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<title>Chapter 21. Boost.Lexical_Cast 1.0</title>
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<div><h2 class="title">
<a name="boost_lexical_cast"></a>Chapter 21. 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 © 2011-2021 Antony Polukhin</p></div>
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<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>)
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<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.gnu_c___version_6_1_1_20160511">GNU
C++ version 6.1.1 20160511</a></span></dt>
<dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.gnu_c___version_4_8_5">GNU
C++ version 4.8.5</a></span></dt>
<dt><span class="section"><a href="boost_lexical_cast/performance.html#boost_lexical_cast.performance.clang_version_3_6_0__tags_release_360_final_">Clang
version 3.6.0 (tags/RELEASE_360/final)</a></span></dt>
</dl></dd>
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</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>
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<td align="left"><p><small>Last revised: April 13, 2021 at 16:32:33 GMT</small></p></td>
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