210 lines
20 KiB
HTML
210 lines
20 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::fpos</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_io_fpos 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>fpos</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-io.html">io</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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<td>
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<div>定义于头文件 <code><a href="cpp-header-ios.html"><ios></a></code></div>
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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> State <span class="sy1">></span><br />
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<span class="kw1">class</span> fpos<span class="sy4">;</span></span></div>
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<p>类模板 <span class="t-lc"><strong class="selflink">std::fpos</strong></span> 的特化标识流或文件中的绝对位置。每个 <code>fpos</code> 类型对象保有流中的字节位置（常为 <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> 类型的私有成员）和当前迁移状态， <code>State</code> 类型值（常为 <span class="t-lc"><a href="cpp-string-multibyte-mbstate_t.html">std::mbstate_t</a></span> ）。</p>
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<p>提供下列 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">></span></span></span> 的 typedef 名（尽管它们在标准中拼写方式不同，它们代表同一类型）：</p>
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<table class="t-dsc-begin">
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<tr class="t-dsc-header">
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<td colspan="2">
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<div>定义于头文件 <code><a href="cpp-header-iosfwd.html"><iosfwd></a></code></div>
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</td>
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</tr>
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<tr class="t-dsc-hitem">
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<td>类型</td>
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<td>定义</td>
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</tr>
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<tr class="t-dsc">
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<td><code>streampos</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">></span></span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>wstreampos</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span><span class="kw4">wchar_t</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">></span></span></span></td>
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<tr class="t-dsc">
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<td><code>u8streampos</code> <span class="t-mark-rev t-since-cxx20">(C++20)</span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span>char8_t<span class="sy1">></span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">></span></span></span></td>
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<tr class="t-dsc">
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<td><code>u16streampos</code> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span><span class="kw4">char16_t</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">></span></span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>u32streampos</code> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-char_traits.html"><span class="kw1267">std::<span class="me2">char_traits</span></span></a><span class="sy1"><</span><span class="kw4">char32_t</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">state_type</span><span class="sy1">></span></span></span></td>
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<p><code>fpos</code> 的所有特化都符合<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> 、<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 、<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 、<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 和<a href="cpp-named_req-EqualityComparable.html"><span style="font-style:italic">可相等比较</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EqualityComparable)</span></a> 要求。</p>
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<p>若 <code>State</code> 可平凡复制构造，则 <code>fpos</code> 拥有平凡默认构造函数。 若 <code>State</code> 可平凡复制赋值，则 <code>fpos</code> 拥有平凡复制赋值运算符。 若 <code>State</code> 可平凡析构，则 <code>fpos</code> 拥有平凡析构函数。</p>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>State</td>
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<td>-</td>
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<td>表示迁移状态的类型</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 class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>State</code> 必须满足<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 、 <a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 、 <a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 和 <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> 的要求。</td>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</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-io-fpos-state.html"><span class="t-lines"><span>state</span></span></a></div>
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</div>
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</td>
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<td>获取/设置迁移状态的值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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<p>另外，提供支持下列操作的成员和非成员函数：</p>
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<ul>
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<li>默认构造函数，存储零偏移并值初始化状态对象。</li>
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</ul>
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<ul>
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<li>非 explicit 的构造函数，接受一个（可为 const 的） <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> 类型参数，存储其偏移并值初始化状态对象。此构造函数必须亦接受特殊值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-streamoff.html"><span class="kw1803">std::<span class="me2">streamoff</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span></span></span> ：某些流操作返回以此方式构造的 <code>std::fpos</code> ，以指示错误。</li>
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</ul>
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<ul>
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<li>从（可为 const 的） <code>fpos</code> 到 <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> 的显式转换。结果是存储的偏移。</li>
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</ul>
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<ul>
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<li>比较二个（可为 const 的） <code>std::fpos</code> 类型，返回可转换为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 的类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">==</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy3">!</span><span class="sy1">=</span></span></span> 。 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy3">!</span><span class="sy1">=</span> q</span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">(</span>p <span class="sy1">==</span> q<span class="br0">)</span></span></span> 。</li>
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</ul>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span></span></span> ，使得对于（可为 const 的） <code>fpos<State></code> 对象 <code>p</code> 和（可为 const 的） <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> 对象 <code>o</code></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><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">+</span> o</span></span> 拥有 <code>fpos<State></code> 类型，而存储的偏移为将 <code>o</code> 加到 <code>p</code> 的结果</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">o <span class="sy2">+</span> p</span></span> 拥有可转换为 <code>fpos<State></code> 的类型，而转换结果等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">+</span> o</span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy2">-</span> o</span></span> 拥有 <code>fpos<State></code> 类型，而存储的偏移为从 <code>p</code> 的偏移减去 <code>o</code> 的结果</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span><span class="sy1">=</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span><span class="sy1">=</span></span></span> ，能接受一个（可为 const 的） <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> 并分别对存储的偏移加/减之。</li>
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</ul>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">-</span></span></span> ，能对二个（可为 const 的） <code>std::fpos</code> 对象相减，产生 <span class="t-lc"><a href="cpp-io-streamoff.html">std::streamoff</a></span> ，使得对于二个这种对象 <code>p</code> 与 <code>q</code> 有 <span class="t-c"><span class="mw-geshi cpp source-cpp">p <span class="sy1">==</span> q <span class="sy2">+</span> <span class="br0">(</span>p <span class="sy2">-</span> q<span class="br0">)</span></span></span> 。</li>
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</ul>
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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>一些 <a href="cpp-io.html">I/O 流成员函数</a>返回并操纵成员 typedef <code>pos_type</code> 类型对象。对于流，这些成员 typedef 由模板形参 <code>Traits</code> 提供，默认为 <span class="t-lc"><a href="cpp-string-char_traits.html">std::char_traits</a></span> ，它定义其 <code>pos_type</code> 为 <code>std::fpos</code> 的特化。 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">pos_type</span></span></span> 不是 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">fpos</span><span class="sy1"><</span><a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">></span></span></span> （别名为 <code>std::streampos</code>、 <code>std::wstreampos</code> 等等）时， I/O 流库的行为是实现定义的。</p>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue57">LWG 57</a></td>
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<td>C++98</td>
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<td><code>streampos</code> 与 <code>wstreampos</code> 被矛盾地在要求相同时允许不同</td>
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<td>澄清为相同</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P0759R1">P0759R1</a></td>
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<td>C++98</td>
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<td>规定不清晰且不完整</td>
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<td>已整理</td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P1148R0">P1148R0</a></td>
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<td>C++11</td>
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<td>不明确 <code>u16streampos</code> 和 <code>u32streampos</code> 的定义是什么以及在哪个头文件</td>
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<td>使之明确</td>
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</tr>
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</table>
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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-io-streamoff.html"><span class="t-lines"><span>streamoff</span></span></a></div>
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</div>
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</td>
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<td>表示相对的文件/流位置（距 fpos 的偏移），足以表示任何文件大小<br />
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<span class="t-mark">(typedef)</span></td>
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</tr>
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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-io-basic_ostream-tellp.html"><span class="t-lines"><span>tellp</span></span></a></div>
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</div>
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</td>
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<td>返回输出位置指示器<br />
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<span class="t-mark">(<code>std::basic_ostream<CharT,Traits></code> 的公开成员函数)</span></td>
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</tr>
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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-io-basic_ostream-seekp.html"><span class="t-lines"><span>seekp</span></span></a></div>
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</div>
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</td>
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<td>设置输出位置指示器<br />
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<span class="t-mark">(<code>std::basic_ostream<CharT,Traits></code> 的公开成员函数)</span></td>
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</tr>
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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-io-c-fgetpos.html"><span class="t-lines"><span>fgetpos</span></span></a></div>
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</div>
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</td>
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<td>获取文件位置指示器<br />
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<span class="t-mark">(函数)</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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