204 lines
18 KiB
HTML
204 lines
18 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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<title>std::scalbn, std::scalbnf, std::scalbnl, std::scalbln, std::scalblnf, std::scalblnl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_math_scalbn 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>scalbn, <span style="font-size:0.7em; line-height:130%">std::</span>scalbnf, <span style="font-size:0.7em; line-height:130%">std::</span>scalbnl, <span style="font-size:0.7em; line-height:130%">std::</span>scalbln, <span style="font-size:0.7em; line-height:130%">std::</span>scalblnf, <span style="font-size:0.7em; line-height:130%">std::</span>scalblnl</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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<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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> scalbn <span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">float</span> scalbnf<span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">int</span> exp <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><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> scalbn <span class="br0">(</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <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><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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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">long</span> <span class="kw4">double</span> scalbn <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">long</span> <span class="kw4">double</span> scalbnl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <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><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> scalbn <span class="br0">(</span> IntegralType x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(4)</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 t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> scalbln <span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">float</span> scalblnf<span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(5)</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 t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> scalbln <span class="br0">(</span> <span class="kw4">double</span> x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(6)</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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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> scalbln <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">long</span> <span class="kw4">double</span> scalblnl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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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>
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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> scalbln <span class="br0">(</span> IntegralType x, <span class="kw4">long</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(8)</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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<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">1-3,5-7)</span> 将浮点值 <code>arg</code> 乘以 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 的 <code>exp</code> 次幂。</div>
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<div class="t-li1"><span class="t-li">4,8)</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">(2)</span> 或 <span class="t-v">(6)</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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<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>浮点值</td>
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</tr>
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<tr class="t-par">
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<td>exp</td>
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<td>-</td>
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<td>整数值</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>若不出现错误，则返回 <code>arg</code> 乘 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 的 <code>exp</code> 次幂（ <span class="texhtml" style="white-space: nowrap;">arg×FLT_RADIX<span class="t-su">exp<br /></span></span> ）。</p>
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<p>若出现上溢所致的值域错误，则返回 <code>±HUGE_VAL</code> 、 <code>±HUGE_VALF</code> 或 <code>±HUGE_VALL</code> 。</p>
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<p>若出现下溢所致的值域错误，则返回（舍入后的）正确结果。</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>报告 <span class="t-lc"><a href="cpp-numeric-math-math_errhandling.html">math_errhandling</a></span> 中指定的错误。</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>决不引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_INEXACT</a></span> ，除非出现值域错误（结果准确）</li>
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<li>忽略<a href="cpp-numeric-fenv-FE_round.html">当前舍入模式</a>，除非出现值域错误</li>
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<li>若 <code>arg</code> 为 ±0 ，则返回不修改的参数</li>
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<li>若 <code>arg</code> 为 ±∞ ，则返回不修改的参数</li>
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<li>若 <code>exp</code> 为 0 ，则返回不修改的 <code>arg</code></li>
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<li>若 <code>arg</code> 为 NaN ，则返回 NaN</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>二进制系统上（其中 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 为 <code>2</code> ）， <code>std::scalbn</code> 等价于 <code>std::ldexp</code> 。</p>
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<p>尽管指定 <code>std::scalbn</code> 和 <code>std::scalbln</code> 高效进行运算，多数实现上它们的效率低于用算术运算符乘或除以二的幂。</p>
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<p>函数名代表“新 scalb ”，其中 <code>scalb</code> 是旧的非标准函数名，其第二参数拥有浮点类型。</p>
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<p>提供 <code>scalbln</code> 函数，因为从最小正浮点值放大到最大正有限值的因子可能大于标准保证的 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 32767 。特别是对于 80 位 <code>long double</code> ，因子是 32828 。</p>
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<p>GNU 实现不设置 <code>errno</code> ，无关乎 <code>math_errhandling</code> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F_2">注意</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="co2">#include <cerrno></span>
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<span class="co2">#include <cstring></span>
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<span class="co2">#include <cfenv></span>
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<span class="co2">#pragma STDC FENV_ACCESS ON</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="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"scalbn(7, -4) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><span class="nu0">7</span>, <span class="sy2">-</span><span class="nu0">4</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">"scalbn(1, -1074) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><span class="nu0">1</span>, <span class="sy2">-</span><span class="nu0">1074</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" (minimum positive subnormal double)<span class="es1">\n</span>"</span>
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<span class="sy1"><<</span> <span class="st0">"scalbn(nextafter(1,0), 1024) = "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><a href="cpp-numeric-math-nextafter.html"><span class="kw1363">std::<span class="me2">nextafter</span></span></a><span class="br0">(</span><span class="nu0">1</span>,<span class="nu0">0</span><span class="br0">)</span>, <span class="nu0">1024</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" (largest finite double)<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="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"scalbn(-0, 10) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.0</span>, <span class="nu0">10</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">"scalbn(-Inf, -1) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><span class="sy2">-</span><a href="cpp-numeric-math-INFINITY.html"><span class="kw1375">INFINITY</span></a>, <span class="sy2">-</span><span class="nu0">1</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">// 错误处理</span>
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<span class="kw2">errno</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<a href="cpp-numeric-fenv-feclearexcept.html"><span class="kw1448">std::<span class="me2">feclearexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1459">FE_ALL_EXCEPT</span></a><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"scalbn(1, 1024) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scalbn</span><span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">1024</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="kw1">if</span> <span class="br0">(</span><span class="kw2">errno</span> <span class="sy1">==</span> <a href="cpp-error-errno_macros.html"><span class="kw862">ERANGE</span></a><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" errno == ERANGE: "</span> <span class="sy1"><<</span> <a href="cpp-string-byte-strerror.html"><span class="kw1161">std::<span class="me2">strerror</span></span></a><span class="br0">(</span><span class="kw2">errno</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="kw1">if</span> <span class="br0">(</span><a href="cpp-numeric-fenv-fetestexcept.html"><span class="kw1449">std::<span class="me2">fetestexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1463">FE_OVERFLOW</span></a><span class="br0">)</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" FE_OVERFLOW raised<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">scalbn(7, -4) = 0.4375
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scalbn(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
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scalbn(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
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scalbn(-0, 10) = -0
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scalbn(-Inf, -1) = -inf
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scalbn(1, 1024) = inf
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errno == ERANGE: Numerical result out of range
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FE_OVERFLOW raised</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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<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-frexp.html"><span class="t-lines"><span>frexp</span><span>frexpf</span><span>frexpl</span></span></a></div>
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<div><span class="t-lines"><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>将数分解为有效数字和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 的幂次<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-ldexp.html"><span class="t-lines"><span>ldexp</span><span>ldexpf</span><span>ldexpl</span></span></a></div>
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<div><span class="t-lines"><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>将数乘以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 的幂次<br />
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<span class="t-mark">(函数)</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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</html>
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