What does the "::" mean in "::tolower"?

Question

I've seen code like this:

std::string str = "wHatEver";
std::transform(str.begin(), str.end(), str.begin(), ::tolower);

And I have a question: what does mean :: before tolower?

and std::tolower not works, but ::tolower works OK

Answer 1

Means that it is explicitly using the tolower in the global namespace (which is presumably the stdc lib one).

Example:

void foo() {
    // This is your global foo
}

namespace bar {
    void foo() {
        // This is bar's foo
    }
}

using namespace bar;

void test() {
    foo();   // Ambiguous - which one is it?
    ::foo(); // This is the global foo()
}