126 lines
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126 lines
12 KiB
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<title>std::fixed, std::scientific, std::hexfloat, std::defaultfloat</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>fixed, <span style="font-size:0.7em; line-height:130%">std::</span>scientific, <span style="font-size:0.7em; line-height:130%">std::</span>hexfloat, <span style="font-size:0.7em; line-height:130%">std::</span>defaultfloat</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-io.html">io</a>‎ | <a href="cpp-io-manip.html">manip</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ios.html"><ios></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> fixed<span class="br0">(</span> <a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> str <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> scientific<span class="br0">(</span> <a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> str <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> hexfloat<span class="br0">(</span> <a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> str <span class="br0">)</span><span class="sy4">;</span></span></div>
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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"><a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> defaultfloat<span class="br0">(</span> <a href="cpp-io-ios_base.html"><span class="kw1669">std::<span class="me2">ios_base</span></span></a><span class="sy3">&</span> str <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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<p>修改浮点输入/输出的默认格式化。</p>
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<div class="t-li1"><span class="t-li">1)</span> 如同以调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">str.<span class="me1">setf</span><span class="br0">(</span><a href="cpp-io-ios_base-fmtflags.html"><span class="kw1688">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">fixed</span></span></a>, <a href="cpp-io-ios_base-fmtflags.html"><span class="kw1689">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">floatfield</span></span></a><span class="br0">)</span></span></span> ，设置流 <code>str</code> 的 <code>floatfield</code> 为 <code>fixed</code></div>
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<div class="t-li1"><span class="t-li">2)</span> 如同以调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">str.<span class="me1">setf</span><span class="br0">(</span><a href="cpp-io-ios_base-fmtflags.html"><span class="kw1687">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">scientific</span></span></a>, <a href="cpp-io-ios_base-fmtflags.html"><span class="kw1689">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">floatfield</span></span></a><span class="br0">)</span></span></span> ，设置流 <code>str</code> 的 <code>floatfield</code> 为 <code>scientific</code></div>
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<div class="t-li1"><span class="t-li">3)</span> 如同以调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">str.<span class="me1">setf</span><span class="br0">(</span><a href="cpp-io-ios_base-fmtflags.html"><span class="kw1688">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">fixed</span></span></a> <span class="sy3">|</span> <a href="cpp-io-ios_base-fmtflags.html"><span class="kw1687">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">scientific</span></span></a>, <a href="cpp-io-ios_base-fmtflags.html"><span class="kw1689">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">floatfield</span></span></a><span class="br0">)</span></span></span> ，设置流 <code>str</code> 的 <code>floatfield</code> 同时为 <code>fixed</code> 和 <code>scientific</code> 。这启用十六进制浮点格式化。</div>
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<div class="t-li1"><span class="t-li">4)</span> 如同以调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">str.<span class="me1">unsetf</span><span class="br0">(</span><a href="cpp-io-ios_base-fmtflags.html"><span class="kw1689">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">floatfield</span></span></a><span class="br0">)</span></span></span> ，设置流 <code>str</code> 的 <code>floatfield</code> 为零，这异于 fixed 和 scientific 。</div>
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<p>这是一个 I/O 操纵符，可用如 <span class="t-c"><span class="mw-geshi cpp source-cpp">out <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">fixed</span></span></span> 的表达式对任何 <span class="t-lc"><a href="cpp-io-basic_ostream.html">std::basic_ostream</a></span> 类型的 <code>out</code> 或用如 <span class="t-c"><span class="mw-geshi cpp source-cpp">in <span class="sy1">>></span> std<span class="sy4">::</span><span class="me2">scientific</span></span></span> 的表达式对任何 <span class="t-lc"><a href="cpp-io-basic_istream.html">std::basic_istream</a></span> 类型的 <code>in</code> 调用。</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>str</td>
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<td>-</td>
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<td>到 I/O 流的引用</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><code>str</code> （到操纵后的流的引用）</p>
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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>十六进制浮点格式化忽略流精度规定，如 <span class="t-lc"><a href="cpp-locale-num_put-put.html">std::num_put::do_put</a></span> 的规定所要求。</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 <iostream></span>
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<span class="co2">#include <sstream></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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The number 0.01 in fixed: "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">fixed</span> <span class="sy1"><<</span> <span class="nu16">0.01</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">"The number 0.01 in scientific: "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">scientific</span> <span class="sy1"><<</span> <span class="nu16">0.01</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">"The number 0.01 in hexfloat: "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">hexfloat</span> <span class="sy1"><<</span> <span class="nu16">0.01</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">"The number 0.01 in default: "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">defaultfloat</span> <span class="sy1"><<</span> <span class="nu16">0.01</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">double</span> f<span class="sy4">;</span>
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<a href="cpp-io-basic_istringstream.html"><span class="kw1735">std::<span class="me2">istringstream</span></span></a><span class="br0">(</span><span class="st0">"0x1P-1022"</span><span class="br0">)</span> <span class="sy1">>></span> std<span class="sy4">::</span><span class="me2">hexfloat</span> <span class="sy1">>></span> f<span class="sy4">;</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">"Parsing 0x1P-1022 as hex gives "</span> <span class="sy1"><<</span> f <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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<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">The number 0.01 in fixed: 0.010000
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The number 0.01 in scientific: 1.000000e-02
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The number 0.01 in hexfloat: 0x1.47ae147ae147bp-7
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The number 0.01 in default: 0.01
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Parsing 0x1P-1022 as hex gives 2.22507e-308</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-io-manip-setprecision.html"><span class="t-lines"><span>setprecision</span></span></a></div>
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</div>
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</td>
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<td>更改浮点精度<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<div class="visualClear"></div>
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