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130 lines
10 KiB
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<title>std::chrono::treat_as_floating_point</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_chrono_treat_as_floating_point 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>treat_as_floating_point</h1>
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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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> Rep<span class="sy1">></span><br />
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<span class="kw1">struct</span> treat_as_floating_point <span class="sy4">:</span> <a href="cpp-types-is_floating_point.html"><span class="kw458">std::<span class="me2">is_floating_point</span></span></a><span class="sy1"><</span>Rep<span class="sy1">></span> <span class="br0">{</span><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><code>std::chrono::treat_as_floating_point</code> 特性帮助确定时期是否能转换成拥有另一种不同计次周期的时期。</p>
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<p>二个时期间的隐式转换通常依赖于时期的计次周期。然而若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">treat_as_floating_point</span><span class="sy1"><</span>Rep<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> 则可发生隐式转换，不管计次周期如何。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.8F.98.E9.87.8F.E6.A8.A1.E6.9D.BF">辅助变量模板</span></h3>
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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> Rep <span class="sy1">></span><br />
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> treat_as_floating_point_v <span class="sy1">=</span> treat_as_floating_point<span class="sy1"><</span>Rep<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<p>可对程序定义类型特化 <code>std::chrono::treat_as_floating_point</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="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 <thread></span>
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<span class="kw4">void</span> timed_piece_of_code<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<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> simulated_work<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-thread-sleep_for.html"><span class="kw2147">std::<span class="me2">this_thread</span><span class="sy4">::</span><span class="me2">sleep_for</span></span></a><span class="br0">(</span>simulated_work<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> start <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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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Running some timed piece of code..."</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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timed_piece_of_code<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> stop <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">// 浮点毫秒类型</span>
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<span class="kw1">using</span> FpMilliseconds <span class="sy1">=</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">float</span>, std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span><span class="sy4">::</span><span class="me2">period</span><span class="sy1">></span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">treat_as_floating_point</span><span class="sy1"><</span>FpMilliseconds<span class="sy4">::</span><span class="me2">rep</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span>,
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<span class="st0">"Rep required to be floating point"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 注意此处不允许隐式转换 </span>
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<span class="kw4">auto</span> i_ms <span class="sy1">=</span> <a href="cpp-chrono-duration-duration_cast.html"><span class="kw1000">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration_cast</span></span></a><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>stop <span class="sy2">-</span> start<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 注意此处允许隐式转换</span>
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<span class="kw4">auto</span> f_ms <span class="sy1">=</span> FpMilliseconds<span class="br0">(</span>stop <span class="sy2">-</span> start<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Time in milliseconds, using default rep: "</span>
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<span class="sy1"><<</span> i_ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1756">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Time in milliseconds, using floating point rep: "</span>
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<span class="sy1"><<</span> f_ms.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</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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<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">Running some timed piece of code...
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Timing stats:
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Time in milliseconds, using default rep: 2
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Time in milliseconds, using floating point rep: 2.57307</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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