256 lines
15 KiB
HTML
256 lines
15 KiB
HTML
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<title>std::abs(float), std::fabs, std::fabsf, std::fabsl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_math_fabs 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>abs<span style="font-size:0.7em; line-height:130%">(float)</span>, <span style="font-size:0.7em; line-height:130%">std::</span>fabs, <span style="font-size:0.7em; line-height:130%">std::</span>fabsf, <span style="font-size:0.7em; line-height:130%">std::</span>fabsl</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>‎ | <a href="cpp-numeric-math.html">math</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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<td>
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<div>定义于头文件 <code><a href="cpp-header-cmath.html"><cmath></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-dsc-header">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> abs<span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td class="t-dcl-nopad"></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> abs<span class="br0">(</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td class="t-dcl-nopad"></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> abs<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td class="t-dcl-nopad"></td>
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</tr>
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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-cmath.html"><cmath></a></code></div>
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</td>
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<td></td>
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<td></td>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(4)</td>
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<td></td>
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<tr class="t-dcl">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> fabs <span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> fabsf<span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> fabs <span class="br0">(</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(5)</td>
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<td class="t-dcl-nopad"></td>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td></td>
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<td rowspan="142">(6)</td>
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<td></td>
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<tr class="t-dcl">
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> fabs <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> fabsl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</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-cxx11">(C++11 起)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> fabs <span class="br0">(</span> IntegralType arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 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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<div class="t-li1"><span class="t-li">1-6)</span> 计算浮点值 <code>arg</code> 的绝对值。</div>
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<div class="t-li1"><span class="t-li">7)</span> 接受任何<a href="cpp-types-is_integral.html">整数类型</a>参数的重载集或函数模板。等价于 <span class="t-v">(5)</span> （将参数转型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span> ）。</div>
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<p>对于整数参数， <a href="cpp-numeric-math-abs.html"><code>std::abs</code> 的整数重载</a>更可能是较好的匹配。若以不能由<a href="cpp-language-implicit_conversion.html#.E6.95.B4.E5.9E.8B.E6.8F.90.E5.8D.87">整型提升</a>转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> 的无符号整型参数调用 <code>std::abs</code> ，则程序为谬构。</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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<tr class="t-par">
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<td>arg</td>
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<td>-</td>
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<td>浮点或<a href="cpp-types-is_integral.html">整数类型</a>值</td>
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</tr>
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</table>
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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>若成功，则返回 <code>arg</code> 的绝对值（ <code>|arg|</code> ）。值是准确的，且不依赖任何舍入模式。</p>
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<h3><span class="mw-headline" id=".E9.94.99.E8.AF.AF.E5.A4.84.E7.90.86">错误处理</span></h3>
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<p>此函数不受制于任何指定于 <span class="t-lc"><a href="cpp-numeric-math-math_errhandling.html">math_errhandling</a></span> 的错误条件。</p>
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<p>若实现支持 IEEE 浮点算术（ IEC 60559 ），则</p>
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<ul>
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<li>若参数为 ±0 ，则返回 +0</li>
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<li>若参数为 ±∞ ，则返回 +∞</li>
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<li>若参数为 NaN ，则返回 NaN</li>
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</ul>
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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 <cmath></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="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"abs(+3.0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">abs</span><span class="br0">(</span><span class="sy2">+</span><span class="nu16">3.0</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"abs(-3.0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">abs</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">3.0</span><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="co1">// 特殊值</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"abs(-0.0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">abs</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.0</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"abs(-Inf) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">abs</span><span class="br0">(</span><span class="sy2">-</span><a href="cpp-numeric-math-INFINITY.html"><span class="kw1376">INFINITY</span></a><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"abs(-NaN) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">abs</span><span class="br0">(</span><span class="sy2">-</span><a href="cpp-numeric-math-NAN.html"><span class="kw1377">NAN</span></a><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></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">abs(+3.0) = 3
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abs(-3.0) = 3
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abs(-0.0) = 0
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abs(-Inf) = inf
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abs(-NaN) = nan</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://cplusplus.github.io/LWG/issue2192">LWG 2192</a></td>
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<td>C++98</td>
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<td><code>std::abs</code> 的重载不一致地声明于二个头文件</td>
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<td>在两个头文件中都声明这些重载</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2735">LWG 2735</a></td>
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<td>C++11</td>
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<td>错误地要求了 <code>std::abs</code> 的对整数类型返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span> 的重载</td>
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<td>移除该要求</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-abs.html"><span class="t-lines"><span>abs<span class="t-dsc-small">(int)</span></span><span>labs</span><span>llabs</span></span></a></div>
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<div><span class="t-lines"><span></span><span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>计算整数值的绝对值（ <span class="mjax" style="display:none">\(\small{|x|}\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">|x|</span> ）<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-copysign.html"><span class="t-lines"><span>copysign</span><span>copysignf</span><span>copysignl</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>复制浮点值的符号<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-signbit.html"><span class="t-lines"><span>signbit</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>检查给定数是否为负<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-complex-abs.html"><span class="t-lines"><span>abs<span class="t-dsc-small">(std::complex)</span></span></span></a></div>
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</div>
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</td>
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<td>返回复数的模<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-valarray-abs.html"><span class="t-lines"><span>abs<span class="t-dsc-small">(std::valarray)</span></span></span></a></div>
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</div>
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</td>
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<td>应用函数 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">abs</span></span></span> 到 valarray 的每个元素<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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</div>
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</div>
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</body>
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