x3fe9000000000000ull<span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> f64v2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bit_cast</span><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="br0">(</span>u64v2<span class="br0">)</span><span class="sy4">;</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>
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<span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span>f64v <span class="sy1"><<</span> <span class="st0">"f64.to_bits() == 0x"</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> u64v <span class="sy1"><<</span> <span class="st0">"u64<span class="es1">\n</span>"</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>
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<span class="sy1"><<</span> <span class="st0">"f64::from_bits(0x"</span> <span class="sy1"><<</span> <a href="cpp-io-manip-hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> u64v2 <span class="sy1"><<</span> <span class="st0">"u64) == "</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span> f64v2 <span class="sy1"><<</span> <span class="st0">"f64<span class="es1">\n</span>"</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>可能的输出：</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">19880124.000000f64.to_bits() == 0x4172f58bc0000000u64
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f64::from_bits(0x3fe9000000000000u64) == 0.781250f64</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/issue2482">CWG 2482</a></td>
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<td>C++20</td>
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<td>在涉及不确定位时未指定 UB 是否会出现</td>
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<td>已指定</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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