Enhancement: provide custom locale to fmt::format

[kivadiu]

The library is extremely useful so that I would like to use it everywhere but it uses snprintf to print floats which uses the global locale.

One problem is when the global locale is set to a language specific locale and you still want to write some numbers with the C locale. Or you may just want to write with different locales in the same program.

Changing the global locale when you call fmt::format is not thread safe, so:

1. is there a solution today to force the C locale in fmt::format if the global locale is different?
2. if not, would it be possible to have something like fmt::format(locale_name, "format", arg1, arg2, ...);
   but I guess this would mean to rewrite snprintf?

[vitaut]

The std branch of the library allows passing a locale via a buffer. Unfortunately it doesn't solve the issue with formatting of floating-point numbers because they are formatted with snprintf which uses the global locale. To solve this we need a custom implementation of floating-point formatting (#147). Note that formatting of integers is already locale-independent.

[foonathan]

A couple of years ago I wrote such an implementation, I can look tomorrow whether I can find it

[kivadiu]

Could we copy the implementation of snprintf from the GNU libc?

[beojan]

No, fmt is BSD licensed. I suppose you could copy it from BSD libc, if you really wanted.

[vitaut]

Closing in favor of #147, since the problem is in floating-point formatting not locale itself.