182 lines
16 KiB
HTML
182 lines
16 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>浮点字面量</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_floating_literal 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">浮点字面量</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-language.html">language</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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<p>浮点字面量定义了其值于源文件中指定的编译时常量。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><span class="t-spar">数字序列</span> <span class="t-spar">十进制指数</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-spar">数字序列</span> <code><b>.</b></code> <span class="t-spar">十进制指数</span><span class="t-mark">(可选)</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-spar">数字序列</span><span class="t-mark">(可选)</span> <code><b>.</b></code> <span class="t-spar">数字序列</span> <span class="t-spar">十进制指数</span><span class="t-mark">(可选)</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(3)</td>
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<td class="t-sdsc-nopad"></td>
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<td><code><b>0x</b></code> | <code><b>0X</b></code> <span class="t-spar">十六进制数字序列</span> <span class="t-spar">十六进制指数</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><code><b>0x</b></code> | <code><b>0X</b></code> <span class="t-spar">十六进制数字序列</span> <code><b>.</b></code> <span class="t-spar">十六进制指数</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(5)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><code><b>0x</b></code> | <code><b>0X</b></code> <span class="t-spar">十六进制数字序列</span><span class="t-mark">(可选)</span> <code><b>.</b></code> <span class="t-spar">十六进制数字序列</span> <span class="t-spar">十六进制指数</span> <span class="t-spar">后缀</span><span class="t-mark">(可选)</span></td>
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<td>(6)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> <span class="t-spar">数字序列</span> 表示无小数点的整数，此时指数不是可选的：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu19">1e10</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp">1e<span class="sy2">-</span>5L</span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-spar">数字序列</span> 表示带小数点的整数，此时指数是可选的：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span>.</span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu19">1.<span class="me1">e</span>-2</span></span></span></div>
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<div class="t-li1"><span class="t-li">3)</span> <span class="t-spar">数字序列</span> 表示小数。指数是可选的：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">3.14</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu17">.1f</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp">0.1e<span class="sy2">-</span>1L</span></span></div>
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<div class="t-li1"><span class="t-li">4)</span> 十六进制 <span class="t-spar">数字序列</span> 表示无基底分隔符的整数。十六进制浮点字面量的指数总是必选的：<span class="t-c"><span class="mw-geshi cpp source-cpp">0x1ffp10</span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp">0X0p<span class="sy2">-</span><span class="nu0">1</span></span></span></div>
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<div class="t-li1"><span class="t-li">5)</span> 十六进制 <span class="t-spar">数字序列</span> 表示带基底分隔符的整数。十六进制浮点字面量的指数总是必选的：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu12">0x1</span>.<span class="me1">p0</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu12">0xf</span>.<span class="me1">p</span><span class="sy2">-</span><span class="nu0">1</span></span></span></div>
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<div class="t-li1"><span class="t-li">6)</span> 十六进制 <span class="t-spar">数字序列</span> 表示带基底分隔符的小数。十六进制浮点字面量的指数总是必选的：<span class="t-c"><span class="mw-geshi cpp source-cpp">0x0.123p<span class="sy2">-</span><span class="nu0">1</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu12">0xa</span>.<span class="me1">bp10l</span></span></span></div>
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<p><span class="t-spar">十进制指数</span> 拥有下列形式</p>
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<table class="t-sdsc-begin">
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<td class="t-sdsc-nopad"><code><b>e</b></code> | <code><b>E</b></code> <span class="t-spar">指数正负号</span><span class="t-mark">(可选)</span> <span class="t-spar">数字序列</span></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<p><span class="t-spar">十六进制指数</span> 拥有下列形式</p>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>p</b></code> | <code><b>P</b></code> <span class="t-spar">指数正负号</span><span class="t-mark">(可选)</span> <span class="t-spar">数字序列</span></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<p><span class="t-spar">指数正负号</span> 若存在则为 <code><b>+</b></code> 或 <code><b>-</b></code>。</p>
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<p><span class="t-spar">后缀</span> 若存在，则为 <code><b>f</b></code>、<code><b>F</b></code>、<code><b>l</b></code> 或 <code><b>L</b></code> 之一。后缀决定浮点字面量的类型：</p>
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<dl>
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<dd>
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<ul>
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<li>（无后缀）定义 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span></li>
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<li><code><b>f F</b></code> 定义 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span></span></span></li>
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<li><code><b>l L</b></code> 定义 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span></span></span></li>
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</ul>
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</dd>
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</dl>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx14">
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<td>
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<p>数位间可插入作为分隔符的单引号（<code>'</code>），在编译时忽略它们。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>使用十进制科学计数法，表示浮点字面量的值是有效数字乘以 10 的 <span class="t-spar">十进制指数</span> 次幂。例如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu19">123e4</span></span></span> 的数学含义是 <i>123×10<sup>4</sup></i>。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<p>若浮点字面量以字符序列 <code>0x</code> 或 <code>0X</code> 开始，则该浮点字面量是<i>十六进制浮点字面量</i>。否则，它是<i>十进制浮点字面量</i>。</p>
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<p>对于<i>十六进制浮点字面量</i>，其有效数字被解释为十六进制有理数，而指数的 <span class="t-spar">数字序列</span> 被解释成有效数字要放大的 2 的整数幂次。</p>
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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="kw4">double</span> d <span class="sy1">=</span> 0x1.4p3<span class="sy4">;</span> <span class="co1">// 十六进制分数 1.4（十进制 1.25）的 2^3 倍，即 10.0</span></pre></div>
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</div>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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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>虽然十六进制浮点字面量直到 C++17 才成为（标准）C++ 的一部分，但是从 C++11 开始它们已经可以通过输入/输出函数被解析和打印：开启 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-manip-fixed.html"><span class="kw1786">std::<span class="me2">hexfloat</span></span></a></span></span> 的 C++ 输入/输出流和 C 输入/输出流：<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-c-fprintf.html"><span class="kw1845">std::<span class="me2">printf</span></span></a></span></span>，<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-c-fscanf.html"><span class="kw1839">std::<span class="me2">scanf</span></span></a></span></span> 等。格式描述请参考 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-string-byte-strtof.html"><span class="kw1135">std::<span class="me2">strtof</span></span></a></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 <iostream></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="nu0">58</span>. <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="nu19">4e2</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="nu19">123.456e-67</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> .1E4f <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> 0x10.1p0 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="sy1"><<</span> <span class="nu12">0x1e5</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 整数字面量，非浮点字面量</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">58
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400
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1.23456e-65
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1000
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16.0625
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485</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><a href="cpp-language-user_literal.html">用户定义字面量</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>拥有用户定义后缀的字面量</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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