Question

Consider the following function:

template<class T1, class T2, class T3 = /* SOMETHING */> 
T3 f(const T1& x, const T2& y);

I want T3 to be equal to the return type of T1+T2. How to do that with C++11 ?

Note: I don’t want the result of std::common_type<T1, T2>::type, I want real type of T1+T2, considering that the operator+ can be a non-member function or can be a member function of T1.

Answer 1

The usual way is decltype and a trailing-return-type:

template<class T1, class T2> 
auto f(const T1& x, const T2& y) -> decltype(x+y);