boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Frequently Asked Questions</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../boost_lexical_cast.html" title="Chapter 17. Boost.Lexical_Cast 1.0"> <link rel="prev" href="synopsis.html" title="Synopsis"> <link rel="next" href="changes.html" title="Changes"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="synopsis.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_lexical_cast.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="changes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <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"> Question: Why does <code class="computeroutput">lexical_cast<int8_t>("127")</code> throw <code class="computeroutput">bad_lexical_cast</code>? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: The type <code class="computeroutput">int8_t</code> is a <code class="computeroutput">typedef</code> to <code class="computeroutput">char</code> or <code class="computeroutput">signed char</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">int</code> or <code class="computeroutput">short int</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">boost::numeric_cast</code></a>: <code class="computeroutput">numeric_cast<int8_t>(lexical_cast<int>("127"));</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"> Question: Why does <code class="computeroutput">lexical_cast<unsigned char>("127")</code> throw <code class="computeroutput">bad_lexical_cast</code>? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: 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">int</code> or <code class="computeroutput">short int</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">boost::numeric_cast</code></a>: <code class="computeroutput">numeric_cast<unsigned char>(lexical_cast<int>("127"));</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"> Question: What does <code class="computeroutput">lexical_cast<std::string></code> of an <code class="computeroutput">int8_t</code> or <code class="computeroutput">uint8_t</code> not do what I expect? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: 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">lexical_cast<std::string>(static_cast<int>(n));</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"> Question: The implementation always resets the <code class="computeroutput">ios_base::skipws</code> flag of an underlying stream object. It breaks my <code class="computeroutput">operator>></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"> Answer: 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">operator>></code> more general. Read a good C++ book, study <code class="computeroutput">std::sentry</code> and <a href="../../../libs/io/doc/ios_state.html" target="_top"><code class="computeroutput">ios_state_saver</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"> Question: Why <code class="computeroutput">std::cout << boost::lexical_cast<unsigned int>("-1");</code> does not throw, but outputs 4294967295? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: <code class="computeroutput">boost::lexical_cast</code> has the behavior of <code class="computeroutput">std::stringstream</code>, which uses <code class="computeroutput">num_get</code> functions of <code class="computeroutput">std::locale</code> to convert numbers. If we look at the Programming languages — C++, we'll see, that <code class="computeroutput">num_get</code> uses the rules of <code class="computeroutput">scanf</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"> Question: Why <code class="computeroutput">boost::lexical_cast<int>(L'A');</code> outputs 65 and <code class="computeroutput">boost::lexical_cast<wchar_t>(L"65");</code> does not throw? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: If you are using an old version of Visual Studio or compile code with /Zc:wchar_t- flag, <code class="computeroutput">boost::lexical_cast</code> sees single <code class="computeroutput">wchar_t</code> character as <code class="computeroutput">unsigned short</code>. It is not a <code class="computeroutput">boost::lexical_cast</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"> Question: Why <code class="computeroutput">boost::lexical_cast<double>("-1.#IND");</code> throws <code class="computeroutput">boost::bad_lexical_cast</code>? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: <code class="computeroutput">"-1.#IND"</code> is a compiler extension, that violates standard. You shall input <code class="computeroutput">"-nan"</code>, <code class="computeroutput">"nan"</code>, <code class="computeroutput">"inf"</code> , <code class="computeroutput">"-inf"</code> (case insensitive) strings to get NaN and Inf values. <code class="computeroutput">boost::lexical_cast<string></code> outputs <code class="computeroutput">"-nan"</code>, <code class="computeroutput">"nan"</code>, <code class="computeroutput">"inf"</code>, <code class="computeroutput">"-inf"</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"> Question: What is the fastest way to convert a non zero terminated string or a substring using <code class="computeroutput">boost::lexical_cast</code>? <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"> Answer: Use <code class="computeroutput">boost::iterator_range</code> for conversion or <code class="computeroutput">lexical_cast</code> overload with two parameters. For example, if you whant to convert to <code class="computeroutput">int</code> two characters from a string <code class="computeroutput">str</code>, you shall write <code class="computeroutput">lexical_cast<int>(make_iterator_range(str.data(), str.data() + 2));</code> or <code class="computeroutput">lexical_cast<int>(str.data(), 2);</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 Copyright © 2006-2010 Alexander Nasonov Copyright © 2011-2014 Antony Polukhin 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>) </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="synopsis.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_lexical_cast.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="changes.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>