Accessing type inside std::pair passes as template parameter

Question

I want to write a function

template <class T>
X foo (T);

where T is of type std::pair<X, X>.

How can I deduce the return type X from T?

Answer 1

In your example X has to be templated, too:

template <class T, class X = typename T::first_type >
 X foo (T t); 

But better would be:

template <class T >
 typename T::first_type foo (T t);