93 lines
9.9 KiB
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93 lines
9.9 KiB
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<title>std::basic_string_view 的推导指引</title>
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<h1 id="firstHeading" class="firstHeading"><code>std::basic_string_view</code> 的推导指引</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>定义于头文件 <code><a href="cpp-header-string_view.html"><string_view></a></code></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> It, <span class="kw1">class</span> End<span class="sy1">></span><br />
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basic_string_view<span class="br0">(</span>It, End<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> basic_string_view<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_value_t</span><span class="sy1"><</span>It<span class="sy1">>></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">template</span><span class="sy1"><</span><span class="kw1">class</span> R<span class="sy1">></span><br />
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basic_string_view<span class="br0">(</span>R<span class="sy3">&&</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> basic_string_view<span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">range_value_t</span><span class="sy1"><</span>R<span class="sy1">>></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-cxx23">(C++23 起)</span></td>
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<p>为 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::basic_string_view</a></span> 提供这些<a href="cpp-language-class_template_argument_deduction.html">推导指引</a>。</p>
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<div class="t-li1"><span class="t-li">1)</span> 此推导指引允许从迭代器-哨位对推导字符类型。此重载仅若 <code>It</code> 满足 <a href="cpp-iterator-contiguous_iterator.html"><code>contiguous_iterator</code></a> 且 <code>End</code> 满足 <code>It</code> 的 <a href="cpp-iterator-sized_sentinel_for.html"><code>sized_sentinel_for</code></a> 才参与重载决议。</div>
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<div class="t-li1"><span class="t-li">2)</span> 此推导指引允许从范围推导字符类型。此重载仅若 <code>R</code> 满足 <a href="cpp-ranges-contiguous_range.html"><code>contiguous_range</code></a> 才参与重载决议。</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="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_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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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> a1 <span class="br0">{</span><span class="st0">'n'</span>, <span class="st0">'u'</span>, <span class="st0">'c'</span>, <span class="st0">'l'</span>, <span class="st0">'e'</span>, <span class="st0">'o'</span>, <span class="st0">'n'</span>, <span class="st0">'s'</span>, <span class="st0">':'</span>, <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1246">std::<span class="me2">basic_string_view</span></span></a> s1<span class="br0">(</span>a1.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, a1.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 推导： CharT -> char</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>s1<span class="br0">)</span><span class="sy4">::</span><span class="me2">value_type</span>, <span class="kw4">char</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</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> s1<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> a2 <span class="br0">{</span>L<span class="st0">'p'</span>, L<span class="st0">'r'</span>, L<span class="st0">'o'</span>, L<span class="st0">'t'</span>, L<span class="st0">'o'</span>, L<span class="st0">'n'</span>, L<span class="st0">'s'</span>, L<span class="st0">'<span class="es1">\n</span>'</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1246">std::<span class="me2">basic_string_view</span></span></a> s2<span class="br0">(</span>a2.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, a2.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 推导： CharT -> wchar_t</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>s2<span class="br0">)</span><span class="sy4">::</span><span class="me2">value_type</span>, <span class="kw4">wchar_t</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1759">std::<span class="me2">wcout</span></span></a> <span class="sy1"><<</span> s2<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><span class="sy1"><</span><span class="kw4">long</span>, <span class="nu0">9</span><span class="sy1">></span> a3 <span class="br0">{</span><span class="st0">'n'</span>, <span class="st0">'e'</span>, <span class="st0">'u'</span>, <span class="st0">'t'</span>, <span class="st0">'r'</span>, <span class="st0">'o'</span>, <span class="st0">'n'</span>, <span class="st0">'s'</span>, <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string_view.html"><span class="kw1246">std::<span class="me2">basic_string_view</span></span></a> s3<span class="br0">(</span>a3.<span class="me1">cbegin</span><span class="br0">(</span><span class="br0">)</span>, a3.<span class="me1">cend</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 推导： CharT -> long</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>s3<span class="br0">)</span><span class="sy4">::</span><span class="me2">value_type</span>, <span class="kw4">long</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> e <span class="sy4">:</span> s3<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span><span class="br0">(</span>e<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<div class="text source-text">
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<pre class="de1">nucleons:
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protons
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neutrons</pre></div>
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