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.

Answer 2

In the first case (cctype) the locale is set implicitely:

Converts the given character to lowercase according to the character conversion rules defined by the currently installed C locale.

http://en.cppreference.com/w/cpp/string/byte/tolower

In the second (locale's) case you have to explicitely set the locale:

Converts parameter c to its lowercase equivalent if c is an uppercase letter and has a lowercase equivalent, as determined by the ctype facet of locale loc. If no such conversion is possible, the value returned is c unchanged.

http://www.cplusplus.com/reference/locale/tolower/