143 lines
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143 lines
15 KiB
HTML
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<title>C++ 具名要求：老式向前迭代器 (LegacyForwardIterator)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-named_req.html">named req</a></span></div>
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<p><strong class="selflink"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></strong> 是能从所指向的元素读取数据的<a href="cpp-named_req-Iterator.html"><span style="font-style:italic">老式迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyIterator)</span></a> 。</p>
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<p>不同于<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 和<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></a> ，它可被用于多趟算法。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>若达成下列条件，则类型 <code>It</code> 满足<span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></p>
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<ul>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a></li>
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<li>类型 <code>It</code> 满足<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a></li>
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<li>类型 <code>It</code> 的对象提供下文所述的<i>多趟保证</i></li>
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<li>类型 <code>std::iterator_traits<It>::reference</code> 必须恰为</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>若 <code>It</code> 满足<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></a> （<code>It</code> 可变）则为 <code>T&</code></li>
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<li>否则（<code>It</code> 为常量），为 <code>const T&</code>，</li>
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</ul>
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</dd>
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<dd>（其中 <code>T</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>It<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span></span></span> 所指代的类型）</dd>
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</dl>
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<ul>
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<li>在同一底层序列上的所有迭代器<span class="t-rev-inl t-since-cxx14"><span>和值初始化迭代器</span> <span><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></span></span>中定义等于和不等于比较。</li>
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</ul>
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<p>而且，给定</p>
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<ul>
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<li><code>i</code>，为 <code>It</code> 类型的可解引用迭代器</li>
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<li><code>reference</code>，为 <code>std::iterator_traits<It>::reference</code> 所指代的类型</li>
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</ul>
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<p>则下列表达式必须合法且拥有其指定的效果</p>
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<table class="wikitable">
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<th>表达式</th>
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<th>返回类型</th>
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<th>等价表达式</th>
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<th>注意</th>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">i<span class="sy2">++</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">It ip<span class="sy1">=</span>i<span class="sy4">;</span> <span class="sy2">++</span>i<span class="sy4">;</span> <span class="kw1">return</span> ip<span class="sy4">;</span></span></span></td>
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<td></td>
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</tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>i<span class="sy2">++</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">reference</span></span></td>
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<td></td>
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<td></td>
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</tr>
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</table>
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<p><i>可变的（mutable）</i><strong class="selflink"><span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span></strong> 是还额外满足<a href="cpp-named_req-OutputIterator.html"><span style="font-style:italic">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></a> 要求的<span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span> 。</p>
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<h3><span class="mw-headline" id=".E5.A4.9A.E8.B6.9F.E4.BF.9D.E8.AF.81">多趟保证</span></h3>
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<p>给定 <code>It</code> 类型的可解引用迭代器 <code>a</code> 和 <code>b</code></p>
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<ul>
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<li>若 <code>a</code> 与 <code>b</code> 比较相等（<code><b>a == b</b></code> 可按语境转换为 <code>true</code>），则要么都不可解引用，要么 <code>*a</code> 与 <code>*b</code> 是绑定到同一对象的引用</li>
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<li>若 <code>*a</code> 与 <code>*b</code> 指代相同的对象，则 <code></code>。</li>
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<li>通过可变<code>ForwardIterator</code>进行赋值不会使迭代器失效（隐含地因为 <code>reference</code> 定义为真引用）</li>
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<li>自增 <code>a</code> 的副本不改变从 <code>a</code> 读取的值（正式而言，要么 <code>It</code> 是原生指针类型，要么表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy2">++</span>It<span class="br0">(</span>a<span class="br0">)</span>, <span class="sy2">*</span>a</span></span> 等价于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>a</span></span>）</li>
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<li><code><b>a == b</b></code> 蕴含 <code><b>++a == ++b</b></code></li>
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</ul>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx14">
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<h3><span class="mw-headline" id=".E5.AD.A4.E7.AB.8B.E8.BF.AD.E4.BB.A3.E5.99.A8">孤立迭代器</span></h3>
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<p><a href="cpp-language-value_initialization.html">值初始化</a>的<span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span> 表现类似某个未指定空容器的末尾后迭代器：它与所有值初始化的同类型<span style="font-style:italic">老式向前迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyForwardIterator)</span> 比较为相等。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<h3><span class="mw-headline" id=".E6.A6.82.E5.BF.B5">概念</span></h3>
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<p>为了定义 <span class="t-lc"><a href="cpp-iterator-iterator_traits.html">std::iterator_traits</a></span>，定义了以下仅用于阐释的概念：</p>
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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 __LegacyForwardIterator <span class="sy1">=</span><br />
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__LegacyInputIterator<span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">constructible_from</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br />
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<a href="cpp-types-is_lvalue_reference.html"><span class="kw465">std::<span class="me2">is_lvalue_reference_v</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<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><br />
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std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<span class="sy1">>></span>,<br />
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<span class="kw1">typename</span> std<span class="sy4">::</span><span class="me2">readable_traits</span><span class="sy1"><</span>I<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span><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> i<span class="sy2">++</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span><span class="kw4">const</span> I<span class="sy3">&</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="br0">{</span> <span class="sy2">*</span>i<span class="sy2">++</span> <span class="br0">}</span> <span class="sy2">-</span><span class="sy1">></span> std<span class="sy4">::</span><span class="me2">same_as</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">iter_reference_t</span><span class="sy1"><</span>I<span class="sy1">>></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>
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<td class="t-dcl-nopad"></td>
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<td class="t-dcl-nopad"></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>其中仅用于阐释的概念 <code>__LegacyInputIterator<T></code> 描述于 <a href="cpp-named_req-InputIterator.html#.E6.A6.82.E5.BF.B5">LegacyInputIterator#概念</a>。</p>
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</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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<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-forward_iterator.html"><span class="t-lines"><span>forward_iterator</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>
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<td>指定 <a href="cpp-iterator-input_iterator.html"><code>input_iterator</code></a> 为向前迭代器，支持相等比较与多趟操作<br />
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<span class="t-mark">(概念)</span></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-iterator.html"><b>迭代器库</b></a></td>
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<td>提供迭代器的定义、迭代器表征、适配器及工具函数</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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</div>
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</div>
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</body>
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