120 lines
8.7 KiB
HTML
120 lines
8.7 KiB
HTML
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<title>std::optional<T>::value_or</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_value_or 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::optional<T>::</span>value_or</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</a></span></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> U <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T value_or<span class="br0">(</span> U<span class="sy3">&&</span> default_value <span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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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> U <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T value_or<span class="br0">(</span> U<span class="sy3">&&</span> default_value <span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 拥有值则返回其所含的值，否则返回 <code>default_value</code> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span> <span class="sy4">?</span> <span class="sy2">**</span>this <span class="sy4">:</span> <span class="kw1">static_cast</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>default_value<span class="br0">)</span><span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span> <span class="sy4">?</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span><span class="sy2">**</span>this<span class="br0">)</span> <span class="sy4">:</span> <span class="kw1">static_cast</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>default_value<span class="br0">)</span><span class="br0">)</span></span></span></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>default_value</td>
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<td>-</td>
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<td>用于 <code>*this</code> 为空情况的值</td>
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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为使用重载 (1) ， <code>T</code> 必须满足<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 的要求。</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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为使用重载 (2) ， <code>T</code> 必须满足<a href="cpp-named_req-MoveConstructible.html"><span style="font-style:italic">可移动构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span></a> 的要求。</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>U&&</code> 必须可转换为 <code>T</code></td>
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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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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 拥有值则为其当前值，否则为 <code>default_value</code> 。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>返回值 <code>T</code> 的被选择构造函数所抛的任何异常。</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 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 <optional></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <cstdlib></span>
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<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span> maybe_getenv<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> n<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> x <span class="sy1">=</span> <a href="cpp-utility-program-getenv.html"><span class="kw975">std::<span class="me2">getenv</span></span></a><span class="br0">(</span>n<span class="br0">)</span><span class="br0">)</span>
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<span class="kw1">return</span> x<span class="sy4">;</span>
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<span class="kw1">else</span>
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<span class="kw1">return</span> <span class="br0">{</span><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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> maybe_getenv<span class="br0">(</span><span class="st0">"MYPWD"</span><span class="br0">)</span>.<span class="me1">value_or</span><span class="br0">(</span><span class="st0">"(none)"</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>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">(none)</pre></div>
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</div>
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</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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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-value.html"><span class="t-lines"><span>value</span></span></a></div>
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</div>
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</td>
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<td>返回所含值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</div>
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<div class="visualClear"></div>
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</body>
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