74 lines
6.3 KiB
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74 lines
6.3 KiB
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<title>std::basic_string<CharT,Traits,Allocator>::npos</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_string<CharT,Traits,Allocator>::</span>npos</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-string.html">string</a>‎ | <a href="cpp-string-basic_string.html">basic string</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">const</span> size_type npos <span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span></span></div>
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<p>这是特殊值，等于 <code>size_type</code> 类型可表示的最大值。准确含义依赖于语境，但通常，期待 string 下标的函数以之为字符串尾指示器，返回 string 下标的函数以之为错误指示器。</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>虽然定义使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">1</span></span></span> ，由于<a href="cpp-language-implicit_conversion.html#.E6.95.B4.E6.95.B0.E8.BD.AC.E6.8D.A2">有符号到无符号隐式转换</a>，且 <a href="cpp-string-basic_string.html"><tt>size_type</tt></a> 是无符号整数类型， <code>npos</code> 的值是其所能保有的最大正值。这是指定任何无符号类型的最大值的可移植方式。</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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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <bitset></span>
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<span class="co2">#include <string></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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<span class="co1">// 若找不到内容则字符串搜索函数返回 npos</span>
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<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"test"</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>s.<span class="me1">find</span><span class="br0">(</span><span class="st0">'a'</span><span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">string</span><span class="sy4">::</span><span class="me2">npos</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1749">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"no 'a' in 'test'<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 以字符串子集为参数的函数以 npos 为“所有到达终点的方式”指示器</span>
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<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> s2<span class="br0">(</span>s, <span class="nu0">2</span>, std<span class="sy4">::</span><span class="me2">string</span><span class="sy4">::</span><span class="me2">npos</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1749">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> s2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-utility-bitset.html"><span class="kw1027">std::<span class="me2">bitset</span></span></a><span class="sy1"><</span><span class="nu0">5</span><span class="sy1">></span> b<span class="br0">(</span><span class="st0">"aaabb"</span>, std<span class="sy4">::</span><span class="me2">string</span><span class="sy4">::</span><span class="me2">npos</span>, <span class="st0">'a'</span>, <span class="st0">'b'</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1749">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> b <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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<p>输出：</p>
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<div class="text source-text">
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<pre class="de1">no 'a' in 'test'
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st
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00011</pre></div>
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