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<title>std::experimental::ranges::BidirectionalIterator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_experimental_ranges_iterator_BidirectionalIterator 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>BidirectionalIterator</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> I <span class="sy1">></span><br /></span>
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<p>concept <span class="kw4">bool</span> BidirectionalIterator <span class="sy1">=</span><br />
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<a href="cpp-experimental-ranges-iterator-ForwardIterator.html"><span class="kw2571">ranges::<span class="me2">ForwardIterator</span></span></a><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br />
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<a href="cpp-experimental-ranges-concepts-DerivedFrom.html"><span class="kw2531">ranges::<span class="me2">DerivedFrom</span></span></a><span class="sy1"><</span><br />
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ranges<span class="sy4">::</span><span class="me2">iterator_category_t</span><span class="sy1"><</span>I<span class="sy1">></span>, ranges<span class="sy4">::</span><span class="me2">bidirectional_iterator_tag</span><br />
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<span class="sy1">></span> <span class="sy3">&&</span><br />
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requires<span class="br0">(</span>I i<span class="br0">)</span> <span class="br0">{</span><br />
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<span class="br0">{</span> <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>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> <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>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><span class="t-mark-rev t-since-ranges-ts t-mark-ts">(范围 TS)</span></td>
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<p>概念 <code>BidirectionalIterator<I></code> 细化 <a href="cpp-experimental-ranges-iterator-ForwardIterator.html"><code>ForwardIterator</code></a> ，添加后移迭代器的能力。</p>
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<p>称双向迭代器 <code>r</code> <i>可自减</i>，若且唯若存在某个 <code>s</code> 使得 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>s <span class="sy1">==</span> r</span></span> 。所有可自减迭代器 <code>r</code> 应当在表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span>r</span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">r<span class="sy2">--</span></span></span> 的定义域中。</p>
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<p>令 <code>a</code> 与 <code>b</code> 为 <code>I</code> 类型的可自减对象。 <code>BidirectionalIterator<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>前自减生成指代运算数的左值： <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span><span class="sy2">--</span>a<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-memory-addressof.html"><span class="kw757">std::<span class="me2">addressof</span></span></a><span class="br0">(</span>a<span class="br0">)</span></span></span> ；</li>
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<li>后自减生成运算数先前的值：若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a<span class="sy2">--</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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<li>后自减和前自减在运算数上进行相同操作：若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则求值 <code>a--</code> 与 <code>--b</code> 两者后， <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> 仍然成立。</li>
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<li>自增和自减互逆：</li>
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<li>若 <code>a</code> 可自增且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">--</span><span class="br0">(</span><span class="sy2">++</span>a<span class="br0">)</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>a <span class="sy1">==</span> b<span class="br0">)</span></span></span> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span><span class="sy2">++</span><span class="br0">(</span><span class="sy2">--</span>a<span class="br0">)</span> <span class="sy1">==</span> b<span class="br0">)</span></span></span> 。</li>
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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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<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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