<!-- saved from url=(0022)http://internet.e-mail --> <!doctype html public "-//W3C//DTD HTML Transitional 4.0//EN"> <html> <head> <title>lexical_cast</title> <meta name="author" content="Kevlin Henney, mailto:kevlin@curbralan.com"> <meta name="generator" content="Microsoft FrontPage 5.0"> </head> <body bgcolor="#FFFFFF" text="#000000"> <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align="center" width="277" height="86">Header <a href="../../boost/lexical_cast.hpp">boost/lexical_cast.hpp</a></h1> <ul type="square"> <li> <a href="#motivation">Motivation</a></li> <li> <a href="#examples">Examples</a></li> <li> <a href="#synopsis">Synopsis</a></li> <li> <a href="#lexical_cast"><code>lexical_cast</code></a></li> <li> <a href="#bad_lexical_cast"><code>bad_lexical_cast</code></a></li> <li> <a href="#faq">Frequently Asked Questions</a></li> <li> <a href="#references">References</a></li> <li> <a href="#changes">Changes</a></li> </ul> <hr> <h2><a name="motivation">Motivation</a></h2> Sometimes a value must be converted to a literal text form, such as an <code>int</code> represented as a <code>string</code>, or vice-versa, when a <code>string</code> is interpreted as an <code>int</code>. Such examples are common when converting between data types internal to a program and representation external to a program, such as windows and configuration files. <p> The standard C and C++ libraries offer a number of facilities for performing such conversions. However, they vary with their ease of use, extensibility, and safety. <p> For instance, there are a number of limitations with the family of standard C functions typified by <code>atoi</code>: <ul type="square"> <li> Conversion is supported in one direction only: from text to internal data type. Converting the other way using the C library requires either the inconvenience and compromised safety of the <code>sprintf</code> function, or the loss of portability associated with non-standard functions such as <code>itoa</code>. </li> <li> The range of types supported is only a subset of the built-in numeric types, namely <code>int</code>, <code>long</code>, and <code>double</code>. </li> <li> The range of types cannot be extended in a uniform manner. For instance, conversion from string representation to <code>complex</code> or <code>rational</code>. </li> </ul> The standard C functions typified by <code>strtol</code> have the same basic limitations, but offer finer control over the conversion process. However, for the common case such control is often either not required or not used. The <code>scanf</code> family of functions offer even greater control, but also lack safety and ease of use. <p> The standard C++ library offers <code>stringstream</code> for the kind of in-core formatting being discussed. It offers a great deal of control over the formatting and conversion of I/O to and from arbitrary types through text. However, for simple conversions direct use of <code>stringstream</code> can be either clumsy (with the introduction of extra local variables and the loss of infix-expression convenience) or obscure (where <code>stringstream</code> objects are created as temporary objects in an expression). Facets provide a comprehensive concept and facility for controlling textual representation, but their perceived complexity and high entry level requires an extreme degree of involvement for simple conversions, and excludes all but a few programmers. <p> The <code>lexical_cast</code> function template offers a convenient and consistent form for supporting common conversions to and from arbitrary types when they are represented as text. The simplification it offers is in expression-level convenience for such conversions. For more involved conversions, such as where precision or formatting need tighter control than is offered by the default behavior of <code>lexical_cast</code>, the conventional <code> stringstream</code> approach is recommended. Where the conversions are numeric to numeric, <code><a href="../numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html">numeric_cast</a></code> may offer more reasonable behavior than <code>lexical_cast</code>. <p> For a good discussion of the options and issues involved in string-based formatting, including comparison of <code>stringstream</code>, <code>lexical_cast</code>, and others, see Herb Sutter's article, <a href="http://www.gotw.ca/publications/mill19.htm"> <i>The String Formatters of Manor Farm</i></a>. <p> <hr> <h2><a name="examples">Examples</a></h2> The following example treats command line arguments as a sequence of numeric data: <blockquote> <pre>int main(int argc, char * argv[]) { using boost::lexical_cast; using boost::bad_lexical_cast; std::vector<short> args; while(*++argv) { try { args.push_back(lexical_cast<short>(*argv)); } catch(bad_lexical_cast &) { args.push_back(0); } } ... } </pre> </blockquote>The following example uses numeric data in a string expression: <blockquote> <pre>void log_message(const std::string &); void log_errno(int yoko) { log_message("Error " + boost::lexical_cast<std::string>(yoko) + ": " + strerror(yoko)); } </pre> </blockquote> <hr> <h2><a name="synopsis">Synopsis</a></h2> Library features defined in <a href="../../boost/lexical_cast.hpp"><code>"boost/lexical_cast.hpp"</code></a>: <blockquote> <pre>namespace boost { class <a href="#bad_lexical_cast">bad_lexical_cast</a>; template<typename Target, typename Source> Target <a href="#lexical_cast">lexical_cast</a>(const Source& arg); } </pre> </blockquote>Unit test defined in <a href="lexical_cast_test.cpp"><code>"lexical_cast_test.cpp"</code></a>. <p> <hr> <h2><a name="lexical_cast"><code>lexical_cast</code></a></h2> <blockquote> <pre>template<typename Target, typename Source> Target lexical_cast(const Source& arg); </pre> </blockquote>Returns the result of streaming <code>arg</code> into a standard library string-based stream and then out as a <code>Target</code> object. Where <code>Target</code> is either <code>std::string</code> or <code>std::wstring</code>, stream extraction takes the whole content of the string, including spaces, rather than relying on the default <code>operator>></code> behavior. If the conversion is unsuccessful, a <a href="#bad_lexical_cast"> <code>bad_lexical_cast</code></a> exception is thrown. <p> The requirements on the argument and result types are: <ul type="square"> <li> <code>Source</code> is <i>OutputStreamable</i>, meaning that an <code>operator<<</code> is defined that takes a <code>std::ostream</code> or <code>std::wostream</code> object on the left hand side and an instance of the argument type on the right. </li> <li> <code>Target</code> is <i>InputStreamable</i>, meaning that an <code>operator>></code> is defined that takes a <code>std::istream</code> or <code>std::wistream</code> object on the left hand side and an instance of the result type on the right. </li> <li> <code>Target</code> is <i>CopyConstructible</i> [20.1.3]. </li> <li> <code>Target</code> is <i>DefaultConstructible</i>, meaning that it is possible to <i>default-initialize</i> an object of that type [8.5, 20.1.4]. </li> </ul> The character type of the underlying stream is assumed to be <code>char</code> unless either the <code>Source</code> or the <code>Target</code> requires wide-character streaming, in which case the underlying stream uses <code>wchar_t</code>. <code>Source</code> types that require wide-character streaming are <code>wchar_t</code>, <code>wchar_t *</code>, and <code>std::wstring</code>. <code>Target</code> types that require wide-character streaming are <code>wchar_t</code> and <code>std::wstring</code>. <p> Where a higher degree of control is required over conversions, <code>std::stringstream</code> and <code>std::wstringstream</code> offer a more appropriate path. Where non-stream-based conversions are required, <code>lexical_cast</code> is the wrong tool for the job and is not special-cased for such scenarios. <p> <hr> <h2><a name="bad_lexical_cast"><code>bad_lexical_cast</code></a></h2> <blockquote> <pre>class bad_lexical_cast : public std::bad_cast { public: ... // <i>same member function interface as</i> std::exception }; </pre> </blockquote>Exception used to indicate runtime <a href="#lexical_cast"><code>lexical_cast</code></a> failure. <hr> <h2><a name="faq">Frequently Asked Questions</h2> <p> Q: Why does <code>lexical_cast<int8_t>("127")</code> throw <code>bad_lexical_cast</code>? <br> A: The type <code>int8_t</code> is a typedef to <code>char</code> or <code>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. <p>Please use other integer types such as <code>int</code> or <code>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">numeric_cast</a>: <pre><a href="../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html">numeric_cast</a><int8_t>(lexical_cast<int>("127"));</pre> <p> Q: What does <code>lexical_cast<std::string></code> of an <code>int8_t</code> or <code>uint8_t</code> not do what I expect? <br> A: As above, note that <code>int8_t</code> and <code>uint8_t</code> are actually chars and are formatted as such. To avoid this, cast to an integer type first: <pre>lexical_cast<std::string>(static_cast<int>(n));</pre> <p> Q: The implementation always resets the <code>ios_base::skipws</code> flag of an underlying stream object. It breaks my <code>operator>></code> that works only in presence of this flag. Can you remove code that resets the flag? <br> A: May be in a future version. There is no requirement in <a href="#n1973">[N1973]</a> to reset the flag but remember that <a href="#n1973">[N1973]</a> is not yet accepted by the committee. By the way, it's a great opportunity to make your <code>operator>></code> conform to the standard. Read a good C++ book, study <code>std::sentry</code> and <a href="../../libs/io/doc/ios_state.html">ios_state_saver</a>. </ul> <h2><a name="references">References</h2> <ul type="square"> <a name="n1973"></a><li> [N1973] Kevlin Henney, Beman Dawes, Lexical Conversion Library Proposal for TR2, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html">N1973</a>. <a name="tuning"></a><li> [Tuning] Alexander Nasonov, Fine Tuning for lexical_cast, <a href="http://www.accu.org/var/uploads/journals/overload74.pdf">Overload #74</a>, August 2006.</li> </ul> <h2><a name="changes">Changes</a></h2> <h3>August, October 2006:</h3> <ul type="square"> <li>Better performance for many combinations of <code>Source</code> and <code>Target</code> types. Refer to <a href="#tuning">[Tuning]</a> for more details. </li> </ul> <h3>June 2005:</h3> <ul type="square"> <li>Call-by-const reference for the parameters. This requires partial specialization of class templates, so it doesn't work for MSVC 6, and it uses the original pass by value there.<br> </li> <li>The MSVC 6 support is deprecated, and will be removed in a future Boost version. </li> </ul> <h3>Earlier:</h3> <ul type="square"> <li>The previous version of <code>lexical_cast</code> used the default stream precision for reading and writing floating-point numbers. For numerics that have a corresponding specialization of <code>std::numeric_limits</code>, the current version now chooses a precision to match. <br> <li>The previous version of <code>lexical_cast</code> did not support conversion to or from any wide-character-based types. For compilers with full language and library support for wide characters, <code>lexical_cast</code> now supports conversions from <code>wchar_t</code>, <code>wchar_t *</code>, and <code>std::wstring</code> and to <code>wchar_t</code> and <code>std::wstring</code>. <br> <li>The previous version of <code>lexical_cast</code> assumed that the conventional stream extractor operators were sufficient for reading values. However, string I/O is asymmetric, with the result that spaces play the role of I/O separators rather than string content. The current version fixes this error for <code>std::string</code> and, where supported, <code>std::wstring</code>: <code>lexical_cast<std::string>("Hello, World")</code> succeeds instead of failing with a <code>bad_lexical_cast</code> exception. <br> <li>The previous version of <code>lexical_cast</code> allowed unsafe and meaningless conversions to pointers. The current version now throws a <code>bad_lexical_cast</code> for conversions to pointers: <code>lexical_cast<char *>("Goodbye, World")</code> now throws an exception instead of causing undefined behavior. </ul> <p> <hr> <div align="right"><small><i>© Copyright Kevlin Henney, 2000–2005</i></small></div> </body> </html>