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122 lines
14 KiB
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<title>std::chrono::operator+, std::chrono::operator- (std::chrono::year_month)</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::year_month)</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-year_month.html">year month</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">year_month</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">year_month</span><span class="sy3">&</span> ym,<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">years</span><span class="sy3">&</span> dy<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">year_month</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">years</span><span class="sy3">&</span> dy,<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">year_month</span><span class="sy3">&</span> ym<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">year_month</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">year_month</span><span class="sy3">&</span> ym,<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">months</span><span class="sy3">&</span> dm<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">year_month</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">months</span><span class="sy3">&</span> dm,<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">year_month</span><span class="sy3">&</span> ym<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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<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_month</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">year_month</span><span class="sy3">&</span> ym,<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">years</span><span class="sy3">&</span> dy<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(5)</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">year_month</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">year_month</span><span class="sy3">&</span> ym,<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">months</span><span class="sy3">&</span> dm<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(6)</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">months</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">year_month</span><span class="sy3">&</span> ym1,<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">year_month</span><span class="sy3">&</span> ym2<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1-2)</span> 加上 <code>dy.count()</code> 年到 <code>ym</code> 。</div>
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<div class="t-li1"><span class="t-li">3-4)</span> 加上 <code>dm.count()</code> 月到 <code>ym</code> 。</div>
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<div class="t-li1"><span class="t-li">5)</span> 从 <code>ym</code> 减去 <code>dy.count()</code> 年。</div>
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<div class="t-li1"><span class="t-li">6)</span> 从 <code>ym</code> 减去 <code>dm.count()</code> 月。</div>
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<div class="t-li1"><span class="t-li">7)</span> 返回 <code>ym1</code> 与 <code>ym2</code> 所表示的二个时间点间以 <code>months</code> 计量的差。</div>
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<p>对于能转换到 <span class="t-lc">std::chrono::years</span> 与 <span class="t-lc">std::chrono::months</span> 两者的时长，若调用有歧义，则偏好 <code>years</code> 重载 <span class="t-v">(1,2,5)</span> 。</p>
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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">1-2)</span> <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">year_month</span><span class="br0">(</span>ym.<span class="me1">year</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> dy, ym.<span class="me1">month</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">3-4)</span> A <code>year_month</code> value <code>z</code> such that <span class="t-c"><span class="mw-geshi cpp source-cpp">z <span class="sy2">-</span> ym <span class="sy1">==</span> dm</span></span></div>
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<div class="t-li1"><span class="t-li">5)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ym <span class="sy2">+</span> <span class="sy2">-</span>dy</span></span></div>
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<div class="t-li1"><span class="t-li">6)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ym <span class="sy2">+</span> <span class="sy2">-</span>dm</span></span></div>
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<div class="t-li1"><span class="t-li">7)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ym1.<span class="me1">year</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> ym2.<span class="me1">year</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">months</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">(</span><span class="kw4">unsigned</span><span class="br0">(</span>ym1.<span class="me1">month</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">-</span> <span class="kw4">int</span><span class="br0">(</span><span class="kw4">unsigned</span><span class="br0">(</span>ym2.<span class="me1">month</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span></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>二个 <code>year_month</code> 值相减的结果是 <span class="t-lc">std::chrono::months</span> 类型时长。此时长单位表示格里高利月的平均长度（ 30.436875 日），而产生的时长与问题中的实际日数无关。例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">2017y<span class="sy2">/</span><span class="nu0">3</span> <span class="sy2">-</span> 2017y<span class="sy2">/</span><span class="nu0">2</span></span></span> 的结果是 <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">months</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span></span></span> ，即使 2017 年二月只含 28 日。</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 class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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><a href="cpp-chrono-year_month-operator_arith.html"><span class="t-lines"><span>operator+=</span><span>operator-=</span></span></a></div>
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<td>以一定量的月和年为程度修改 <code>year_month</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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