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<title>std::codecvt&lt;InternT,ExternT,State&gt;::length, do_length</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::codecvt&lt;InternT,ExternT,State&gt;::</span>length, do_length</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-locale.html">locale</a>&lrm; | <a href="cpp-locale-codecvt.html">codecvt</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-locale.html">&lt;locale&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">public</span><span class="sy4">:</span><br /></span>
<p><span class="kw4">int</span> length<span class="br0">(</span> StateT<span class="sy3">&amp;</span> state,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <span class="kw4">const</span> ExternT<span class="sy2">*</span> from,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <span class="kw4">const</span> ExternT<span class="sy2">*</span> from_end,<br /></p>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> max <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></div>
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<td>(1)</td>
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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> <span class="kw4">int</span> do_length<span class="br0">(</span> StateT<span class="sy3">&amp;</span> state,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<span class="kw4">const</span> ExternT<span class="sy2">*</span> from,<br />
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<span class="kw4">const</span> ExternT<span class="sy2">*</span> from_end,<br /></p>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> max <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></div>
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<td>(2)</td>
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<div class="t-li1"><span class="t-li">1)</span> &#20844;&#24320;&#25104;&#21592;&#20989;&#25968;&#65292;&#35843;&#29992;&#26368;&#32456;&#23548;&#20986;&#31867;&#30340;&#25104;&#21592;&#20989;&#25968; <code>do_length</code> &#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#32473;&#23450;&#21021;&#22987;&#36716;&#25442;&#29366;&#24577; <code>state</code> &#65292;&#35797;&#22270;&#36716;&#25442;&#26469;&#33258; <code>[from, from_end)</code> &#25152;&#23450;&#20041;&#30340;&#23383;&#31526;&#25968;&#32452;&#30340; <code>externT</code> &#23383;&#31526;&#65292;&#20026;&#33267;&#22810; <code>max</code> &#20010; <code>internT</code> &#23383;&#31526;&#65292;&#24182;&#36820;&#22238;&#36825;&#31181;&#36716;&#25442;&#20250;&#28040;&#32791;&#30340; <code>externT</code> &#23383;&#31526;&#25968;&#12290;&#22914;&#21516;&#20197;&#23545;&#26576;&#34394;&#26500;&#30340; <code>[to, to+max)</code> &#36755;&#20986;&#32531;&#20914;&#21306;&#25191;&#34892; <span class="t-c"><span class="mw-geshi cpp source-cpp">do_in<span class="br0">(</span>state, from, from_end, from, to, to<span class="sy2">+</span>max, to<span class="br0">)</span></span></span> &#19968;&#33324;&#20462;&#25913; <code>state</code> &#12290;</div>
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<p>&#20551;&#22914;&#20197; <span class="t-lc"><a href="cpp-locale-codecvt-in.html">do_in()</a></span> &#36716;&#25442;&#30452;&#33267;&#28040;&#32791;&#25152;&#26377; <span class="t-c"><span class="mw-geshi cpp source-cpp">from_end<span class="sy2">-</span>from</span></span> &#20010;&#23383;&#31526;&#65292;&#25110;&#20135;&#29983; <code>max</code> &#20010; <code>internT</code> &#23383;&#31526;&#65292;&#25110;&#20986;&#29616;&#36716;&#25442;&#38169;&#35823;&#65292;&#21017;&#20250;&#28040;&#32791;&#30340; <code>externT</code> &#23383;&#31526;&#25968;&#12290;</p>
<p>&#38750;&#36716;&#25442;&#29305;&#21270; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-locale-codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="kw4">char</span>, <a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">&gt;</span></span></span> &#36820;&#22238; <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-min.html"><span class="kw1642">std::<span class="me2">min</span></span></a><span class="br0">(</span>max, from_end<span class="sy2">-</span>from<span class="br0">)</span></span></span> &#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;locale&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// &#31364;&#22810;&#23383;&#33410;&#32534;&#30721;</span>
<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">&quot;z<span class="es3">\u00df</span><span class="es3">\u6c34</span><span class="es4">\U0001d10b</span>&quot;</span><span class="sy4">;</span> <span class="co1">// &#25110; u8&quot;z&szlig;&#27700;&#119051;&quot;</span>
<span class="co1">// &#25110; &quot;\x7a\xc3\x9f\xe6\xb0\xb4\xf0\x9d\x84\x8b&quot;;</span>
<a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a> mb <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="br0">(</span><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;Only the first &quot;</span> <span class="sy1">&lt;&lt;</span>
<a href="cpp-locale-use_facet.html"><span class="kw1897">std::<span class="me2">use_facet</span></span></a><span class="sy1">&lt;</span><a href="cpp-locale-codecvt.html"><span class="kw1939">std::<span class="me2">codecvt</span></span></a><span class="sy1">&lt;</span><span class="kw4">wchar_t</span>, <span class="kw4">char</span>, <a href="cpp-string-multibyte-mbstate_t.html"><span class="kw1179">std::<span class="me2">mbstate_t</span></span></a><span class="sy1">&gt;&gt;</span><span class="br0">(</span>
<a href="cpp-locale-locale.html"><span class="kw1894">std::<span class="me2">locale</span></span></a><span class="br0">(</span><span class="st0">&quot;en_US.utf8&quot;</span><span class="br0">)</span>
<span class="br0">)</span>.<span class="me1">length</span><span class="br0">(</span>mb, <span class="sy3">&amp;</span>s<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, <span class="sy3">&amp;</span>s<span class="br0">[</span>s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">]</span>, <span class="nu0">2</span><span class="br0">)</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; bytes out of &quot;</span> <span class="sy1">&lt;&lt;</span> s.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot; would be consumed &quot;</span>
<span class="st0">&quot; to produce the first 2 characters<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<pre class="de1">Only the first 3 bytes out of 10 would be consumed to produce the first 2 characters</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-locale-codecvt-in.html"><span class="t-lines"><span>do_in</span></span></a></div>
<div><span class="t-lines"><span><span class="t-cmark">[&#34394;]</span></span></span></div>
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<td>&#20174; externT &#36716;&#25442;&#23383;&#31526;&#20018;&#20026; internT &#65292;&#22914;&#22312;&#20174;&#25991;&#20214;&#35835;&#21462;&#26102;<br />
<span class="t-mark">(&#34394;&#21463;&#20445;&#25252;&#25104;&#21592;&#20989;&#25968;)</span></td>
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