79 lines
9.2 KiB
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79 lines
9.2 KiB
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<title>std::chrono::year_month_day::operator sys_days, std::chrono::year_month_day::operator local_days</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::chrono::year_month_day::</span>operator sys_days, <span style="font-size:0.7em; line-height:130%">std::chrono::year_month_day::</span>operator local_days</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><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> operator std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">sys_days</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</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="kw1">explicit</span> <span class="kw4">constexpr</span> operator std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">local_days</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</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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<p>将 <code>*this</code> 转换成与此 <code>year_month_day</code> 表示同一日期的 <span class="t-lc"><a href="cpp-chrono-time_point.html">std::chrono::time_point</a></span> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 若 <span class="t-lc">ok()</span> 为 true ，则返回值保有从 <span class="t-lc"><a href="cpp-chrono-system_clock.html">std::chrono::system_clock</a></span> 纪元 (1970-01-01) 到 <code>*this</code> 的日数总计。若 <code>*this</code> 表示的日期先于纪元，则结果为负。</div>
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<div class="t-li1"><span class="t-li"></span> 否则，若存储的 <code>year</code> 和 <code>month</code> 合法（ <span class="t-c"><span class="mw-geshi cpp source-cpp">year<span class="br0">(</span><span class="br0">)</span>.<span class="me1">ok</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span> month<span class="br0">(</span><span class="br0">)</span>.<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">true</span></span></span> ），则返回值为 <span class="t-c"><span class="mw-geshi cpp source-cpp">sys_days<span class="br0">(</span>year<span class="br0">(</span><span class="br0">)</span><span class="sy2">/</span>month<span class="br0">(</span><span class="br0">)</span><span class="sy2">/</span>1d<span class="br0">)</span> <span class="sy2">+</span> <span class="br0">(</span>day<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> 1d<span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li"></span> 否则（若 <span class="t-c"><span class="mw-geshi cpp source-cpp">year<span class="br0">(</span><span class="br0">)</span>.<span class="me1">ok</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span> month<span class="br0">(</span><span class="br0">)</span>.<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> ），返回值未指定。</div>
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<div class="t-li1"><span class="t-li"></span> 范围 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</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="sy2">-</span><span class="nu0">12687428</span><span class="br0">}</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">11248737</span><span class="br0">}</span><span class="br0">]</span></span></span> 中的 <code>sys_days</code> ，转换到 <code>year_month_day</code> 再转换回来后，生成相同值。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，但替而返回 <code>local_days</code> 。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> local_days<span class="br0">(</span>sys_days<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span>.<span class="me1">time_since_epoch</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></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>转换到 <code>sys_days</code> 再转换回来，能用于正规化含有非法 <code>day</code> 但有合法 <code>year</code> 和 <code>month</code> 的 <code>year_month_day</code> ：</p>
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<pre class="de1"><span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">;</span>
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<span class="kw4">auto</span> ymd <span class="sy1">=</span> 2017y<span class="sy2">/</span>January<span class="sy2">/</span><span class="nu0">0</span><span class="sy4">;</span>
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ymd <span class="sy1">=</span> sys_days<span class="br0">{</span>ymd<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// ymd 现在是 2016y/December/31</span></pre></div>
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<p>能通过加（或减）零个 <span class="t-lc">std::chrono::months</span> 正规化年和月：</p>
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<pre class="de1"><span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> year_month_day normalize<span class="br0">(</span>year_month_day ymd<span class="br0">)</span><span class="br0">{</span>
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ymd <span class="sy2">+</span><span class="sy1">=</span> months<span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 正规化年和月</span>
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<span class="kw1">return</span> sys_days<span class="br0">{</span>ymd<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 正规化日</span>
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<span class="br0">}</span>
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static_assert<span class="br0">(</span>normalize<span class="br0">(</span>2017y<span class="sy2">/</span><span class="nu0">33</span><span class="sy2">/</span><span class="nu0">59</span><span class="br0">)</span> <span class="sy1">==</span> 2019y<span class="sy2">/</span><span class="nu0">10</span><span class="sy2">/</span><span class="nu0">29</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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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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