<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Chapter 17. Boost.Lexical_Cast 1.0</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="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> <link rel="prev" href="lambda/s10.html" title="Rationale for some of the design decisions"> <link rel="next" href="boost_lexical_cast/examples.html" title="Examples"> </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="lambda/s10.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_lexical_cast/examples.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> <div class="chapter"> <div class="titlepage"><div> <div><h2 class="title"> <a name="boost_lexical_cast"></a>Chapter 17. Boost.Lexical_Cast 1.0</h2></div> <div><p class="copyright">Copyright © 2000-2005 Kevlin Henney</p></div> <div><p class="copyright">Copyright © 2006-2010 Alexander Nasonov</p></div> <div><p class="copyright">Copyright © 2011-2014 Antony Polukhin</p></div> <div><div class="legalnotice"> <a name="boost_lexical_cast.legal"></a><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></div> </div></div> <div class="toc"> <p><b>Table of Contents</b></p> <dl class="toc"> <dt><span class="section"><a href="boost_lexical_cast.html#boost_lexical_cast.motivation">Motivation</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html">Examples</a></span></dt> <dd><dl> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.strings_to_numbers_conversion">Strings to numbers conversion</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.numbers_to_strings_conversion">Numbers to strings conversion</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.converting_to_string_without_dynamic_memory_allocation">Converting to string without dynamic memory allocation</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.converting_part_of_the_string">Converting part of the string</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.generic_programming__boost_fusion_">Generic programming (Boost.Fusion)</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/examples.html#boost_lexical_cast.examples.generic_programming__boost_variant_">Generic programming (Boost.Variant)</a></span></dt> </dl></dd> <dt><span class="section"><a href="boost_lexical_cast/synopsis.html">Synopsis</a></span></dt> <dd><dl> <dt><span class="section"><a href="boost_lexical_cast/synopsis.html#boost_lexical_cast.synopsis.lexical_cast">lexical_cast</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/synopsis.html#boost_lexical_cast.synopsis.bad_lexical_cast">bad_lexical_cast</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/synopsis.html#boost_lexical_cast.synopsis.try_lexical_convert">try_lexical_convert</a></span></dt> </dl></dd> <dt><span class="section"><a href="boost_lexical_cast/frequently_asked_questions.html">Frequently Asked Questions</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/changes.html">Changes</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html">Performance</a></span></dt> <dd><dl> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.tests_description">Tests description</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.clang_version_3_0__tags_release_30_final_">Clang version 3.0 (tags/RELEASE_30/final)</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.gnu_c___version_4_6_3">GNU C++ version 4.6.3</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.gnu_c___version_4_5_3">GNU C++ version 4.5.3</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.gnu_c___version_4_4_7">GNU C++ version 4.4.7</a></span></dt> <dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.microsoft_visual_c___version_11_0">Microsoft Visual C++ version 11.0</a></span></dt> </dl></dd> </dl> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="boost_lexical_cast.motivation"></a><a class="link" href="boost_lexical_cast.html#boost_lexical_cast.motivation" title="Motivation">Motivation</a> </h2></div></div></div> <p> Sometimes a value must be converted to a literal text form, such as an <code class="computeroutput"><span class="keyword">int</span></code> represented as a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>, or vice-versa, when a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code> is interpreted as an <code class="computeroutput"><span class="keyword">int</span></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> <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> <p> For instance, there are a number of limitations with the family of standard C functions typified by <code class="computeroutput"><span class="identifier">atoi</span></code>: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> 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 class="computeroutput"><span class="identifier">sprintf</span></code> function, or the loss of portability associated with non-standard functions such as <code class="computeroutput"><span class="identifier">itoa</span></code>. </li> <li class="listitem"> The range of types supported is only a subset of the built-in numeric types, namely <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="keyword">long</span></code>, and <code class="computeroutput"><span class="keyword">double</span></code>. </li> <li class="listitem"> The range of types cannot be extended in a uniform manner. For instance, conversion from string representation to complex or rational. </li> </ul></div> <p> The standard C functions typified by <code class="computeroutput"><span class="identifier">strtol</span></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 class="computeroutput"><span class="identifier">scanf</span></code> family of functions offer even greater control, but also lack safety and ease of use. </p> <p> The standard C++ library offers <code class="computeroutput"><span class="identifier">stringstream</span></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 class="computeroutput"><span class="identifier">stringstream</span></code> can be either clumsy (with the introduction of extra local variables and the loss of infix-expression convenience) or obscure (where <code class="computeroutput"><span class="identifier">stringstream</span></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> <p> The <code class="computeroutput"><span class="identifier">lexical_cast</span></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 class="computeroutput"><span class="identifier">lexical_cast</span></code>, the conventional <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code> approach is recommended. Where the conversions are numeric to numeric, <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> may offer more reasonable behavior than <code class="computeroutput"><span class="identifier">lexical_cast</span></code>. </p> <p> For a good discussion of the options and issues involved in string-based formatting, including comparison of <code class="computeroutput"><span class="identifier">stringstream</span></code>, <code class="computeroutput"><span class="identifier">lexical_cast</span></code>, and others, see Herb Sutter's article, <a href="http://www.gotw.ca/publications/mill19.htm" target="_top">The String Formatters of Manor Farm</a>. Also, take a look at the <a class="link" href="boost_lexical_cast/performance.html" title="Performance">Performance</a> section. </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"><p><small>Last revised: December 14, 2015 at 09:19:01 GMT</small></p></td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="lambda/s10.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.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="boost_lexical_cast/examples.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>