c++ - 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

As to why the :: is necessary: the standard defines two tolower's, a function template in std::, and a simple function in both :: and std::. Depending on which headers have been included (and that includes headers indirectly included from other headers, which you may not know about), either one, the other, or both may be visible. Using :: ensures that the older one, from the C standard, is used. (If the one in std:: is considered, the call will be ambiguous, since transform is a template itself, and the compiler will not be able to deduce the template arguments.)

While I'm at it, I might mention that using ::tolower like this is undefined behavior, at least if plain char is signed. The input to ::tolower is an int, and must be in the range 0...UCHAR_MAX, or EOF. If plain char is signed, some of the characters may have negative encodings, which results in undefined behavior. In practice, most implementations make this work. For all characters except 0xFF (? in Latin 1). If you're not concerned with portability, some compilers have a switch to make char unsigned---use it. Otherwise, write a small functional object to handle it correctly, either:

struct ToLower
{
    char operator()( char ch ) const
    {
        return ::tolower( static_cast<unsigned char>(ch) );
    }
};

or (better, but significantly more work---only worth it if your using it a lot), a functional object whose constructor takes a locale (defaulting to the global locale) and contains a reference to an std::ctype, which it uses for the tolower function. (Of course, if you're really internationalized, tolower probably doesn't have any meaning. And you'll be using UTF-8, which is a multi-byte encoding, and doesn't work with any of the available possibilities.)