112 lines
8.6 KiB
HTML
112 lines
8.6 KiB
HTML
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<title>std::literals::chrono_literals::operator""y</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_chrono_operator_y 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::literals::chrono_literals::</span>operator""y</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-chrono.html">chrono</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-chrono.html"><chrono></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">year</span> operator <span class="st0">""</span>y<span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</span> y<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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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<p>构成表示外推格里高利历中年份的 <span class="t-lc">std::chrono::year</span> 字面量。</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>y</td>
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<td>-</td>
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<td>年份值</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>从 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span><span class="br0">(</span>y<span class="br0">)</span></span></span> 初始化的 <span class="t-lc">std::chrono::year</span> 。若转型结果不在 [-32767, 32767] 中，则存储值未指定。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">year</span> operator <span class="st0">""</span>y<span class="br0">(</span><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</span> y<span class="br0">)</span> <span class="kw1">noexcept</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">year</span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span>y<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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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>此运算符声明于命名空间 <code>std::literals::chrono_literals</code> ，其中 <code>literals</code> 与 <code>chrono_literals</code> 为内联命名空间。能通过 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">chrono_literals</span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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">chrono_literals</span></span></span> 取得对此运算符的访问。</p>
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<p>另外，在命名空间 <code>std::chrono</code> 中，标准库提供 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> <span class="kw1">namespace</span> literals<span class="sy4">::</span><span class="me2">chrono_literals</span><span class="sy4">;</span></span></span> 指令，故若程序员使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">;</span></span></span> 取得对 chrono 库中的类的访问，则对应的字面量运算符亦变为可见。</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 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 <chrono></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>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>
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<span class="sy1"><<</span> <span class="kw4">int</span><span class="br0">(</span>2020y<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> <span class="kw4">int</span><span class="br0">(</span><span class="sy2">-</span>220y<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> <span class="kw4">int</span><span class="br0">(</span>3000y<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> <span class="kw4">int</span><span class="br0">(</span>32768y<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// 未指定</span>
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<span class="sy1"><<</span> <span class="kw4">int</span><span class="br0">(</span>65578y<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 未指定</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</p>
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<div class="text source-text">
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<pre class="de1">2020 -220 3000 -32768 42</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 t-dsc-member-nobold-div">
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<div><a href="cpp-chrono-year-year.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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<td>构造 <code>year</code><br />
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<span class="t-mark">(<code>std::chrono::year</code> 的公开成员函数)</span></td>
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