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<title>std::chrono::operator+, std::chrono::operator- (std::chrono::year_month_day)</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_day)</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_day.html">year month day</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="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">year_month_day</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_day</span><span class="sy3">&</span> ymd,<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><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_day</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_day</span><span class="sy3">&</span> ymd<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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_day</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_day</span><span class="sy3">&</span> ymd,<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><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_day</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_day</span><span class="sy3">&</span> ymd<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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_day</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_day</span><span class="sy3">&</span> ymd,<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><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_day</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_day</span><span class="sy3">&</span> ymd,<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><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> 将 <span class="t-c"><span class="mw-geshi cpp source-cpp">dm.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span> 月加到 <code>ymd</code> 所表示的日期。结果与 <code>ymd</code> 拥有相同的 <code>day()</code> ，且与 <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>ymd.<span class="me1">year</span><span class="br0">(</span><span class="br0">)</span>, ymd.<span class="me1">month</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy2">+</span> dm</span></span> 拥有相同的 <code>year()</code> 和 <code>month()</code> 。</div>
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<div class="t-li1"><span class="t-li">3-4)</span> 将 <span class="t-c"><span class="mw-geshi cpp source-cpp">dy.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span> 年加到 <code>ymd</code> 所表示的日期。结果等价于 <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_day</span><span class="br0">(</span>ymd.<span class="me1">year</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> dy, ymd.<span class="me1">month</span><span class="br0">(</span><span class="br0">)</span>, ymd.<span class="me1">day</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">5)</span> 从 <code>ymd</code> 所表示的日期减去 <span class="t-c"><span class="mw-geshi cpp source-cpp">dm.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span> 月。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ymd <span class="sy2">+</span> <span class="sy2">-</span>dm</span></span> 。</div>
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<div class="t-li1"><span class="t-li">5)</span> 从 <code>ymd</code> 所表示的日期减去 <span class="t-c"><span class="mw-geshi cpp source-cpp">dy.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span></span></span> 年。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ymd <span class="sy2">+</span> <span class="sy2">-</span>dy</span></span> 。</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，若调用本会有歧义则偏好 <code>years</code> 重载 <span class="t-v">(3,4,6)</span> 。</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>即使 <code>ymd.ok()</code> 为 true ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ymd.<span class="me1">day</span><span class="br0">(</span><span class="br0">)</span></span></span> 为 29 、 30 或 31 ，则产生的 <code>year_month_day</code> 可能不表示合法日期。</p>
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