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<title>std::basic_filebuf&lt;CharT,Traits&gt;::seekpos</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_filebuf&lt;CharT,Traits&gt;::</span>seekpos</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-io.html">io</a>&lrm; | <a href="cpp-io-basic_filebuf.html">basic filebuf</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>
<p><span class="kw1">virtual</span> pos_type seekpos<span class="br0">(</span> pos_type sp,<br /></p>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <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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<p>&#33509;&#21487;&#33021;&#65292;&#21017;&#37325;&#23547;&#20301;&#25991;&#20214;&#25351;&#38024;&#21040; <code>sp</code> &#25152;&#25351;&#31034;&#30340;&#20301;&#32622;&#12290;</p>
<p>&#33509;&#20851;&#32852;&#25991;&#20214;&#26410;&#25171;&#24320;&#65288; <span class="t-c"><span class="mw-geshi cpp source-cpp">is_open<span class="br0">(</span><span class="br0">)</span><span class="sy1">==</span><span class="kw2">false</span></span></span> &#65289;&#65292;&#21017;&#31435;&#21363;&#22833;&#36133;&#12290;</p>
<p>&#33509;&#25991;&#20214;&#20026;&#20889;&#20837;&#25171;&#24320;&#65292;&#21017;&#39318;&#20808;&#29992; <span class="t-lc"><a href="cpp-io-basic_streambuf-overflow.html">overflow()</a></span> &#20889;&#20837;&#25918;&#32622;&#21306;&#21644;&#20219;&#20309;&#24403;&#21069;&#24863;&#26579;&#30340; locale &#25152;&#35201;&#27714;&#30340;&#21453;&#36801;&#31227;&#24207;&#21015;&#12290;</p>
<p>&#28982;&#21518;&#22914;&#21516;&#36890;&#36807;&#35843;&#29992; <span class="t-lc"><a href="cpp-io-c-fsetpos.html">std::fsetpos()</a></span> &#37325;&#23547;&#20301;&#25351;&#38024;&#12290;</p>
<p>&#33509;&#25991;&#20214;&#20026;&#35835;&#21462;&#25171;&#24320;&#65292;&#21017;&#33509;&#38656;&#35201;&#21017;&#26356;&#26032;&#33719;&#21462;&#21306;&#12290;</p>
<p>&#33509; <code>sp</code> &#19981;&#26159;&#30001;&#22312;&#21516;&#19968;&#25991;&#20214;&#19978;&#35843;&#29992; <span class="t-lc"><a href="cpp-io-basic_streambuf-pubseekoff.html">seekoff()</a></span> &#25110; <code>seekpos()</code> &#33719;&#24471;&#65292;&#21017;&#34892;&#20026;&#26410;&#23450;&#20041;&#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
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<td>sp</td>
<td>-</td>
<td>&#20043;&#21069;&#22312;&#21516;&#19968;&#25991;&#20214;&#19978;&#35843;&#29992; <span class="t-lc"><a href="cpp-io-basic_streambuf-pubseekoff.html">seekoff()</a></span> &#25110; <code>seekpos()</code> &#33719;&#24471;&#30340;&#25991;&#20214;&#20301;&#32622;</td>
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<td>which</td>
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<td>&#30830;&#23450;&#35201;&#24433;&#21709;&#36755;&#20837;&#21644;/&#25110;&#36755;&#20986;&#24207;&#21015;&#30340;&#21738;&#20010;&#12290;&#23427;&#33021;&#20026;&#19979;&#21015;&#24120;&#37327;&#20043;&#19968;&#25110;&#20854;&#32452;&#21512;&#65306;
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<td>&#24120;&#37327;</td>
<td>&#35299;&#37322;</td>
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<td><a href="cpp-io-ios_base-openmode.html"><tt>in</tt></a></td>
<td>&#24433;&#21709;&#36755;&#20837;&#24207;&#21015;</td>
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<td><a href="cpp-io-ios_base-openmode.html"><tt>out</tt></a></td>
<td>&#24433;&#21709;&#36755;&#20986;&#24207;&#21015;</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#25104;&#21151;&#26102;&#20026; <code>sp</code> &#65292;&#22833;&#36133;&#26102;&#20026; <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> &#12290;</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">&#27880;&#24847;</span></h3>
<p><code>seekpos()</code> &#20026; <span class="t-lc"><a href="cpp-io-basic_streambuf-pubseekpos.html">std::basic_streambuf::pubseekpos()</a></span> &#25152;&#35843;&#29992;&#65292;&#21518;&#32773;&#20026; <span class="t-lc"><a href="cpp-io-basic_istream-seekg.html">std::basic_istream::seekg()</a></span> &#21644; <span class="t-lc"><a href="cpp-io-basic_ostream-seekp.html">std::basic_ostream::seekp()</a></span> &#30340;&#21333;&#21442;&#25968;&#29256;&#26412;&#35843;&#29992;&#12290;</p>
<p>&#35768;&#22810;&#23454;&#29616;&#19981;&#20110; <code>seekpos()</code> &#20013;&#26356;&#26032;&#33719;&#21462;&#21306;&#65292;&#32780;&#26159;&#22996;&#25176;&#32473;&#19979;&#27425; <span class="t-lc"><a href="cpp-io-basic_streambuf-sgetc.html">sgetc()</a></span> &#25152;&#35843;&#29992;&#30340; <span class="t-lc"><a href="cpp-io-basic_streambuf-underflow.html">underflow()</a></span> &#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<p>&#19968;&#20123;&#23454;&#29616;&#19978;&#65292; seekpos() &#28165;&#31354;&#33719;&#21462;&#21306;&#24182;&#38656;&#35201;&#31532;&#20108;&#20010; underflow() &#20197;&#35266;&#27979;&#25928;&#26524;</p>
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<pre class="de1"><span class="co2">#include &lt;fstream&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> mybuf <span class="sy4">:</span> <a href="cpp-io-basic_filebuf.html"><span class="kw1725">std::<span class="me2">filebuf</span></span></a>
<span class="br0">{</span>
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>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;Before seekpos(&quot;</span> <span class="sy1">&lt;&lt;</span> sp <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;), size of the get area is &quot;</span>
<span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&quot; with &quot;</span>
<span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&quot; read positions available<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
pos_type rc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">filebuf</span><span class="sy4">::</span><span class="me2">seekpos</span><span class="br0">(</span>sp, which<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;seekpos() returns &quot;</span> <span class="sy1">&lt;&lt;</span> rc <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;.<span class="es1">\n</span>After the call, &quot;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot;size of the get area is &quot;</span>
<span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&quot; with &quot;</span>
<span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&quot; read positions available<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="co1">// &#33509; seekpos() &#28165;&#31354;&#33719;&#21462;&#21306;&#21017;&#21453;&#27880;&#37322;</span>
<span class="co1">// std::filebuf::underflow();</span>
<span class="co1">// std::cout &lt;&lt; &quot;after forced underflow(), size of the get area is &quot;</span>
<span class="co1">// &lt;&lt; egptr()-eback() &lt;&lt; &quot; with &quot;</span>
<span class="co1">// &lt;&lt; egptr()-gptr() &lt;&lt; &quot; read positions available\n&quot;;</span>
<span class="kw1">return</span> rc<span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
mybuf buf<span class="sy4">;</span>
buf.<span class="me1">open</span><span class="br0">(</span><span class="st0">&quot;test.txt&quot;</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="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-basic_istream.html"><span class="kw1727">std::<span class="me2">istream</span></span></a> stream<span class="br0">(</span><span class="sy3">&amp;</span>buf<span class="br0">)</span><span class="sy4">;</span>
stream.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// &#35835;&#19968;&#20010;&#23383;&#31526;&#20197;&#24378;&#21046; underflow()</span>
stream.<span class="me1">seekg</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
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<pre class="de1">Before seekpos(2), size of the get area is 110 with 109 read positions available
seekpos() returns 2.
After the call, size of the get area is 110 with 108 read positions available</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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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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<td>&#35843;&#29992; <span class="t-c"><span class="mw-geshi cpp source-cpp">seekpos<span class="br0">(</span><span class="br0">)</span></span></span><br />
<span class="t-mark">(<code>std::basic_streambuf&lt;CharT,Traits&gt;</code> &#30340;&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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<div><a href="cpp-io-basic_filebuf-seekoff.html"><span class="t-lines"><span>seekoff</span></span></a></div>
<div><span class="t-lines"><span><span class="t-cmark">[&#34394;]</span></span></span></div>
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<td>&#29992;&#30456;&#23545;&#23547;&#22336;&#37325;&#23547;&#20301;&#25991;&#20214;&#20301;&#32622;<br />
<span class="t-mark">(&#34394;&#21463;&#20445;&#25252;&#25104;&#21592;&#20989;&#25968;)</span></td>
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<div><a href="cpp-io-c-fseek.html"><span class="t-lines"><span>fseek</span></span></a></div>
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<td>&#31227;&#21160;&#25991;&#20214;&#20301;&#32622;&#25351;&#31034;&#22120;&#21040;&#25991;&#20214;&#20013;&#30340;&#25351;&#23450;&#20301;&#32622;<br />
<span class="t-mark">(&#20989;&#25968;)</span></td>
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