224 lines
13 KiB
HTML
224 lines
13 KiB
HTML
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<title>std::basic_format_arg</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_format_basic_format_arg 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>basic_format_arg</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>定义于头文件 <code><a href="cpp-header-format.html"><format></a></code></div>
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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> Context<span class="sy1">></span><br />
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<span class="kw1">class</span> basic_format_arg<span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<p>提供到格式化参数的访问。</p>
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<p><code>basic_format_arg</code> 对象通常由 <span class="t-lc">std::make_format_args</span> 创建并通过 <span class="t-lc">std::visit_format_arg</span> 访问。</p>
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<p><code>basic_format_arg</code> 对象表现为如同存储一个下列类型的 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> ：</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-variant-monostate.html"><span class="kw1095">std::<span class="me2">monostate</span></span></a></span></span> （仅若对象为默认构造的）</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">Context<span class="sy4">::</span><span class="me2">char_type</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">int</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">long</span> <span class="kw4">int</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</span> <span class="kw4">int</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span></li>
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<li><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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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> Context<span class="sy4">::</span><span class="me2">char_type</span><span class="sy2">*</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-string-basic_string_view.html"><span class="kw1246">std::<span class="me2">basic_string_view</span></span></a><span class="sy1"><</span>Context<span class="sy4">::</span><span class="me2">char_type</span><span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">basic_format_arg<span class="sy4">::</span><span class="me2">handle</span></span></span></li>
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</ul>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB">成员类</span></h3>
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<table class="t-dsc-begin">
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<td>类型</td>
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<td>定义</td>
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>handle</span></span></div>
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</div>
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</td>
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<td>允许格式化用户定义类型的对象的类型擦除包装<br />
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<span class="t-mark">(类)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><span class="t-lines"><span>(构造函数)</span></span></div>
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</div>
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</td>
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<td>构造 <code>std::basic_format_arg</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>operator bool</span></span></div>
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</div>
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</td>
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<td>检查当前对象是否保有格式化参数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
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<div><a href="cpp-utility-format-visit_format_arg.html"><span class="t-lines"><span>visit_format_arg</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>用户定义格式化器的参数观览接口<br />
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<span class="t-mark">(函数模板)</span></td>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_format_arg::handle"><span style="font-size:0.7em; line-height:130%">std::basic_format_arg::</span>handle</span></h2>
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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> Context<span class="sy1">></span><br />
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<span class="kw1">class</span> basic_format_arg<span class="sy1"><</span>Context<span class="sy1">></span><span class="sy4">::</span><span class="me2">handle</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<p>允许格式化用户定义类型的对象。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0_2">成员函数</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><span class="t-lines"><span>format</span></span></div>
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</td>
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<td>以给定的环境格式化被引用对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_format_arg::handle::format"><span style="font-size:0.7em; line-height:130%">std::basic_format_arg::handle::</span>format</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> format<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">basic_format_parse_context</span><span class="sy1"><</span>Context<span class="sy4">::</span><span class="me2">char_type</span><span class="sy1">></span><span class="sy3">&</span> parse_ctx,<br />
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Context<span class="sy3">&</span> format_ctx<span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<p>令 <code>T</code> 为关联的格式化参数的类型， <code>ref</code> 为指代该格式化参数的 <code>const T&</code> 。等价于：</p>
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<p><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> Context<span class="sy4">::</span><span class="kw1">template</span> formatter_type<span class="sy1"><</span>T<span class="sy1">></span> f<span class="sy4">;</span><br />
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parse_ctx.<span class="me1">advance_to</span><span class="br0">(</span>f.<span class="me1">parse</span><span class="br0">(</span>parse_ctx<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span><br />
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format_ctx.<span class="me1">advance_to</span><span class="br0">(</span>f.<span class="me1">format</span><span class="br0">(</span>ref, format_ctx<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></span></p>
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</div>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_format_arg::basic_format_arg"><span style="font-size:0.7em; line-height:130%">std::basic_format_arg::</span>basic_format_arg</span></h2>
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<div><span class="mw-geshi cpp source-cpp">basic_format_arg<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>默认构造函数。构造不保有格式化参数的 <code>basic_format_arg</code> 。存储的对象拥有 <span class="t-lc"><a href="cpp-utility-variant-monostate.html">std::monostate</a></span> 类型。</p>
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<p>为创建保有格式化参数的 <code>basic_format_arg</code> ，必须使用 <span class="t-lc">std::make_format_args</span> 。</p>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_format_arg::operator_bool"><span style="font-size:0.7em; line-height:130%">std::basic_format_arg::</span>operator bool</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>检查 <code>*this</code> 是否保有格式化参数。</p>
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<p>若 <code>*this</code> 保有格式化参数（即存储的对象不拥有 <span class="t-lc"><a href="cpp-utility-variant-monostate.html">std::monostate</a></span> 类型）则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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</div>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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="visualClear"></div>
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