171 lines
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171 lines
12 KiB
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<title>std::chrono::time_point_cast</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::chrono::</span>time_point_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-time_point.html">time point</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> Clock, <span class="kw1">class</span> Duration<span class="sy1">></span><br /></span>
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<p><a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, ToDuration<span class="sy1">></span><br /></p>
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time_point_cast<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, Duration<span class="sy1">></span> <span class="sy3">&</span>t<span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx14">(C++14 前)</span></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> ToDuration, <span class="kw1">class</span> Clock, <span class="kw1">class</span> Duration<span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, ToDuration<span class="sy1">></span><br /></p>
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time_point_cast<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, Duration<span class="sy1">></span> <span class="sy3">&</span>t<span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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<p>将 <span class="t-lc"><a href="cpp-chrono-time_point.html">std::chrono::time_point</a></span> 从一个时长转换到另一个。</p>
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<p><code>time_point_cast</code> 仅若 <code>ToDuration</code> 为 <span class="t-lc"><a href="cpp-chrono-duration.html">std::chrono::duration</a></span> 的特化才参与重载决议。</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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<td>t</td>
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<td>-</td>
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<td>转换来源的 <code>time_point</code></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><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, ToDuration<span class="sy1">></span><span class="br0">(</span><br />
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<a href="cpp-chrono-duration-duration_cast.html"><span class="kw999">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration_cast</span></span></a><span class="sy1"><</span>ToDuration<span class="sy1">></span><span class="br0">(</span>t.<span class="me1">time_since_epoch</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B_2">示例</span></h3>
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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="kw1">using</span> Clock <span class="sy1">=</span> <a href="cpp-chrono-high_resolution_clock.html"><span class="kw1007">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span></span></a><span class="sy4">;</span>
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<span class="kw1">using</span> Ms <span class="sy1">=</span> <a href="cpp-chrono-duration.html"><span class="kw995">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="sy4">;</span>
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<span class="kw1">using</span> Sec <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">seconds</span></span></a><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> Duration<span class="sy1">></span>
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<span class="kw1">using</span> TimePoint <span class="sy1">=</span> <a href="cpp-chrono-time_point.html"><span class="kw991">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point</span></span></a><span class="sy1"><</span>Clock, Duration<span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">void</span> print_ms<span class="br0">(</span><span class="kw4">const</span> TimePoint<span class="sy1"><</span>Ms<span class="sy1">></span><span class="sy3">&</span> time_point<span class="br0">)</span>
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<span class="br0">{</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> time_point.<span class="me1">time_since_epoch</span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" ms<span class="es1">\n</span>"</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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TimePoint<span class="sy1"><</span>Sec<span class="sy1">></span> time_point_sec<span class="br0">(</span>Sec<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 隐式转换，无精度损失</span>
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TimePoint<span class="sy1"><</span>Ms<span class="sy1">></span> time_point_ms<span class="br0">(</span>time_point_sec<span class="br0">)</span><span class="sy4">;</span>
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print_ms<span class="br0">(</span>time_point_ms<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 4000 ms</span>
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time_point_ms <span class="sy1">=</span> TimePoint<span class="sy1"><</span>Ms<span class="sy1">></span><span class="br0">(</span>Ms<span class="br0">(</span><span class="nu0">5756</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 显式转型，在精度损失可能发生时需要</span>
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<span class="co1">// 5756 truncated to 5000</span>
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time_point_sec <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">time_point_cast</span><span class="sy1"><</span>Sec<span class="sy1">></span><span class="br0">(</span>time_point_ms<span class="br0">)</span><span class="sy4">;</span>
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print_ms<span class="br0">(</span>time_point_sec<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 5000 ms</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">4000 ms
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5000 ms</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">
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<div><a href="cpp-chrono-time_point.html"><span class="t-lines"><span>time_point</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>时间中的点<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-floor.html"><span class="t-lines"><span>floor<span class="t-dsc-small">(std::chrono::time_point)</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 time_point 到另一个，向下取整<br />
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<span class="t-mark">(函数模板)</span></td>
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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-ceil.html"><span class="t-lines"><span>ceil<span class="t-dsc-small">(std::chrono::time_point)</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 time_point 到另一个，向上取整<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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<div class="t-dsc-member-div">
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<div><a href="cpp-chrono-time_point-round.html"><span class="t-lines"><span>round<span class="t-dsc-small">(std::chrono::time_point)</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 time_point 到另一个，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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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-chrono-duration-duration_cast.html"><span class="t-lines"><span>duration_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>转换时长到另一个拥有不同嘀嗒间隔的时长<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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