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138 lines
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<title>std::bit_ceil</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_bit_ceil 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_ceil</h1>
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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>定义于头文件 <code><a href="cpp-header-bit.html"><bit></a></code></div>
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<td></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_ceil<span class="br0">(</span>T x<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>不小于 <code>x</code> 的最小的二的整数次幂。</p>
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<p>若值不能以 <code>T</code> 表示，则行为未定义。仅若不出现未定义行为才容许在<a href="cpp-language-constant_expression.html">常量求值</a>中调用此函数。</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=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>x</td>
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<td>-</td>
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<td>无符号整数类型的值</td>
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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>不小于 <code>x</code> 的最小的二的整数次幂。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>不抛出。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<p>见 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/62c25d7adb1a5664982449dda0e7f9ca63cf4735/libstdc%2B%2B-v3/include/std/bit#L217-L248">libstdc++ (gcc)</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/bit#L439-L456">libc++ (clang)</a> 中的可能实现。</p>
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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>std<span class="sy4">::</span><span class="me2">unsigned_integral</span> T<span class="sy1">></span>
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requires <span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, <span class="kw4">bool</span><span class="sy1">></span> <span class="sy3">&&</span> <span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, <span class="kw4">char</span><span class="sy1">></span> <span class="sy3">&&</span>
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<span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, char8_t<span class="sy1">></span> <span class="sy3">&&</span> <span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, <span class="kw4">char16_t</span><span class="sy1">></span> <span class="sy3">&&</span>
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<span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, <span class="kw4">char32_t</span><span class="sy1">></span> <span class="sy3">&&</span> <span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, <span class="kw4">wchar_t</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> T bit_ceil<span class="br0">(</span>T x<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>x <span class="sy1"><=</span> 1u<span class="br0">)</span>
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<span class="kw1">return</span> T<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>T, decltype<span class="br0">(</span><span class="sy2">+</span>x<span class="br0">)</span><span class="sy1">></span><span class="br0">)</span>
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<span class="kw1">return</span> T<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">bit_width</span><span class="br0">(</span>T<span class="br0">(</span>x <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="br0">{</span> <span class="co1">// 对于要经受整数提升的类型</span>
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<span class="kw4">constexpr</span> <span class="kw4">int</span> offset_for_ub <span class="sy1">=</span>
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<a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><span class="kw4">unsigned</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">digits</span> <span class="sy2">-</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="sy4">;</span>
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<span class="kw1">return</span> T<span class="br0">(</span>1u <span class="sy1"><<</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">bit_width</span><span class="br0">(</span>T<span class="br0">(</span>x <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">+</span> offset_for_ub<span class="br0">)</span> <span class="sy1">>></span> offset_for_ub<span class="br0">)</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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<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">using</span> bin <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">8</span><span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">unsigned</span> x<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span> x <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">10</span><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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<span class="kw4">auto</span> <span class="kw4">const</span> z <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">bit_ceil</span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// P1956R1 前为 `ceil2`</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">"bit_ceil("</span> <span class="sy1"><<</span> bin<span class="br0">(</span>x<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">") = "</span> <span class="sy1"><<</span> bin<span class="br0">(</span>z<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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<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_ceil(00000000) = 00000001
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bit_ceil(00000001) = 00000001
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bit_ceil(00000010) = 00000010
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bit_ceil(00000011) = 00000100
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bit_ceil(00000100) = 00000100
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bit_ceil(00000101) = 00001000
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bit_ceil(00000110) = 00001000
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bit_ceil(00000111) = 00001000
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bit_ceil(00001000) = 00001000
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bit_ceil(00001001) = 00010000</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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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-bit_floor.html"><span class="t-lines"><span>bit_floor</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>寻找不大于给定值的最大的二的整数次幂<br />
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<span class="t-mark">(函数模板)</span></td>
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