181 lines
15 KiB
HTML
181 lines
15 KiB
HTML
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::indirectly_readable_traits</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_indirectly_readable_traits 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::</span>indirectly_readable_traits</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-iterator.html">iterator</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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<td></td>
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<td></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> I <span class="sy1">></span><br />
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<span class="kw1">struct</span> indirectly_readable_traits <span class="br0">{</span> <span class="br0">}</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> T <span class="sy1">></span><br />
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>T<span class="sy2">*</span><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-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> I <span class="sy1">></span><br /></span>
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<p> requires <a href="cpp-types-is_array.html"><span class="kw461">std::<span class="me2">is_array_v</span></span></a><span class="sy1"><</span>I<span class="sy1">></span><br /></p>
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">;</span></div>
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<td>(3)</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-cxx20">
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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> T <span class="sy1">></span><br /></span>
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<p><span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span><span class="kw4">const</span> T<span class="sy1">></span> <span class="sy4">:</span><br /></p>
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indirectly_readable_traits<span class="sy1"><</span>T<span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="coMULTI">/*has-member-value-type*/</span> T<span class="sy1">></span><br />
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span></span></div>
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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 class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="coMULTI">/*has-member-element-type*/</span> T<span class="sy1">></span><br />
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>T<span class="sy1">></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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="coMULTI">/*has-member-value-type*/</span> T<span class="sy1">></span><br /></span>
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<p> requires <span class="coMULTI">/*has-member-element-type*/</span><span class="sy1"><</span>T<span class="sy1">></span><br /></p>
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>T<span class="sy1">></span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></div>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="coMULTI">/*has-member-value-type*/</span> T<span class="sy1">></span><br /></span>
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<p> requires <span class="coMULTI">/*has-member-element-type*/</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">element_type</span><span class="sy1">></span>,<br />
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<a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">value_type</span><span class="sy1">>></span><br /></p>
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<span class="kw1">struct</span> indirectly_readable_traits<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span></div>
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<td>(8)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>计算类型 <code>I</code> 的关联值类型，若它存在。用户可对程序定义类型特化 <code>indirectly_readable_traits</code> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 主模板是无成员 <code>value_type</code> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 对指针的特化。若 <code>T</code> 是对象类型，则提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 的成员类型 <code>value_type</code> 。否则无成员 <code>value_type</code> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 对数组类型的特化。提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><a href="cpp-types-remove_extent.html"><span class="kw621">std::<span class="me2">remove_extent_t</span></span></a><span class="sy1"><</span>I<span class="sy1">>></span></span></span> 的成员类型 <code>value_type</code> 。</div>
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<div class="t-li1"><span class="t-li">4)</span> 对 const 限定类型的特化。</div>
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<div class="t-li1"><span class="t-li">5)</span> 对定义公开可访问成员类型 <code>value_type</code> 的类型（例如 <span class="t-lc"><a href="cpp-iterator-reverse_iterator.html">std::reverse_iterator</a></span> ）的特化。若 <code>T::value_type</code> 是对象类型，则提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">value_type</span><span class="sy1">></span></span></span> 的成员类型 <code>value_type</code> 。否则无成员 <code>value_type</code> 。</div>
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<div class="t-li1"><span class="t-li">6)</span> 对定义公开可访问成员类型 <code>element_type</code> 的类型（例如 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> ）的特化。若 <code>T::element_type</code> 是对象类型，则提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">element_type</span><span class="sy1">></span></span></span> 的 <code>value_type</code> 。否则无成员 <code>value_type</code> 。</div>
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<div class="t-li1"><span class="t-li">7-8)</span> 对定义公开可访问成员类型 <code>value_type</code> 与 <code>element_type</code> 的类型（例如 <a href="cpp-container-span.html"><tt>std::span</tt></a> ）的特化。若 <code>T::value_type</code> 与 <code>T::element_type</code> 均为对象类型且在移除顶层 cv 限定符后为同一类型，则提供等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_cv.html"><span class="kw597">std::<span class="me2">remove_cv_t</span></span></a><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">value_type</span><span class="sy1">></span></span></span> 的成员类型 <code>value_type</code> 。否则无成员 <code>value_type</code> 。</div>
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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>有意将 <code>value_type</code> 对如迭代器的 <a href="cpp-iterator-indirectly_readable.html"><code>indirectly_readable</code></a> 类型使用。并非有意将它对范围使用。</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-empty-cell"></td>
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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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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3446">LWG 3446</a></td>
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<td>C++20</td>
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<td>特化对含有 <code>value_type</code> 和 <code>element_type</code> 两个成员类型的类型有歧义</td>
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<td>添加了消歧义特化</td>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3541">LWG 3541</a></td>
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<td>C++20</td>
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<td>LWG 3446 对 <code>value_type</code> 与 <code>element_type</code> 相异的情况引入了硬错误</td>
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<td>使之导致替换失败</td>
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</tr>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-indirectly_readable.html"><span class="t-lines"><span>indirectly_readable</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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<td>指定类型通过应用运算符 <code>*</code> 可读<br />
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<span class="t-mark">(概念)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-iter_t.html"><span class="t-lines"><span>iter_value_t</span><span>iter_reference_t</span><span>iter_difference_t</span><span>iter_rvalue_reference_t</span><span>iter_common_reference_t</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>计算迭代器的关联类型<br />
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<span class="t-mark">(别名模板)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-iterator_traits.html"><span class="t-lines"><span>iterator_traits</span></span></a></div>
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</div>
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</td>
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<td>为迭代器各项性质提供统一接口<br />
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<span class="t-mark">(类模板)</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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