81 lines
7.3 KiB
HTML
81 lines
7.3 KiB
HTML
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<title>std::ranges::sized_range, std::ranges::disable_sized_range</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_sized_range 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::ranges::</span>sized_range, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>disable_sized_range</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-ranges.html">ranges</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></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> T <span class="sy1">></span><br /></span>
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<p>concept sized_range <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">range</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>T<span class="sy3">&</span> t<span class="br0">)</span> <span class="br0">{</span><br />
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ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>t<span class="br0">)</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span><span class="sy4">;</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><span class="sy1">></span><br />
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> disable_sized_range <span class="sy1">=</span> <span class="kw2">false</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 class="t-li1"><span class="t-li">1)</span> <code>sized_range</code> 概念指定在常数时间内以 <code>size</code> 函数知晓其大小的 <a href="cpp-ranges-range.html"><code>range</code></a> 类型。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>disable_sized_range</code> 为允许使用（成员或非成员）提供 <code>size</code> 函数但不实际实现 <code>sized_range</code> 的范围类型存在。用户可对无 cv 限定的程序定义类型特化 <code>disable_sized_range</code> 。这些特化应当可用于<a href="cpp-language-constant_expression.html">常量表达式</a>并拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">bool</span></span></span> 类型。</div>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 给定 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 类型左值 <code>t</code> ， <code>T</code> 实现 <code>sized_range</code> 仅若
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<ul>
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<li><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>t<span class="br0">)</span></span></span></li>
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</ul>
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<dl>
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<dd>
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<li>拥有非均摊的常数时间复杂度，</li>
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<li>不以对于保持相等性的表达式可观察的方式更改 <code>t</code> 的值，且</li>
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<li>等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>t<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> ，而</li>
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</dd>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">iterator_t</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 实现 <a href="cpp-iterator-forward_iterator.html"><code>forward_iterator</code></a> ，则 <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>t<span class="br0">)</span></span></span> 为良定义，而无关乎 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>t<span class="br0">)</span></span></span> 的求值（换言之，单趟有大小范围可以只在首次调用 <code>begin</code> 前支持调用 <code>size</code> ，但向前范围必须在所有时间支持）</li>
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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><code>disable_sized_range</code> 不能用于手动禁用迭代器与哨位满足 <a href="cpp-iterator-sized_sentinel_for.html"><code>sized_sentinel_for</code></a> 的范围；必须用 <code>disable_sized_sentinel_for</code> 代替。</p>
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<p>不能对数组类型或引用类型特化 <code>disable_sized_range</code> 。</p>
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