So every library which uses lexical_cast would have to provide the 'imbue' and store the locale everywhere? I think per-thread global variable is better.
I think you missed my point. All operations that involves string handling (sorting, parsing etc) need to do that within a certain context which is called locale. However, all string handling desen't necessary work with the same locale. That is why the STL streams, string_algo etc all allow you to specify the locale to use per instance or function. If you introduce another global (or per-thread global) variable it will not solve anything since lexical_cast might be used in different libraries where I want to use different locales. A (thread-) global variable calld program_options_locale could work but it would be just as easy to make it a class-member. You don't allow differnt locales in your library and neither does lecxical_cast so they work nice together. But if you want to support different locales in your library you can't use lexical_cast and that is my whole point with this discussion. Lexical_cast is supposed to make a common task easy and reduce the risk of errors but instead it gives you a false sense of security and motivates others to also ignore locale support.