Changes

boost 1.48.0 :

Added code to work with Inf and NaN on any platform.
Better performance and less memory usage for conversions to float type (and to double type, if sizeof(double) < sizeof(long double)).

boost 1.47.0 :

Optimizations for "C" and other locales without number grouping.
Better performance and less memory usage for unsigned char and signed char conversions.
Better performance and less memory usage for conversions to arithmetic types.
Better performance and less memory usage for conversions from arithmetic type to arithmetic type.
Directly construct Target from Source on some conversions (like conversions from string to string, from char array to string, from char to char and others).

boost 1.34.0 :

Better performance for many combinations of Source and Target types. For more details refer to Alexander Nasonovs article Fine Tuning for lexical_cast, Overload #74, August 2006 (PDF).

boost 1.33.0 :

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.

Boost C++ Libraries