string - Which tolower in C++?

Question

Given string foo, I've written answers on how to use cctype's tolower to convert the characters to lowercase

transform(cbegin(foo), cend(foo), begin(foo), static_cast<int (*)(int)>(tolower))

But I've begun to consider locale's tolower, which could be used like this:

use_facet<ctype<char>>(cout.getloc()).tolower(data(foo), next(data(foo), foo.size()));

• Is there a reason to prefer one of these over the other?
• Does their functionality differ at all?
• I mean other than the fact that tolower accepts and returns an int which I assume is just some antiquated C stuff?

Answer 1

Unfortunately,both are equally bad. Although std::string pretends to be a utf-8 encoded string, non of the methods/function (including tolower), are really utf-8 aware. So, tolower / tolower + locale may work with characters which are single byte (= ASCII), they will fail for every other set of languages.

On Linux, I'd use ICU library. On Windows, I'd use CharUpper function.