129 lines
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129 lines
15 KiB
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<title>std::chrono::operator+, std::chrono::operator- (std::chrono::weekday)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::chrono::</span>operator+, <span style="font-size:0.7em; line-height:130%">std::chrono::</span>operator- <span style="font-size:0.7em; line-height:130%">(std::chrono::weekday)</span></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-weekday.html">weekday</a></span></div>
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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">weekday</span> operator<span class="sy2">+</span><span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="sy3">&</span> wd,<br />
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="sy3">&</span> d<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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<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">weekday</span> operator<span class="sy2">+</span><span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="sy3">&</span> d,<br />
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="sy3">&</span> wd<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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">weekday</span> operator<span class="sy2">-</span><span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="sy3">&</span> wd,<br />
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="sy3">&</span> d<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></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">days</span> operator<span class="sy2">-</span><span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="sy3">&</span> wd1,<br />
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<span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="sy3">&</span> wd2<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<div class="t-li1"><span class="t-li">1-2)</span> 向 <code>wd</code> 加上 <code>d.count()</code> 日。通过首先求值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">long</span> <span class="kw4">long</span><span class="sy1">></span><span class="br0">(</span>wd.<span class="me1">c_encoding</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">+</span> d.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span> 再将它对 7 取余，得到范围 [0, 6] 中的整数，计算结果中保有的星期之日值。</div>
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<div class="t-li1"><span class="t-li">3)</span> 从 <code>wd</code> 减去 <code>d.count()</code> 日。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> wd <span class="sy2">+</span> <span class="sy2">-</span>d<span class="sy4">;</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp">wd1.<span class="me1">ok</span><span class="br0">(</span><span class="br0">)</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">wd2.<span class="me1">ok</span><span class="br0">(</span><span class="br0">)</span></span></span> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，则返回 <span class="t-lc">std::chrono::days</span> 值 <code>d</code> ，满足 <code>d.count()</code> 在范围 [0, 6] 中且 <span class="t-c"><span class="mw-geshi cpp source-cpp">wd2 <span class="sy2">+</span> d <span class="sy1">==</span> wd1</span></span> 。否则返回值未指定。</div>
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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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<div class="t-li1"><span class="t-li">1-3)</span> 保有按上述方式计算的星期之日值的 <span class="t-lc">std::chrono::weekday</span> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 表示 <code>wd1</code> 与 <code>wd2</code> 间距离的 <span class="t-lc">std::chrono::days</span> 。</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>只要计算不溢出， <span class="t-v">(1-3)</span> 就始终返回合法的 <code>weekday</code> ，即使 <span class="t-c"><span class="mw-geshi cpp source-cpp">wd.<span class="me1">ok</span><span class="br0">(</span><span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</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 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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span> wd <span class="br0">{</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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wd <span class="sy1">=</span> wd <span class="sy2">+</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</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> <span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="br0">(</span><span class="nu0">6</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Saturday</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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wd <span class="sy1">=</span> wd <span class="sy2">-</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="br0">(</span><span class="nu0">3</span><span class="br0">)</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> <span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">weekday</span><span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Wednesday</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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wd <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Tuesday</span><span class="sy4">;</span>
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wd <span class="sy1">=</span> wd <span class="sy2">+</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">days</span><span class="br0">{</span><span class="nu0">8</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// (((2 + 8) == 10) % 7) == 3;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Wednesday</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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wd <span class="sy1">=</span> wd <span class="sy2">+</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Sunday</span> <span class="sy2">-</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Thursday</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// (3 + 3) == 6</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="br0">(</span>wd <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">Saturday</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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</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">true true true true true true</pre></div>
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</div>
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</div>
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<p><br /></p>
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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-weekday-operator_inc_dec.html"><span class="t-lines"><span>operator++</span><span>operator++<span class="t-dsc-small">(int)</span></span><span>operator--</span><span>operator--<span class="t-dsc-small">(int)</span></span></span></a></div>
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</div>
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</td>
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<td>自增或自减 <code>weekday</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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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-chrono-weekday-operator_arith.html"><span class="t-lines"><span>operator+=</span><span>operator-=</span></span></a></div>
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</div>
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</td>
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<td>加或减日数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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