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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_lexical_cast.frequently_asked_questions"></a><a class="link" href="frequently_asked_questions.html" title="Frequently Asked Questions">Frequently |
| Asked Questions</a> |
| </h2></div></div></div> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">int8_t</span><span class="special">>(</span><span class="string">"127"</span><span class="special">)</span></code> throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> The type <code class="computeroutput"><span class="identifier">int8_t</span></code> |
| is a <code class="computeroutput"><span class="keyword">typedef</span></code> to <code class="computeroutput"><span class="keyword">char</span></code> or <code class="computeroutput"><span class="keyword">signed</span> |
| <span class="keyword">char</span></code>. Lexical conversion to |
| these types is simply reading a byte from source but since the source |
| has more than one byte, the exception is thrown. Please use other |
| integer types such as <code class="computeroutput"><span class="keyword">int</span></code> |
| or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>. |
| If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special"><</span><span class="identifier">int8_t</span><span class="special">>(</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"127"</span><span class="special">));</span></code> |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>(</span><span class="string">"127"</span><span class="special">)</span></code> |
| throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> Lexical conversion to any |
| char type is simply reading a byte from source. But since the source |
| has more than one byte, the exception is thrown. Please use other |
| integer types such as <code class="computeroutput"><span class="keyword">int</span></code> |
| or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>. |
| If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>(</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"127"</span><span class="special">));</span></code> |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> What does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> of an <code class="computeroutput"><span class="identifier">int8_t</span></code> |
| or <code class="computeroutput"><span class="identifier">uint8_t</span></code> not do what |
| I expect? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> As above, note that int8_t |
| and uint8_t are actually chars and are formatted as such. To avoid |
| this, cast to an integer type first: <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">n</span><span class="special">));</span></code> |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> The implementation always resets |
| the <code class="computeroutput"><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">skipws</span></code> flag of an underlying stream object. |
| It breaks my <code class="computeroutput"><span class="keyword">operator</span><span class="special">>></span></code> |
| that works only in presence of this flag. Can you remove code that resets |
| the flag? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> May be in a future version. |
| There is no requirement in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical |
| Conversion Library Proposal for TR2, N1973 by Kevlin Henney and Beman |
| Dawes</a> to reset the flag but remember that <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical |
| Conversion Library Proposal for TR2, N1973</a> is not yet accepted |
| by the committee. By the way, it's a great opportunity to make your |
| <code class="computeroutput"><span class="keyword">operator</span><span class="special">>></span></code> |
| more general. Read a good C++ book, study <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sentry</span></code> |
| and <a href="../../../libs/io/doc/ios_state.html" target="_top"><code class="computeroutput"><span class="identifier">ios_state_saver</span></code></a>. |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">>(</span><span class="string">"-1"</span><span class="special">);</span></code> |
| does not throw, but outputs 4294967295? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> |
| has the behavior of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>, |
| which uses <code class="computeroutput"><span class="identifier">num_get</span></code> |
| functions of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span></code> |
| to convert numbers. If we look at the Programming languages — C++, |
| we'll see, that <code class="computeroutput"><span class="identifier">num_get</span></code> |
| uses the rules of <code class="computeroutput"><span class="identifier">scanf</span></code> |
| for conversions. And in the C99 standard for unsigned input value |
| minus sign is optional, so if a negative number is read, no errors |
| will arise and the result will be the two's complement. |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">L</span><span class="char">'A'</span><span class="special">);</span></code> outputs |
| 65 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">wchar_t</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"65"</span><span class="special">);</span></code> does not throw? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> If you are using an old |
| version of Visual Studio or compile code with /Zc:wchar_t- flag, |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> sees single <code class="computeroutput"><span class="keyword">wchar_t</span></code> character as <code class="computeroutput"><span class="keyword">unsigned</span> <span class="keyword">short</span></code>. |
| It is not a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> |
| mistake, but a limitation of compiler options that you use. |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="string">"-1.#IND"</span><span class="special">);</span></code> throws <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_lexical_cast</span></code>? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> <code class="computeroutput"><span class="string">"-1.#IND"</span></code> |
| is a compiler extension, that violates standard. You shall input |
| <code class="computeroutput"><span class="string">"-nan"</span></code>, <code class="computeroutput"><span class="string">"nan"</span></code>, <code class="computeroutput"><span class="string">"inf"</span></code> |
| , <code class="computeroutput"><span class="string">"-inf"</span></code> (case |
| insensitive) strings to get NaN and Inf values. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code> outputs <code class="computeroutput"><span class="string">"-nan"</span></code>, |
| <code class="computeroutput"><span class="string">"nan"</span></code>, <code class="computeroutput"><span class="string">"inf"</span></code>, <code class="computeroutput"><span class="string">"-inf"</span></code> |
| strings, when has NaN or Inf input values. |
| </li></ul></div> |
| </li></ul></div> |
| <pre class="programlisting"></pre> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <span class="bold"><strong>Question:</strong></span> What is the fastest way to convert |
| a non zero terminated string or a substring using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>? |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> |
| <span class="bold"><strong>Answer:</strong></span> Use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">iterator_range</span></code> |
| for conversion or <code class="computeroutput"><span class="identifier">lexical_cast</span></code> |
| overload with two parameters. For example, if you whant to convert |
| to <code class="computeroutput"><span class="keyword">int</span></code> two characters |
| from a string <code class="computeroutput"><span class="identifier">str</span></code>, |
| you shall write <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">make_iterator_range</span><span class="special">(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">()</span> <span class="special">+</span> <span class="number">2</span><span class="special">));</span></code> |
| or <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> |
| <span class="number">2</span><span class="special">);</span></code>. |
| </li></ul></div> |
| </li></ul></div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2000-2005 Kevlin Henney<br>Copyright © 2006-2010 Alexander Nasonov<br>Copyright © 2011-2014 Antony |
| Polukhin<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
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