228 lines
20 KiB
HTML
228 lines
20 KiB
HTML
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_format_format_to_n 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>format_to_n</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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<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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> OutputIt, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>std<span class="sy4">::</span><span class="me2">format_to_n_result</span><span class="sy1"><</span>OutputIt<span class="sy1">></span><br />
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format_to_n<span class="br0">(</span> OutputIt out, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>OutputIt<span class="sy1">></span> n,<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>(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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> OutputIt, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>std<span class="sy4">::</span><span class="me2">format_to_n_result</span><span class="sy1"><</span>OutputIt<span class="sy1">></span><br />
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format_to_n<span class="br0">(</span> OutputIt out, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>OutputIt<span class="sy1">></span> n,<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>(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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> OutputIt, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>std<span class="sy4">::</span><span class="me2">format_to_n_result</span><span class="sy1"><</span>OutputIt<span class="sy1">></span><br />
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format_to_n<span class="br0">(</span> OutputIt out, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>OutputIt<span class="sy1">></span> n,<br /></p>
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<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, <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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> OutputIt, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p>std<span class="sy4">::</span><span class="me2">format_to_n_result</span><span class="sy1"><</span>OutputIt<span class="sy1">></span><br />
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format_to_n<span class="br0">(</span> OutputIt out, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>OutputIt<span class="sy1">></span> n,<br /></p>
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<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, <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 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> OutputIt <span class="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> format_to_n_result <span class="br0">{</span><br />
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OutputIt out<span class="sy4">;</span><br />
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std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>OutputIt<span class="sy1">></span> size<span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</span></div>
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</td>
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<td>(5)</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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<p>按照格式字符串 <code>fmt</code> 格式化 <code>args</code> ，并写结果到输出迭代器 <code>out</code> 。至多写 <code>n</code> 个字符。 <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>令 <code>CharT</code> 对重载 <span class="t-v">(1,3)</span> 为 <code>char</code> ，对重载 <code>wchar_t</code> 为 <span class="t-v">(2,4)</span> 。</p>
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<p>这些重载仅若 <code>OutputIt</code> 满足概念 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">output_iterator</span><span class="sy1"><</span><span class="kw4">const</span> CharT<span class="sy3">&</span><span class="sy1">></span></span></span> 才参与重载决议。若 <code>OutputIt</code> 不实现（满足其语义要求）概念 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">output_iterator</span><span class="sy1"><</span><span class="kw4">const</span> CharT<span class="sy3">&</span><span class="sy1">></span></span></span> ，或若对 <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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<div class="t-li1"><span class="t-li">5)</span> <code>std::format_to_n_result</code> 无基类或 <code>out</code>、 <code>size</code> 及隐式声明的特殊成员函数以外的成员。</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>out</td>
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<td>-</td>
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<td>指向输出缓冲区的迭代器</td>
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</tr>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>写入输出缓冲区的最大字符数</td>
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</tr>
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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><code>format_to_n_result</code> ，其 <code>out</code> 成员为指向输出范围末尾后一位置的迭代器，而 <code>size</code> 成员为总（不截断）的输出大小。</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>传播格式化器或迭代器操作所抛的任何异常。</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 <string_view></span>
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<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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<span class="kw4">char</span> buffer<span class="br0">[</span><span class="nu0">64</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> result <span class="sy1">=</span>
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std<span class="sy4">::</span><span class="me2">format_to_n</span><span class="br0">(</span>buffer, <a href="cpp-iterator-size.html"><span class="kw684">std::<span class="me2">size</span></span></a><span class="br0">(</span>buffer<span class="br0">)</span>,
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<span class="st0">"Hubble's H{0} {1} {2} miles/sec/mpc."</span>,
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<span class="st0">"<span class="es3">\u2080</span>"</span>, <span class="st0">"<span class="es3">\u2245</span>"</span>, <span class="nu0">42</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> <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, result.<span class="me1">size</span><span class="br0">}</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span><span class="es1">\n</span>"</span>
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<span class="sy1"><<</span> <span class="st0">"Buffer size = "</span> <span class="sy1"><<</span> <a href="cpp-iterator-size.html"><span class="kw684">std::<span class="me2">size</span></span></a><span class="br0">(</span>buffer<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">"Untruncated output size = "</span> <span class="sy1"><<</span> result.<span class="me1">size</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>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">Buffer: "Hubble's H₀ ≅ 42 miles/sec/mpc."
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Buffer size = 64
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Untruncated output size = 35</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.html"><span class="t-lines"><span>format</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>在新 string 中存储参数的格式化表示<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.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-formatted_size.html"><span class="t-lines"><span>formatted_size</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>确定存储其参数的格式化表示所需的字符数<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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