lexical_cast
Author: Kevlin Henney, mailto:kevlin@curbralan.com

# Header boost/lexical_cast.hpp

- Motivation
- Examples
- Synopsis
- `lexical_cast`
- `bad_lexical_cast`
- Frequently Asked Questions
- References
- Changes

## Motivation

Sometimes a value must be converted to a literal text form, such as an `int` represented as a `string`, or vice-versa, when a `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 `stringstream` approach is recommended. Where the conversions are numeric to numeric, `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.

## Examples

The following example treats command line arguments as a sequence of numeric data:

>     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);
>             }
>         }
>         ...
>     }

The following example uses numeric data in a string expression:

>     void log_message(const std::string &);

>     void log_errno(int yoko)
>     {
>         log_message("Error " + boost::lexical_cast<std::string>(yoko) + ": " + strerror(yoko));
>     }

## Synopsis

Library features defined in `"boost/lexical_cast.hpp"`:

>     namespace boost
>     {
>         class bad_lexical_cast;
>         template<typename Target, typename Source>
>           Target lexical_cast(const Source& arg);
>     }

Unit test defined in `"lexical_cast_test.cpp"`.

## `lexical_cast`

>     template<typename Target, typename Source>
>       Target lexical_cast(const Source& arg);

Returns the result of streaming `arg` into a standard library string-based stream and then out as a `Target` object. Where `Target` is either `std::string` or `std::wstring`, stream extraction takes the whole content of the string, including spaces, rather than relying on the default `operator>>` behavior. If the conversion is unsuccessful, a `bad_lexical_cast` exception is thrown.

The requirements on the argument and result types are:

- `Source` is OutputStreamable, meaning that an `operator<<` is defined that takes a `std::ostream` or `std::wostream` object on the left hand side and an instance of the argument type on the right.
- `Target` is InputStreamable, meaning that an `operator>>` is defined that takes a `std::istream` or `std::wistream` object on the left hand side and an instance of the result type on the right.
- `Target` is CopyConstructible [20.1.3].
- `Target` is DefaultConstructible, meaning that it is possible to default-initialize an object of that type [8.5, 20.1.4].

The character type of the underlying stream is assumed to be `char` unless either the `Source` or the `Target` requires wide-character streaming, in which case the underlying stream uses `wchar_t`. `Source` types that require wide-character streaming are `wchar_t`, `wchar_t *`, and `std::wstring`. `Target` types that require wide-character streaming are `wchar_t` and `std::wstring`.

Where a higher degree of control is required over conversions, `std::stringstream` and `std::wstringstream` offer a more appropriate path. Where non-stream-based conversions are required, `lexical_cast` is the wrong tool for the job and is not special-cased for such scenarios.

## `bad_lexical_cast`

>     class bad_lexical_cast : public std::bad_cast
>     {
>     public:
>         ... // same member function interface as std::exception
>     };

Exception used to indicate runtime `lexical_cast` failure.

## Frequently Asked Questions

Q: Why does `lexical_cast<int8_t>("127")` throw `bad_lexical_cast`?
A: The type `int8_t` is a typedef to `char` or `signed char`. 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 `int` or `short int`. If bounds checking is important, you can also call numeric_cast:

    numeric_cast<int8_t>(lexical_cast<int>("127"));

Q: What does `lexical_cast<std::string>` of an `int8_t` or `uint8_t` not do what I expect?
A: 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:

    lexical_cast<std::string>(static_cast<int>(n));

Q: The implementation always resets the `ios_base::skipws` flag of an underlying stream object. It breaks my `operator>>` that works only in presence of this flag. Can you remove code that resets the flag?
A: May be in a future version. There is no requirement in [N1973] to reset the flag but remember that [N1973] is not yet accepted by the committee. By the way, it's a great opportunity to make your `operator>>` conform to the standard. Read a good C++ book, study `std::sentry` and ios_state_saver.

## References

- [N1973] Kevlin Henney, Beman Dawes, Lexical Conversion Library Proposal for TR2, N1973.
- [Tuning] Alexander Nasonov, Fine Tuning for lexical_cast, Overload #74, August 2006.

## Changes

### August, October 2006:

- Better performance for many combinations of `Source` and `Target` types. Refer to [Tuning] for more details.

### June 2005:

- 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.
- The MSVC 6 support is deprecated, and will be removed in a future Boost version.

### Earlier:

- The previous version of `lexical_cast` used the default stream precision for reading and writing floating-point numbers. For numerics that have a corresponding specialization of `std::numeric_limits`, the current version now chooses a precision to match.
- The previous version of `lexical_cast` did not support conversion to or from any wide-character-based types. For compilers with full language and library support for wide characters, `lexical_cast` now supports conversions from `wchar_t`, `wchar_t *`, and `std::wstring` and to `wchar_t` and `std::wstring`.
- The previous version of `lexical_cast` 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 `std::string` and, where supported, `std::wstring`: `lexical_cast<std::string>("Hello, World")` succeeds instead of failing with a `bad_lexical_cast` exception.
- The previous version of `lexical_cast` allowed unsafe and meaningless conversions to pointers. The current version now throws a `bad_lexical_cast` for conversions to pointers: `lexical_cast<char *>("Goodbye, World")` now throws an exception instead of causing undefined behavior.

© Copyright Kevlin Henney, 2000–2005