203 lines
14 KiB
HTML
203 lines
14 KiB
HTML
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<title>std::strtof, std::strtod, std::strtold</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>strtof, <span style="font-size:0.7em; line-height:130%">std::</span>strtod, <span style="font-size:0.7em; line-height:130%">std::</span>strtold</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-byte.html">byte</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></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="kw4">float</span> strtof<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> str, <span class="kw4">char</span><span class="sy2">**</span> str_end <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> strtod<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> str, <span class="kw4">char</span><span class="sy2">**</span> str_end <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> strtold<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> str, <span class="kw4">char</span><span class="sy2">**</span> str_end <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p>转译 <code>str</code> 所指向的字节字符串中的浮点值。</p>
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<p>函数会舍弃任何空白符（由 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">isspace</span><span class="br0">(</span><span class="br0">)</span></span></span> 确定），直至找到首个非空白符。然后它会取用尽可能多的字符，以构成合法的浮点数表示，并将它们转换成浮点值。合法的浮点值可以为下列之一：</p>
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<ul>
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<li>十进制浮点数表达式。它由下列部分组成：</li>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li>非空的十进制数字序列，可选地包含一个小数点字符（由当前的 C <a href="cpp-locale-setlocale.html">本地环境</a>确定）（定义有效数字）</li>
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<li><span class="t-mark">(可选)</span> <code><b>e</b></code> 或 <code><b>E</b></code> ，并跟随可选的正或负号，以及非空十进制数字序列（以 10 为底定义指数）</li>
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<ul>
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<li>十六进制浮点数表达式。它由下列部分组成：</li>
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</ul>
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<ul>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li><code><b>0x</b></code> 或 <code><b>0X</b></code></li>
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<li>非空的十六进制数字序列，选地包含一个小数点字符（由当前的 C <a href="cpp-locale-setlocale.html">本地环境</a>确定）（定义有效数字）</li>
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<li><span class="t-mark">(可选)</span> <code><b>p</b></code> 或 <code><b>P</b></code> ，并跟随可选的正或负号，以及非空十进制数字序列（以 2 为底定义指数）</li>
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</ul>
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<ul>
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<li>无穷大表达式。它由下列部分组成：</li>
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<li><span class="t-mark">(可选)</span> 正或负号</li>
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<li><code><b>INF</b></code> 或 <code><b>INFINITY</b></code> ，忽略大小写</li>
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</ul>
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</dd>
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<ul>
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<li>非数（NaN）表达式。它由下列部分组成：</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-mark">(可选)</span> 正或负号</li>
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<li><code><b>NAN</b></code> 或 <code><b>NAN(</b></code><i>char_sequence</i><code><b>)</b></code> ，忽略 <code><b>NAN</b></code> 部分的大小写。 <i>char_sequence</i> 只能由数字、拉丁字母和下划线构成。结果是一个静态的 NaN 浮点值。</li>
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</ul>
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</dd>
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</dl>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<ul>
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<li>任何其他可由当前 C <a href="cpp-locale-setlocale.html">本地环境</a>接受的表达式</li>
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</ul>
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<p>函数设置 <code>str_end</code> 所指向的指针指向最后被转译字符的后一字符，若 <code>str_end</code> 为空指针，则忽略它。</p>
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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>str</td>
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<td>-</td>
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<td>指向要转译的空终止字节字符串的指针</td>
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</tr>
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<tr class="t-par">
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<td>str_end</td>
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<td>-</td>
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<td>指向指向字符指针的指针。</td>
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</table>
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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>成功时为对应 <code>str</code> 内容的浮点值。若转换出的值落在对应返回类型的范围外，则发生值域错误并返回 <span class="t-lc"><a href="cpp-numeric-math-HUGE_VAL.html">HUGE_VAL</a></span> 、 <span class="t-lc"><a href="cpp-numeric-math-HUGE_VAL.html">HUGE_VALF</a></span> 或 <span class="t-lc"><a href="cpp-numeric-math-HUGE_VAL.html">HUGE_VALL</a></span> 。若无法进行转换，则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 并将 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>str_end</span></span> 设为 <code>str</code> 。</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="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <cerrno></span>
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<span class="co2">#include <cstdlib></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="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> p <span class="sy1">=</span> <span class="st0">"111.11 -2.22 0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz"</span><span class="sy4">;</span>
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<span class="kw4">char</span><span class="sy2">*</span> end<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Parsing <span class="es1">\"</span>"</span> <span class="sy1"><<</span> p <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span>:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">double</span> f <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">strtod</span><span class="br0">(</span>p, <span class="sy3">&</span>end<span class="br0">)</span><span class="sy4">;</span> p <span class="sy3">!</span><span class="sy1">=</span> end<span class="sy4">;</span> f <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">strtod</span><span class="br0">(</span>p, <span class="sy3">&</span>end<span class="br0">)</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"'"</span> <span class="sy1"><<</span> <a href="cpp-string-basic_string.html"><span class="kw1227">std::<span class="me2">string</span></span></a><span class="br0">(</span>p, end<span class="sy2">-</span>p<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"' -> "</span><span class="sy4">;</span>
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p <span class="sy1">=</span> end<span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="kw2">errno</span> <span class="sy1">==</span> <a href="cpp-error-errno_macros.html"><span class="kw862">ERANGE</span></a><span class="br0">)</span><span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"range error, got "</span><span class="sy4">;</span>
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<span class="kw2">errno</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> f <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>
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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">Parsing "111.11 -2.22 0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz":
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'111.11' -> 111.11
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' -2.22' -> -2.22
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' 0X1.BC70A3D70A3D7P+6' -> 111.11
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' 1.18973e+4932' -> range error, got inf</pre></div>
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</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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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-byte-atof.html"><span class="t-lines"><span>atof</span></span></a></div>
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</div>
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</td>
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<td>转换字节字符串为浮点值<br />
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<span class="t-mark">(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-string-wide-wcstof.html"><span class="t-lines"><span>wcstof</span><span>wcstod</span><span>wcstold</span></span></a></div>
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</div>
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</td>
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<td>转换宽字符串为浮点值<br />
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<span class="t-mark">(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-utility-from_chars.html"><span class="t-lines"><span>from_chars</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>转换字符序列到整数或浮点值<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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