std::tolower and Visual Studio 2013

Question

I try to understand how to use std::tolower...

#include <iostream>
#include <string>
#include <algorithm>
#include <locale>

int main()
{
    std::string test = "Hello World";
    std::locale loc;
    for (auto &c : test)
    {
        c = std::tolower(c, loc);
    }

    std::transform(test.begin(), test.end(), test.begin(), ::tolower); // 1) OK
    std::transform(test.begin(), test.end(), test.begin(), std::tolower); // 2) Cryptic compile error
    std::transform(test.begin(), test.end(), test.begin(), static_cast<int(*)(int)>(std::tolower)); // 3) Cryptic compile error. Seems OK with other compilers though

    return 0;
}

So:

1. Why ::tolower version is working?
2. Why std::tolower is not working in std::transform?
3. What static_cast<int(*)(int)>(std::tolower)) really is trying to do? Why does it work with GCC and not with Visual Studio 2013?
4. How could I use std::lower in std::transform with Visual Studio 2013 then?

Answer 1

First off, note, that none of these approaches does the right thing in a portable way! The problem is that char may be signed (and typically is) but the versions of tolower() only accept positive values! That is you really want to use std::tolower() using something like this:

std::transform(test.begin(), test.end(), test.begin(),
               [](unsigned char c) { return std::tolower(c); });

(or, of course, using a corresponding function object if you are stuck with C++03). Using std::tolower() (or ::tolower() for that matter) with a negative value results in undefined behavior. Of course, this only matters on platform where char is signed which seems, however, to be the typical choice.

To answer your questions:

1. When including <cctype> you typically get the various functions and types from the standard C library both in namespace std as well as in the global namespace. Thus, using ::tolower normally works but isn't guaranteed to work.
2. When including <locale>, there are two versions of std::tolower available, one as int(*)(int) and one as char(*)(char, std::locale const&). When using just std::tolower the compiler has generally no way to decide which one to use.
3. Since std::tolower is ambiguous, using static_cast<int(*)(int)>(std::tolower) disambiguates which version to use. Why use of static_cast<...>() with VC++ fails, I don't know.
4. You shouldn't use std::tolower() with a sequences of chars anyway as it will result in undefined behavior. Use a function object using std::tolower internally on an unsigned char.

It is worth noting that using a function object rather than a function pointer is typically a lot faster because it is trivial to inline the function object but not as trivial to inline the function pointer. Compilers are getting better with inlining the use of function pointers where the function is actually known but contemporary compilers certainly don't always inline function calls through function pointers even if all the context would be there.