78 lines
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78 lines
10 KiB
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<title>std::experimental::ranges::SizedSentinel</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_ranges_iterator_SizedSentinel 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::experimental::ranges::</span>SizedSentinel</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-experimental.html">experimental</a>‎ | <a href="cpp-experimental-ranges.html">ranges</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-experimental-ranges-iterator.html"><experimental/ranges/iterator></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> S, <span class="kw1">class</span> I<span class="sy1">></span><br /></span>
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<p>concept <span class="kw4">bool</span> SizedSentinel <span class="sy1">=</span><br />
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<a href="cpp-experimental-ranges-iterator-Sentinel.html"><span class="kw2567">ranges::<span class="me2">Sentinel</span></span></a><span class="sy1"><</span>S, I<span class="sy1">></span> <span class="sy3">&&</span><br />
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<span class="sy3">!</span>ranges<span class="sy4">::</span><span class="me2">disable_sized_sentinel</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>S<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>I<span class="sy1">>></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span><span class="kw4">const</span> I<span class="sy3">&</span> i, <span class="kw4">const</span> S<span class="sy3">&</span> s<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> s <span class="sy2">-</span> i <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-experimental-ranges-concepts-Same.html"><span class="kw2530">ranges::<span class="me2">Same</span></span></a><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">difference_type_t</span><span class="sy1"><</span>I<span class="sy1">>></span><span class="sy3">&&</span><span class="sy4">;</span><br />
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<span class="br0">{</span> i <span class="sy2">-</span> s <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-experimental-ranges-concepts-Same.html"><span class="kw2530">ranges::<span class="me2">Same</span></span></a><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">difference_type_t</span><span class="sy1"><</span>I<span class="sy1">>></span><span class="sy3">&&</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-ranges-ts t-mark-ts">(范围 TS)</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> S, <span class="kw1">class</span> I<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> disable_sized_sentinel <span class="sy1">=</span> <span class="kw2">false</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-ranges-ts t-mark-ts">(范围 TS)</span></td>
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<p><code>SizedSentinel</code> 概念指定能在常数时间内相减迭代器类型 <code>I</code> 的对象和哨位类型 <code>S</code> 的对象，计算其间的距离。</p>
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<p>令 <code>i</code> 为 <code>I</code> 类型迭代器，而 <code>s</code> 为 <code>S</code> 类型哨位，使得 <code>[i, s)</code> 代表范围。令 <code>N</code> 为需要应用 <code>++i</code> 以令 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>i <span class="sy1">==</span> s<span class="br0">)</span></span></span> 为 true 的最小次数。则 <code>SizedSentinel<S, I></code> 仅若符合下列条件才得到满足：</p>
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<ul>
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<li>若 <code>N</code> 能以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">difference_type_t</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 表示，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">s <span class="sy2">-</span> i</span></span> 良定义并等于 <code>N</code> ；且</li>
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<li>若 <code>-N</code> 能以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">difference_type_t</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 表示，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy2">-</span> s</span></span> 良定义并等于 <code>-N</code> 。</li>
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</ul>
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<p>变量模板 <code>disable_sized_sentinel</code> 通过特化变量模板为拥有值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，为能相减但不符合 <code>SizedSentinel</code> 语义要求的迭代器和哨位提供撤出概念的机制。</p>
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<h3><span class="mw-headline" id=".E7.9B.B8.E7.AD.89.E6.80.A7.E4.BF.9D.E6.8C.81">相等性保持</span></h3>
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<p>若表达式对给定的相等输入产生相等输出，则它<i>保持相等性</i>。</p>
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<ul>
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<li>表达式的输入由其运算数组成。</li>
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<li>表达式的输出由其结果和表达式所修改的所有运算数（若存在）组成。</li>
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</ul>
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<p>进一步要求每个要求保持相等性的表达式都<i>稳定</i>：这种表达式带相同输入对象的二次求值必须拥有相等的输出，而无任何对这些输入对象的显式中间修改。</p>
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<p>除非另外提醒，每个用于 <i>requires 表达式</i>中的表达式都要求保持相等性且稳定，而表达式的求值必须只修改其非常运算数。必须不修改常运算数。</p>
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<h3><span class="mw-headline" id=".E9.9A.90.E5.BC.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F.E5.8F.98.E7.A7.8D">隐式表达式变种</span></h3>
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<p>使用不修改某 const 左值运算数的表达式的 <i>requires 表达式</i>亦隐式要求该表达式的额外变种对给定运算数接受非 const 左值或（可为 const 的）右值，除非以有区别的语义显式要求这种表达式变种。这些<i>隐式表达式变种</i>必须符合与声明的表达式的相同的语义。不指定实现以何种程度校验变种的语法。</p>
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