How to change case of latin UTF-8 strings in C++?

Question

In Objective-C, it's dead simple:

NSLog(@"%@", [@"BAÑO" lowercaseString]);  // Outputs "baño".

In C++, what's the equivalent? Can anyone provide valid code for this that produces the same output? Is there a nice STL way to do this without relying on ICU, Boost, or any other 3rd party libs?

My current non-solution is:

using namespace std;
string s = "BAÑO";
wstring w(s.begin(), s.end());
transform(w.begin(), w.end(), w.begin(), towlower);
// w contains "baÑo"

Answer 1

The problem turns out to be incredibly complicated in C++. There's only one library I know of that gets it absolutely right taking into consideration unicode normalization and other non-lower-128-ASCII character point issues.

IBM's ICU

It's massive but it does everything correctly. toupper and tolower fall short in this issue unfortunately and there's no other C++ construct available.

Answer 2

There is tolower, which is locale specific, but I don't think it'll work with UTF-8 strings.

The correct solution will always be locale specific, because the case rules depend on the language. For example, the lowercase version of 'I' is not always 'i'.