Question

SFINAE allows us to detect if a type has certain data members or member functions. Can it also be used to detect if a type exists at all? Background: I want to know whether <vector> was included or not.

Answer 1

It can do, although I’ve only managed to make it work in C++0x.

struct no_type {};
struct is_vector_included {
    template<typename U> static decltype(std::vector<U>::iterator()) func( U* );
    template<typename U> static no_type func( ... );
    static const bool value = !std::is_same<no_type, decltype(func<int>(nullptr))>::value;
};

The structure is very curious and although there should be certain obvious simplifications, they don’t work. More importantly, why don’t you just #include <vector> to be certain?