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## Chapter 17. Boost.Lexical_Cast 1.0

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 http://www.boost.org/LICENSE_1_0.txt)

## Motivation

Sometimes a value must be converted to a literal text form, such as an `int` represented as a `std::string`, or vice-versa, when a `std::string` is interpreted as an `int`. 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.

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.

For instance, there are a number of limitations with the family of standard C functions typified by `atoi`:

- 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 `sprintf` function, or the loss of portability associated with non-standard functions such as `itoa`.
- The range of types supported is only a subset of the built-in numeric types, namely `int`, `long`, and `double`.
- The range of types cannot be extended in a uniform manner. For instance, conversion from string representation to complex or rational.

The standard C functions typified by `strtol` 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 `scanf` family of functions offer even greater control, but also lack safety and ease of use.

The standard C++ library offers `stringstream` 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 `stringstream` can be either clumsy (with the introduction of extra local variables and the loss of infix-expression convenience) or obscure (where `stringstream` 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.

The `lexical_cast` 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 `lexical_cast`, the conventional `std::stringstream` approach is recommended. Where the conversions are numeric to numeric, `boost::numeric_cast` may offer more reasonable behavior than `lexical_cast`.

For a good discussion of the options and issues involved in string-based formatting, including comparison of `stringstream`, `lexical_cast`, and others, see Herb Sutter's article, The String Formatters of Manor Farm. Also, take a look at the Performance section.

Last revised: August 04, 2015 at 09:06:45 GMT |