208 lines
18 KiB
HTML
208 lines
18 KiB
HTML
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<title>std::basic_string_view<CharT,Traits>::basic_string_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_basic_string_view_basic_string_view 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::basic_string_view<CharT,Traits>::</span>basic_string_view</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-string.html">string</a>‎ | <a href="cpp-string-basic_string_view.html">basic string view</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</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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</tr>
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<tr class="t-dcl t-since-cxx17">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span><span class="kw4">const</span> basic_string_view<span class="sy3">&</span> other<span class="br0">)</span> <span class="kw1">noexcept</span><br />
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<span class="sy1">=</span> <span class="kw1">default</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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</tr>
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<tr class="t-dcl t-since-cxx17">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span><span class="kw4">const</span> CharT<span class="sy2">*</span> s, size_type count<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span><span class="kw4">const</span> CharT<span class="sy2">*</span> s<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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> It, <span class="kw1">class</span> End<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span>It first, End last<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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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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</tr>
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<tr class="t-dcl t-since-cxx23">
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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> R<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span>R<span class="sy3">&&</span> r<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(6)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx23">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> basic_string_view<span class="br0">(</span><a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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</td>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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</tr>
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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)</span> 默认构造函数。构造空的 <code>basic_string_view</code> 。构造后， <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <code>nullptr</code> ，而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <code>0</code> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 复制构造函数。构造内容同 <code>other</code> 的视图。构造后， <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <code>other.data()</code> ，而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <code>other.size()</code> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 构造始于 <code>s</code> 所指向元素的字符数组的首 <code>count</code> 个字符的视图。 <code>s</code> 可包含空字符。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span>s, s<span class="sy2">+</span>count<span class="br0">)</span></span></span> 不是合法范围，则行为未定义（尽管构造函数可能不访问此范围的任何元素）。构造后， <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <code>s</code> ，而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <code>count</code> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 构造一个 <code>s</code> 所指向的空终止字符串的视图，不包含终止空字符。视图的长度如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">length</span><span class="br0">(</span>s<span class="br0">)</span></span></span> 确定。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span>s, s<span class="sy2">+</span>Traits<span class="sy4">::</span><span class="me2">length</span><span class="br0">(</span>s<span class="br0">)</span><span class="br0">)</span></span></span> 不是合法范围，则行为未定义。构造后， <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <code>s</code> ，而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <code>Traits::length(s)</code> 。</div>
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<div class="t-li1"><span class="t-li">5)</span> 构造范围 <code>[first, last)</code> 上的 <code>basic_string_view</code> 。若 <code>[first, last)</code> 不是合法范围、若 <code>It</code> 不实际实现 <a href="cpp-iterator-contiguous_iterator.html"><code>contiguous_iterator</code></a> 或若 <code>End</code> 不实际实现 <code>It</code> 的 <a href="cpp-iterator-sized_sentinel_for.html"><code>sized_sentinel_for</code></a> 则行为未定义。构造后 <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">to_address</span><span class="br0">(</span>first<span class="br0">)</span></span></span> 而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">last <span class="sy2">-</span> first</span></span> 。
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<p>此重载仅若</p>
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<dl>
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<dd>
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<ul>
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<li><code>It</code> 满足 <a href="cpp-iterator-contiguous_iterator.html"><code>contiguous_iterator</code></a> ，</li>
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<li><code>End</code> 满足 <code>It</code> 的 <a href="cpp-iterator-sized_sentinel_for.html"><code>sized_sentinel_for</code></a> ，</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>It<span class="sy1">></span></span></span> 与 <code>CharT</code> 为同一类型且</li>
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<li><code>End</code> 不可转换到 <span class="t-lc"><a href="cpp-types-size_t.html">std::size_t</a></span></li>
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</ul>
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</dd>
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</dl>
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才参与重载决议。</div>
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<div class="t-li1"><span class="t-li">6)</span> 构造范围 <code>r</code> 上的 <code>basic_string_view</code> 。构造后 <span class="t-lc"><a href="cpp-string-basic_string_view-data.html">data()</a></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">data</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 而 <span class="t-lc"><a href="cpp-string-basic_string_view-size.html">size()</a></span> 等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 。
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<p>此重载仅若</p>
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<dl>
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<dd>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>R<span class="sy1">></span></span></span> 与 <code>basic_string_view</code> 不是同一类型，</li>
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<li><code>R</code> 实现 <a href="cpp-ranges-contiguous_range.html"><code>contiguous_range</code></a> 与 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a> ，</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">range_value_t</span><span class="sy1"><</span>R<span class="sy1">></span></span></span> 与 <code>CharT</code> 为同一类型，</li>
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<li><code>R</code> 不可转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> CharT<span class="sy2">*</span></span></span> ，</li>
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<li>令 <code>d</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>R<span class="sy1">></span></span></span> 类型左值， <span class="t-c"><span class="mw-geshi cpp source-cpp">d.<span class="me1">operator</span> <span class="sy4">::</span><a href="cpp-string-basic_string_view.html"><span class="kw1246">std::<span class="me2">basic_string_view</span></span></a><span class="sy1"><</span>CharT, Traits<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span></span></span> 不是合法的表达式，且</li>
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<li>若有限定标识 <code>R::traits_type</code> 合法并代表理性，则它与 <code>Traits</code> 相同才参与重载决议。</li>
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</ul>
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</dd>
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</dl>
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</div>
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<div class="t-li1"><span class="t-li">7)</span> 不能从 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span> 构造 <code>basic_string_view</code> 。</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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<table class="t-par-begin">
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<tr class="t-par">
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<td>other</td>
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<td>-</td>
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<td>用以初始化视图的另一视图</td>
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</tr>
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<tr class="t-par">
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<td>s</td>
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<td>-</td>
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<td>用以初始化视图的指向字符数组或 C 字符串的指针</td>
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</tr>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>要包含于视图的字符数</td>
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</tr>
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<tr class="t-par">
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<td>first</td>
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<td>-</td>
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<td>指向序列首字符的迭代器</td>
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</tr>
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<tr class="t-par">
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<td>last</td>
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<td>-</td>
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<td>指向序列尾字符后一位置的迭代器或另一哨位</td>
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</tr>
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<tr class="t-par">
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<td>r</td>
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<td>-</td>
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<td>含有序列的连续范围</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<div class="t-li1"><span class="t-li">1-3,5-6)</span> 常数</div>
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<div class="t-li1"><span class="t-li">4)</span> 与 <code>s</code> 的长度成线性</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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<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 <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <string_view></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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<span class="kw4">constexpr</span> <a href="cpp-string-basic_string_view.html"><span class="kw1250">std::<span class="me2">wstring_view</span></span></a> wcstr_v <span class="sy1">=</span> L<span class="st0">"xyzzy"</span><span class="sy4">;</span>
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<span class="kw4">char</span> array<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'B'</span>, <span class="st0">'a'</span>, <span class="st0">'r'</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> array_v<span class="br0">(</span>array, <a href="cpp-iterator-size.html"><span class="kw684">std::<span class="me2">size</span></span></a><span class="br0">(</span>array<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> cppstr <span class="sy1">=</span> <span class="st0">"Foo"</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> cppstr_v<span class="br0">(</span>cppstr<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> ar <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'P'</span>, <span class="st0">'u'</span>, <span class="st0">'b'</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> ar_v<span class="br0">(</span>ar.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, ar.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C++20</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> cppstr_v <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> array_v <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> ar_v <span class="sy1"><<</span> <span class="st0">' '</span>
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<span class="sy1"><<</span> wcstr_v.<span class="me1">size</span><span class="br0">(</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">Foo Bar Pub 5</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string_view-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>对视图赋值<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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<td>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-string-basic_string-basic_string.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造 <code>basic_string</code><br />
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<span class="t-mark">(<code>std::basic_string<CharT,Traits,Allocator></code> 的公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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