153 lines
12 KiB
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153 lines
12 KiB
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<title>std::chrono::duration_cast</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_chrono_duration_duration_cast 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::chrono::</span>duration_cast</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-chrono.html">chrono</a>‎ | <a href="cpp-chrono-duration.html">duration</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> ToDuration, <span class="kw1">class</span> Rep, <span class="kw1">class</span> Period<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> ToDuration duration_cast<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span>Rep,Period<span class="sy1">></span><span class="sy3">&</span> d<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>转换 <span class="t-lc"><a href="cpp-chrono-duration.html">std::chrono::duration</a></span> 为不同类型 <code>ToDuration</code> 的时长。 函数不参与重载决议，除非 <code>ToDuration</code> 是 <span class="t-lc"><a href="cpp-chrono-duration.html">std::chrono::duration</a></span> 的特化。</p>
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<p>不使用隐式转换。可能的情况下避免乘法和除法，若在编译时已知一或多个参数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> 。以最宽的可用类型进行计算，而如同用 <a href="cpp-language-static_cast.html"><tt>static_cast</tt></a> 到结果类型的转换，只在完成时进行。</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>d</td>
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<td>-</td>
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<td>要转换的 duration</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>转换为 <code>ToDuration</code> 类型时长的 <code>d</code> 。</p>
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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>在源周期能准确地为目标周期所整除的场合（例如小时到分钟），浮点时长和整数时长间转型能经由 <a href="cpp-chrono-duration-duration.html"><code>std::chrono::duration</code> 构造函数</a>隐式进行，无需 <code>duration_cast</code> 。</p>
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<p>从浮点时长转型到整数时长，在浮点值为 NaN 、无穷大或过大而无法以目标的整数类型表示时，<a href="cpp-language-implicit_conversion.html#.E6.B5.AE.E7.82.B9.E6.95.B4.E6.95.B0.E8.BD.AC.E6.8D.A2">导致未定义行为</a>。否则转型到整数时长受制于截断，同任何到整数类型到 <code>static_cast</code> 。</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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<p>此示例度量函数的执行时间</p>
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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="co2">#include <chrono></span>
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<span class="co2">#include <ratio></span>
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<span class="co2">#include <thread></span>
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<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2148">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span><a href="cpp-chrono-duration.html"><span class="kw997">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">seconds</span></span></a><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</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">auto</span> t1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 整数时长：要求 duration_cast</span>
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<span class="kw4">auto</span> int_ms <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration_cast</span><span class="sy1"><</span><a href="cpp-chrono-duration.html"><span class="kw996">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="sy1">></span><span class="br0">(</span>t2 <span class="sy2">-</span> t1<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 小数时长：不要求 duration_cast</span>
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<a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, <a href="cpp-numeric-ratio-ratio.html"><span class="kw1527">std::<span class="me2">milli</span></span></a><span class="sy1">></span> fp_ms <span class="sy1">=</span> t2 <span class="sy2">-</span> t1<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"f() took "</span> <span class="sy1"><<</span> fp_ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" ms, "</span>
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<span class="sy1"><<</span> <span class="st0">"or "</span> <span class="sy1"><<</span> int_ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" whole milliseconds<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>可能的输出：</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">f() took 1000.23 ms, or 1000 whole milliseconds</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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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-chrono-duration.html"><span class="t-lines"><span>duration</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>时间区间<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-chrono-time_point-time_point_cast.html"><span class="t-lines"><span>time_point_cast</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>转换时间点为同一时钟上拥有不同时长的另一时间点<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-chrono-duration-floor.html"><span class="t-lines"><span>floor<span class="t-dsc-small">(std::chrono::duration)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>以向下取整的方式，将一个时长转换为另一个时长<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-chrono-duration-ceil.html"><span class="t-lines"><span>ceil<span class="t-dsc-small">(std::chrono::duration)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>以向上取整的方式，将一个时长转换为另一个时长<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-chrono-duration-round.html"><span class="t-lines"><span>round<span class="t-dsc-small">(std::chrono::duration)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>转换时长到另一个时长，就近取整，偶数优先<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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</body>
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