100 lines
7.0 KiB
HTML
100 lines
7.0 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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<title>std::bit_width</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-cpp_numeric_bit_width 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"><span style="font-size:0.7em; line-height:130%">std::</span>bit_width</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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<tbody>
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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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<td>
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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 <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T bit_width<span class="br0">(</span>T x<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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</tbody>
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</table>
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<p>若 <code>x</code> 非零，则计算存储值 <code>x</code> 所需的位数，即 <span class="mjax" style="display:none">\(1 + \lfloor \log_2(x) \rfloor\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">1 + floor(log<span class="t-su t-su-b"><br />
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2</span>(x))</span> 。若 <code>x</code> 为零，则返回零。</p>
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<p>此重载仅若 <code>T</code> 为无符号整数类型（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">short</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">int</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">long</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</span></span></span> 或扩展无符号整数类型）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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<p>若 <code>x</code> 为零则为零；否则为一加 <code>x</code> 的 2 为底的对数，舍弃任何小数部分。</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>此函数等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">digits</span> <span class="sy2">-</span> std<span class="sy4">::</span><span class="me2">countl_zero</span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span></span></span> 。</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 <bit></span>
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<span class="co2">#include <bitset></span>
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<span class="co2">#include <iostream></span>
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<span class="kw4">auto</span> main<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">int</span>
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<span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">unsigned</span> x<span class="br0">{</span>0u<span class="br0">}</span><span class="sy4">;</span> x <span class="sy3">!</span><span class="sy1">=</span> 8u<span class="sy4">;</span> <span class="sy2">++</span>x<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> <span class="st0">"bit_width( "</span>
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<span class="sy1"><<</span> <a href="cpp-utility-bitset.html"><span class="kw1027">std::<span class="me2">bitset</span></span></a><span class="sy1"><</span><span class="nu0">4</span><span class="sy1">></span><span class="br0">{</span>x<span class="br0">}</span> <span class="sy1"><<</span> <span class="st0">" ) = "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">bit_width</span><span class="br0">(</span>x<span class="br0">)</span> <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="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">bit_width( 0000 ) = 0
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bit_width( 0001 ) = 1
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bit_width( 0010 ) = 2
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bit_width( 0011 ) = 2
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bit_width( 0100 ) = 3
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bit_width( 0101 ) = 3
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bit_width( 0110 ) = 3
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bit_width( 0111 ) = 3</pre></div>
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</div>
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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">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-countl_zero.html"><span class="t-lines"><span>countl_zero</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>从最高位起计量连续的 0 位的数量<br />
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<span class="t-mark">(函数模板)</span></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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</body>
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