161 lines
14 KiB
HTML
161 lines
14 KiB
HTML
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<title>std::strstreambuf::seekpos</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_strstreambuf_seekpos 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::strstreambuf::</span>seekpos</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-io.html">io</a>‎ | <a href="cpp-io-strstreambuf.html">strstreambuf</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">protected</span><span class="sy4">:</span><br /></span>
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<p><span class="kw1">virtual</span> pos_type seekpos<span class="br0">(</span>pos_type sp,<br /></p>
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<a href="cpp-io-ios_base-openmode.html"><span class="kw1671">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">openmode</span></span></a> which <span class="sy1">=</span> <a href="cpp-io-ios_base-openmode.html"><span class="kw1674">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">in</span></span></a> <span class="sy3">|</span> <a href="cpp-io-ios_base-openmode.html"><span class="kw1675">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">out</span></span></a> <span class="br0">)</span><span class="sy4">;</span></div>
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<td></td>
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<p>若可能，则重寻位 <span class="t-lc"><a href="cpp-io-basic_streambuf-gptr.html">std::basic_streambuf::gptr</a></span> 和/或 <span class="t-lc"><a href="cpp-io-basic_streambuf-pptr.html">std::basic_streambuf::pptr</a></span> 到 <code>sp</code> 所指示的位置。</p>
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<p>若 <code>which</code> 中设置了 <span class="t-lc"><a href="cpp-io-ios_base-openmode.html">std::ios_base::in</a></span> ，则试图重寻位 <code>gptr()</code> （获取区中的下一位置指针）。若 <code>which</code> 中设置了 <span class="t-lc"><a href="cpp-io-ios_base-openmode.html">std::ios_base::out</a></span> ，则试图重寻位 <code>pptr()</code> （放置区中的下一位置指针）。若 <code>which</code> 中未设置这些位，则操作失败。</p>
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<p>以下列方式重寻位每个下一位置指针：</p>
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<div class="t-li1"><span class="t-li">1)</span> 若下一位置指针为空，则操作失败</div>
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<div class="t-li1"><span class="t-li">2)</span> 否则，通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">sp.<span class="me1">offset</span><span class="br0">(</span><span class="br0">)</span></span></span> 确定新偏移（ <code>off_type</code> 类型的） <code>newoff</code> 。若 <code>newoff</code> 为负、在缓冲区边界外或非法，则操作失败。</div>
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<div class="t-li1"><span class="t-li">3)</span> 否则，如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> eback<span class="br0">(</span><span class="br0">)</span><span class="sy2">+</span>newoff</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">pptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> pbase<span class="br0">(</span><span class="br0">)</span><span class="sy2">+</span>newoff</span></span> 赋值下一位置指针。</div>
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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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<td>sp</td>
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<td>-</td>
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<td>流位置，由 <span class="t-lc"><a href="cpp-io-strstreambuf-seekoff.html">seekoff()</a></span> 或 <code>seekpos()</code> 所获得者</td>
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<tr class="t-par">
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<td>which</td>
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<td>-</td>
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<td>定义要影响输入序列、输出序列或两者。它能为下列常量之一或其组合：
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<table class="t-dsc-begin">
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<td>常量</td>
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<td>解释</td>
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<td><a href="cpp-io-ios_base-openmode.html"><tt>in</tt></a></td>
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<td>影响输入序列</td>
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<td><a href="cpp-io-ios_base-openmode.html"><tt>out</tt></a></td>
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<td>影响输出序列</td>
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</tr>
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</td>
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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>成功时为转换到 <code>pos_type</code> 的结果偏移，失败时为 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos_type<span class="br0">(</span>off_type<span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p><code>seekpos()</code> 为 <span class="t-lc"><a href="cpp-io-basic_streambuf-pubseekpos.html">std::basic_streambuf::pubseekpos()</a></span> 所调用，后者为 <span class="t-lc"><a href="cpp-io-basic_istream-seekg.html">std::basic_istream::seekg()</a></span> 和 <span class="t-lc"><a href="cpp-io-basic_ostream-seekp.html">std::basic_ostream::seekp()</a></span> 的单参数版本所调用。</p>
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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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <strstream></span>
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<span class="co2">#include <cstring></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> mybuf <span class="sy4">:</span> <a href="cpp-io-strstreambuf.html"><span class="kw1752">std::<span class="me2">strstreambuf</span></span></a>
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<span class="br0">{</span>
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mybuf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> str<span class="br0">)</span> <span class="sy4">:</span> <a href="cpp-io-strstreambuf.html"><span class="kw1752">std::<span class="me2">strstreambuf</span></span></a><span class="br0">(</span>str, <a href="cpp-string-byte-strlen.html"><span class="kw1145">std::<span class="me2">strlen</span></span></a><span class="br0">(</span>str<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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pos_type seekpos<span class="br0">(</span>pos_type sp, <a href="cpp-io-ios_base-openmode.html"><span class="kw1671">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">openmode</span></span></a> which<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Before seekpos("</span> <span class="sy1"><<</span> sp <span class="sy1"><<</span> <span class="st0">"), size of the get area is "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>eback<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" with "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" read positions available<span class="es1">\n</span>"</span><span class="sy4">;</span>
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pos_type rc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">strstreambuf</span><span class="sy4">::</span><span class="me2">seekpos</span><span class="br0">(</span>sp, which<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"seekpos() returns "</span> <span class="sy1"><<</span> rc <span class="sy1"><<</span> <span class="st0">".<span class="es1">\n</span>After the call, "</span>
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<span class="sy1"><<</span> <span class="st0">"size of the get area is "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>eback<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" with "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" read positions available<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> rc<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</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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mybuf buf<span class="br0">(</span><span class="st0">"12345"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-basic_iostream.html"><span class="kw1731">std::<span class="me2">iostream</span></span></a> stream<span class="br0">(</span><span class="sy3">&</span>buf<span class="br0">)</span><span class="sy4">;</span>
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stream.<span class="me1">seekg</span><span class="br0">(</span><span class="nu0">2</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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<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">Before seekpos(2), size of the get area is 5 with 5 read positions available
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seekpos() returns 2.
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After the call, size of the get area is 5 with 3 read positions available</pre></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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_streambuf-pubseekpos.html"><span class="t-lines"><span>pubseekpos</span></span></a></div>
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</div>
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</td>
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<td>调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">seekpos<span class="br0">(</span><span class="br0">)</span></span></span><br />
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<span class="t-mark">(<code>std::basic_streambuf<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_stringbuf-seekpos.html"><span class="t-lines"><span>seekpos</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>用绝对寻址，重定位输入序列、输出序列或两者中的下一位置指针<br />
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<span class="t-mark">(<code>std::basic_stringbuf<CharT,Traits,Allocator></code> 的虚受保护成员函数)</span></td>
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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_filebuf-seekpos.html"><span class="t-lines"><span>seekpos</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>用绝对寻址重寻位文件位置<br />
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<span class="t-mark">(<code>std::basic_filebuf<CharT,Traits></code> 的虚受保护成员函数)</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-strstreambuf-seekoff.html"><span class="t-lines"><span>seekoff</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>用相对寻址重寻位输入序列、输出序列或两者中的下一位置指针<br />
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<span class="t-mark">(虚受保护成员函数)</span></td>
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</tr>
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</table>
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