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<title>std::as_bytes, std::as_writable_bytes</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>as_bytes, <span style="font-size:0.7em; line-height:130%">std::</span>as_writable_bytes</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-container.html">container</a>‎ | <a href="cpp-container-span.html">span</a></span></div>
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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> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span><br />
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std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">byte</span>, S<span class="coMULTI">/* see below */</span><span class="sy1">></span> as_bytes<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span>T, N<span class="sy1">></span> s<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N<span class="sy1">></span><br />
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std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">byte</span>, S<span class="coMULTI">/* see below */</span><span class="sy1">></span> as_writable_bytes<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span>T, N<span class="sy1">></span> s<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<p>获得对 span <code>s</code> 的元素的对象表示的视图。</p>
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<p>若 <code>N</code> 为 <code>std::dynamic_extent</code> ，则返回的 span <code>S</code> 的长度亦为 <code>std::dynamic_extent</code> ；否则它是 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof<span class="br0">(</span>T<span class="br0">)</span> <span class="sy2">*</span> N</span></span> 。</p>
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<p><code>as_writable_bytes</code> 仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_const.html"><span class="kw495">std::<span class="me2">is_const_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 才参与重载决议。</p>
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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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<div class="t-li1"><span class="t-li">1)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">byte</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>s.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, s.<span class="me1">size_bytes</span><span class="br0">(</span><span class="br0">)</span><span class="br0">}</span></span></span> 构造的 span 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">byte</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>s.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, s.<span class="me1">size_bytes</span><span class="br0">(</span><span class="br0">)</span><span class="br0">}</span></span></span> 构造的 span 。</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 <cstddef></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <span></span>
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<span class="kw4">void</span> print<span class="br0">(</span><span class="kw4">float</span> <span class="kw4">const</span> x, std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">byte</span><span class="sy1">></span> <span class="kw4">const</span> bytes<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> <a href="cpp-io-manip-setprecision.html"><span class="kw1796">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">6</span><span class="br0">)</span> <span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">8</span><span class="br0">)</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">" = { "</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> <a href="cpp-io-manip-uppercase.html"><span class="kw1774">std::<span class="me2">uppercase</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setfill.html"><span class="kw1795">std::<span class="me2">setfill</span></span></a><span class="br0">(</span><span class="st0">'0'</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="kw4">const</span> b <span class="sy4">:</span> bytes<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> <a href="cpp-io-manip-setw.html"><span class="kw1797">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">to_integer</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>b<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</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> <a href="cpp-io-manip-hex.html"><span class="kw1781">std::<span class="me2">dec</span></span></a> <span class="sy1"><<</span> <span class="st0">"}<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</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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<span class="coMULTI">/* mutable */</span> <span class="kw4">float</span> data<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span> <span class="br0">{</span> <span class="nu17">3.141592f</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> <span class="kw4">const</span> const_bytes <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">as_bytes</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">span</span><span class="br0">{</span> data <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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print<span class="br0">(</span>data<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, const_bytes<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> <span class="kw4">const</span> writable_bytes <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">as_writable_bytes</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">span</span><span class="br0">{</span> data <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 更改作为最高有效位的符号位（ IEEE 754 浮点标准）</span>
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writable_bytes<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy3">|</span><span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">byte</span><span class="br0">{</span> <span class="nu6">0b1000</span><span class="st0">'0000 };
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print(data[0], const_bytes);
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}</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">
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3.14159 = { D8 0F 49 40 }
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-3.14159 = { D8 0F 49 C0 }</pre></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">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-types-byte.html"><span class="t-lines"><span>byte</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>字节类型<br />
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<span class="t-mark">(枚举)</span></td>
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<div class="visualClear"></div>
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</body>
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</html>
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