202 lines
18 KiB
HTML
202 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::formatted_size</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_format_formatted_size 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"><span style="font-size:0.7em; line-height:130%">std::</span>formatted_size</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-utility.html">utility</a>‎ | <a href="cpp-utility-format.html">format</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-format.html"><format></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-dcl t-since-cxx20">
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<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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> formatted_size<span class="br0">(</span> <span class="coMULTI">/*format_string<Args...>*/</span> fmt, <span class="kw4">const</span> Args<span class="sy3">&</span>... <span class="me1">args</span> <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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> formatted_size<span class="br0">(</span> <span class="coMULTI">/*wformat_string<Args...>*/</span> fmt, <span class="kw4">const</span> Args<span class="sy3">&</span>... <span class="me1">args</span> <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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> formatted_size<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-locale-locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="sy3">&</span> loc,<br /></p>
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<span class="coMULTI">/*format_string<Args...>*/</span> fmt, <span class="kw4">const</span> Args<span class="sy3">&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> formatted_size<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-locale-locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="sy3">&</span> loc,<br /></p>
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<span class="coMULTI">/*wformat_string<Args...>*/</span> fmt, <span class="kw4">const</span> Args<span class="sy3">&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 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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<td></td>
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</tr>
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</tbody>
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</table>
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<p>确定按照格式字符串 <code>fmt</code> 格式化 <code>args</code> 得到的有格式字符串的总字符数。 <code>loc</code> 若存在则用于本地环境特定的格式化。</p>
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<p>若对 <code>Args</code> 中任何 <code>Ti</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">formatter</span><span class="sy1"><</span>Ti, CharT<span class="sy1">></span></span></span> 不满足<a href="cpp-named_req-Formatter.html"><span style="font-style:italic">格式化器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Formatter)</span></a> 要求则行为未定义。</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>fmt</td>
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<td>-</td>
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<td>
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<div class="t-template-editlink" style=""></div>
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未指定类型的形参，其初始化仅若实参可转换成 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::string_view</a></span> （对于 <span class="t-v">(1,3)</span> ）或 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::wstring_view</a></span> （对于 <span class="t-v">(2,4)</span> ），而转换结果是常量表达式和 <code>Args</code> 的合法格式字符串才合法。格式字符串由以下内容组成：
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<ul>
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<li>通常字符（除了 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">}</span></span></span> ），它们被不加修改地复制到输出，</li>
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<li>转义序列 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span><span class="br0">{</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">}</span><span class="br0">}</span></span></span> ，它们在输出中被分别替换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">}</span></span></span> ，以及</li>
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<li>替换域。</li>
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</ul>
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<p>每个替换域拥有下列格式：</p>
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<ul>
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<li>引入的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span></span></span> 字符；</li>
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<li><span class="t-mark">(可选)</span> <span class="t-spar">arg-id</span> ，一个非负数；</li>
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<li><span class="t-mark">(可选)</span> 冒号（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">:</span></span></span> ）后随格式说明；</li>
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<li>终止的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">}</span></span></span> 字符。</li>
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</ul>
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<p><span class="t-spar">arg-id</span> 指定用于格式化其值的 <code>args</code> 中的参数的下标；若省略 <span class="t-spar">arg-id</span> ，则按顺序使用参数。格式字符串中的 <span class="t-spar">arg-id</span> 必须全部存在或全部被省略。混合手动和自动指定下标是错误。</p>
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<p>格式说明由对应参数特化的 <span class="t-lc">std::formatter</span> 定义。</p>
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<ul>
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<li>对于基本类型和标准字符串类型，格式说明为<a href="cpp-utility-format-formatter.html#.E6.A0.87.E5.87.86.E6.A0.BC.E5.BC.8F.E8.AF.B4.E6.98.8E">标准格式说明</a>；</li>
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<li>对于标准日期和时间类型，格式说明为 <a href="cpp-chrono-system_clock-formatter.html#.E6.A0.BC.E5.BC.8F.E8.AF.B4.E6.98.8E">chrono 格式说明</a>；</li>
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<li>对于用户定义类型，格式说明由用户定义的 <span class="t-lc">std::formatter</span> 特化决定。</li>
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</ul>
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</td>
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</tr>
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<tr class="t-par">
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<td>args...</td>
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<td>-</td>
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<td>要格式化的参数</td>
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</tr>
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<tr class="t-par">
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<td>loc</td>
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<td>-</td>
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<td>用于本地环境特定的格式化的 <span class="t-lc"><a href="cpp-locale-locale.html">std::locale</a></span></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>有格式字符串中的总字符数。</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>传播格式化器所抛的任何异常。</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 <format></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <string_view></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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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">::</span><span class="me2">string_view_literals</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> fmt_str <span class="br0">{</span> <span class="st0">"Hubble's H{0} {1} {2:*^4} miles/sec/mpc."</span>sv <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> sub_zero <span class="br0">{</span> <span class="st0">"₀"</span>sv <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// { "\u2080"sv } => { 0xe2, 0x82, 0x80 };</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> aprox_equ <span class="br0">{</span> <span class="st0">"≅"</span>sv <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// { "\u2245"sv } => { 0xe2, 0x89, 0x85 };</span>
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<span class="kw4">constexpr</span> <span class="kw4">int</span> Ho <span class="br0">{</span> <span class="nu0">42</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// H₀</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> min_buffer_size <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">formatted_size</span><span class="br0">(</span>fmt_str, sub_zero, aprox_equ, Ho<span class="br0">)</span><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">"Min buffer size = "</span> <span class="sy1"><<</span> min_buffer_size <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// 以 std::vector 为动态缓冲区。注：缓冲区不包含尾随 '\0' 。</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span> buffer<span class="br0">(</span>min_buffer_size<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">format_to_n</span><span class="br0">(</span>buffer.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span>, buffer.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span>, fmt_str, sub_zero, aprox_equ, Ho<span class="br0">)</span><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">"Buffer: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> <a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a><span class="br0">{</span>buffer.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span>, min_buffer_size<span class="br0">}</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 或者我们能通过添加尾随 '\0' 直接打印缓冲区。</span>
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buffer.<span class="me1">push_back</span><span class="br0">(</span><span class="st0">'<span class="es5">\0</span>'</span><span class="br0">)</span><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">"Buffer: <span class="es1">\"</span>"</span> <span class="sy1"><<</span> buffer.<span class="me1">data</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><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>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">Min buffer size = 37
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Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."
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Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."</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://wg21.link/P2216R3">P2216R3</a></td>
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<td>C++20</td>
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<td>对非法格式字符串抛出 <span class="t-lc">std::format_error</span></td>
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<td>非法格式字符串导致编译时错误</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-utility-format-format_to.html"><span class="t-lines"><span>format_to</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>
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<td>通过输出迭代器写其参数的格式化表示<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-utility-format-format_to_n.html"><span class="t-lines"><span>format_to_n</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>
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<td>通过输出迭代器写其参数的格式化表示，不超出指定的大小<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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</html>
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