204 lines
17 KiB
HTML
204 lines
17 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::distance</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_distance skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>distance</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-iterator.html">iterator</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<tbody>
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<td>
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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</td>
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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> InputIt <span class="sy1">></span><br /></span>
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<p><span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">difference_type</span><br /></p>
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distance<span class="br0">(</span> InputIt first, InputIt last <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><span class="t-mark">(C++17 起为 <code>constexpr</code>)</span></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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</tr>
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</tbody>
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</table>
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<p>返回从 <code>first</code> 到 <code>last</code> 的路程。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx11">
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<td>
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<p>若 <code>last</code> 不可从 <code>first</code> 通过（可以重复）自增 <code>first</code> 抵达，则行为未定义。</p>
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</td>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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</tr>
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<tr class="t-rev t-since-cxx11">
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<p>若 <code>InputIt</code> 不是<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> ，则若 <code>last</code> 不可从 <code>first</code> 通过（可以重复）自增 <code>first</code> 抵达，则行为未定义。 若 <code>InputIt</code> 是<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> ，则若 <code>last</code> 不可从 <code>first</code> 抵达且 <code>first</code> 不可从 <code>last</code> 抵达，则行为未定义。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>first</td>
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<td>-</td>
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<td>指向首元素的迭代器</td>
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</tr>
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<tr class="t-par">
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<td>last</td>
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<td>-</td>
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<td>指向范围尾的迭代器</td>
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</tr>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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</tr>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>InputIt</code> 必须满足<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> 的要求。若 <code>InputIt</code> 额外满足<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> 的要求则操作更高效</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>从 <code>first</code> 走到 <code>last</code> 所需的自增数。<span class="t-rev-inl t-since-cxx11"><span>若使用随机访问迭代器且 <code>first</code> 可从 <code>last</code> 抵达，则值可能为负。</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span></p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>线性。</p>
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<p>然而，若 <code>InputIt</code> 额外满足<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> 的要求，则复杂度是常数。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<p>参阅 <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/d9375e490072d1aae73a93949aa158fcd2a27018/libstdc%2B%2B-v3/include/bits/stl_iterator_base_funcs.h#L135">libstdc++</a> 与 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/a12cb9d211019d99b5875b6d8034617cbc24c2cc/include/iterator#L611">libc++</a> 中的实现。</p>
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<p><br /></p>
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<table class="eq-fun-cpp-table">
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<tr>
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<th>版本一</th>
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</tr>
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<tr>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co1">// 经由标签派发的实现，移除 constexpr 后可用于 C++98</span>
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<span class="kw1">namespace</span> detail <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> It<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++17 起要求</span>
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<span class="kw1">typename</span> <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">difference_type</span>
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do_distance<span class="br0">(</span>It first, It last, <a href="cpp-iterator-iterator_tags.html"><span class="kw652">std::<span class="me2">input_iterator_tag</span></span></a><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">typename</span> <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">difference_type</span> result <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span>first <span class="sy3">!</span><span class="sy1">=</span> last<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">++</span>first<span class="sy4">;</span>
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<span class="sy2">++</span>result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> It<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++17 起要求</span>
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<span class="kw1">typename</span> <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">difference_type</span>
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do_distance<span class="br0">(</span>It first, It last, <a href="cpp-iterator-iterator_tags.html"><span class="kw656">std::<span class="me2">random_access_iterator_tag</span></span></a><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> last <span class="sy2">-</span> first<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span> <span class="co1">// namespace detail</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> It<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="co1">// C++17 起</span>
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<span class="kw1">typename</span> <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">difference_type</span>
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distance<span class="br0">(</span>It first, It last<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> detail<span class="sy4">::</span><span class="me2">do_distance</span><span class="br0">(</span>first, last,
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<span class="kw1">typename</span> <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">iterator_category</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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<tr>
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<th>版本二</th>
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</tr>
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<tr>
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<td>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co1">// 经由 constexpr if 的实现，可用于 C++17</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> It<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">typename</span> <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">difference_type</span>
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distance<span class="br0">(</span>It first, It last<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">using</span> category <span class="sy1">=</span> <span class="kw1">typename</span> <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">iterator_category</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_base_of.html"><span class="kw591">std::<span class="me2">is_base_of_v</span></span></a><span class="sy1"><</span><a href="cpp-iterator-iterator_tags.html"><span class="kw652">std::<span class="me2">input_iterator_tag</span></span></a>, category<span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_base_of.html"><span class="kw591">std::<span class="me2">is_base_of_v</span></span></a><span class="sy1"><</span><a href="cpp-iterator-iterator_tags.html"><span class="kw656">std::<span class="me2">random_access_iterator_tag</span></span></a>, category<span class="sy1">></span><span class="br0">)</span>
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<span class="kw1">return</span> last <span class="sy2">-</span> first<span class="sy4">;</span>
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<span class="kw1">else</span> <span class="br0">{</span>
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<span class="kw1">typename</span> <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">difference_type</span> result <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span>first <span class="sy3">!</span><span class="sy1">=</span> last<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">++</span>first<span class="sy4">;</span>
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<span class="sy2">++</span>result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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</td>
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</table>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <vector></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1268">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span> <span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"distance(first, last) = "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"distance(last, first) = "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">distance(first, last) = 3
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distance(last, first) = -3</pre></div>
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</div>
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</div>
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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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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-advance.html"><span class="t-lines"><span>advance</span></span></a></div>
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</div>
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</td>
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<td>令迭代器前进给定的距离<br />
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<span class="t-mark">(函数模板)</span></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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</html>
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