在 C++98 中实现 decltype

Question

I was wondering if there is a way for implementing the C++11 operator decltype using just C++98 features.

I mean something like

template<class Func>
struct decl_type{
    typedef Func() type;
};

Obviously, my code does not compile.

Answer 1

You can't really implement the decltype since it is not really a function, but a compiler keyword, as you can't implement, for example, return or auto . Actually, decltype(...) , somewhat similarly to auto , is replaced with the result typename by the c++ preprocessor, because, unlike, for example, Haskell or Javascript, you cannot have a type itself stored in a value. But you can overload a function multiple times, so you have something like

void declt(int a) {
    typedef int type;
}
void declt(char a) {
    typedef char type;
}
...