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117 lines
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<title>C++ 具名要求：老式输出迭代器 (LegacyOutputIterator)</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">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</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">老式输出迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyOutputIterator)</span></strong> 是能写入所指向元素的<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>一个实现<strong class="selflink"><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></strong> 的类型的例子是 <a href="cpp-iterator-ostream_iterator.html"><tt>std::ostream_iterator</tt></a>。</p>
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<p>当<a href="cpp-named_req-ForwardIterator.html"><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></a> 、<a href="cpp-named_req-BidirectionalIterator.html"><span style="font-style:italic">老式双向迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyBidirectionalIterator)</span></a> 或<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 在自身的要求之外还满足<strong class="selflink"><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></strong> 的要求时，它即被描述为<b>可变的（mutable）</b>。</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>以下情况下，类型 <code>X</code> 满足<strong class="selflink"><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></strong></p>
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<ul>
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<li>类型 <code>X</code> 满足<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></li>
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<li><code>X</code> 是类类型或指针类型</li>
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</ul>
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<p>且，给定</p>
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<ul>
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<li><code>o</code>，是输出迭代器对应的可写入类型的值（可以有多种可写入的类型，例如 <code>operator=</code> 是模板的情况。并没有像输入迭代器的 <code>value_type</code> 那种记号）</li>
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<li><code>r</code>，为 <code>X</code> 类型的左值，</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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<tr>
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<th>表达式</th>
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<th>返回</th>
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<th>等价表达式</th>
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<th>前条件</th>
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<th>后条件</th>
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<th>注解</th>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>r <span class="sy1">=</span> o</span></span></td>
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<td>（不使用）</td>
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<td></td>
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<td><code>r</code> 可解引用</td>
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<td><code>r</code> 可自增</td>
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<td>在此操作后不要求 <code>r</code> 可解引用，且不再要求任何 <code>r</code> 的先前值的副本可解引用或可自增。</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>r</span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">X<span class="sy3">&</span></span></span></td>
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<td></td>
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<td><code>r</code> 可自增</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">r</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>r</span></span> 指代同一迭代器对象，<code>r</code> 可解引用或为末尾后迭代器</td>
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<td>在此操作后不要求 <code>r</code> 可自增，且不再要求任何 <code>r</code> 的先前值的副本可解引用或可自增。</td>
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</tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">r<span class="sy2">++</span></span></span></td>
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<td>可转换到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> X<span class="sy3">&</span></span></span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">X temp <span class="sy1">=</span> r<span class="sy4">;</span><br /></span></span>
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<p><span class="sy2">++</span>r<span class="sy4">;</span><br />
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<span class="kw1">return</span> temp<span class="sy4">;</span></p>
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</td>
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<td></td>
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<td></td>
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<td></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>r<span class="sy2">++</span> <span class="sy1">=</span> o</span></span></td>
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<td>（不使用）</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>r <span class="sy1">=</span> o<span class="sy4">;</span><br /></span></span>
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<p><span class="sy2">++</span>r<span class="sy4">;</span></p>
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</td>
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<td></td>
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<td></td>
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<td></td>
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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>operator*</code> 对于输出迭代器的仅有的合法用法是在赋值的左侧：<code>operator*</code> 可以返回一个代理对象，它定义了成员 <code>operator=</code>（可以是模板）</p>
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<p>可以不对输出迭代器定义相等和不相等。即使定义了 <code>operator==</code>，<code>x == y</code> 也不需要隐含 <code>++x == ++y</code>。</p>
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<p>通过输出迭代器的同一值进行的赋值只会发生一次：输出迭代器上的算法必须是单趟算法。</p>
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<p>预期通过输出迭代器的赋值与自增交替进行。双重自增是未定义行为（C++ 标准当前声称双重自增得到支持，这与 STL 文档相反；此为 <a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2035">LWG2035</a>）</p>
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<p>允许纯粹仅支持输出的迭代器声明其 <code>iterator_traits<X>::value_type</code>、<code>iterator_traits<X>::difference_type</code>、<code>iterator_traits<X>::pointer</code> 与 <code>iterator_traits<X>::reference</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>（如 <span class="t-lc"><a href="cpp-iterator-back_insert_iterator.html">std::back_insert_iterator</a></span> 这样的迭代器正如此）。</p>
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<h3><span class="mw-headline" id=".E6.A0.87.E5.87.86.E5.BA.93">标准库</span></h3>
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<p>下列的标准库迭代器是输出迭代器而非向前迭代器：</p>
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<ul>
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<li><span class="t-lc"><a href="cpp-iterator-ostream_iterator.html">std::ostream_iterator</a></span></li>
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<li><span class="t-lc"><a href="cpp-iterator-ostreambuf_iterator.html">std::ostreambuf_iterator</a></span></li>
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<li><span class="t-lc"><a href="cpp-iterator-insert_iterator.html">std::insert_iterator</a></span></li>
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<li><span class="t-lc"><a href="cpp-iterator-back_insert_iterator.html">std::back_insert_iterator</a></span></li>
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<li><span class="t-lc"><a href="cpp-iterator-front_insert_iterator.html">std::front_insert_iterator</a></span></li>
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</ul>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-output_iterator.html"><span class="t-lines"><span>output_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>指定类型为给定的值类型的输出迭代器，即可向其写入该类型的值，且可前/后自增<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 class="visualClear"></div>
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</div>
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