Visual Studio: Global namespace takes precedence over local

Question

I got some code sent to me today, and it's using the std::transform on a std::string to make it all lowercase. The sender had written and compiled the code in Visual Studio 2010:

using namespace std;
string test = "TEST";
transform(test.begin(), test.end(), test.begin(), tolower);

Wheras I compiled it on OSx with GCC and/or Clang, and got compile error, as it involves namespace's clashing.

There is indeed a bunch of answered questions that addresses the global namespace vs local (::tolower vs std::tolower), but this is regarding the fact that this piece of code actually works on VS.

Questions I'd like answers to:

1. Why does Visual Studio compile this?
2. How come it defaults (?) to the global namespace :: when std:: should take precedence?

• Heres a ideone snippet (with errors): http://ideone.com/qvUAMw
• Heres a ideone snippet (that works) : http://ideone.com/lk0H5d

Note: Im using GCC 4.2 and Clang (425.0.24 LLVM 3.2svn) on OSx Mountain Lion

Answer 1

The problem here is two-fold. First, if you don't specifically #include <cctype>, you might be getting a tolower that's a macro, not a function (though I don't know whether this is actually a problem these days and whether this would break standards compliance.)

Second, in C++ tolower is overloaded. You need to select a specific overload to use:

transform(test2.begin(), test2.end(), test2.begin(),
          static_cast<int(*)(int)>(tolower));

But I don't know the reason why std:: gets an overloaded version and :: does not. In any event, I'd recommend that you stay away from those C functions and instead use std::ctype from the <locale> header, which provides a better tolower:

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