how to get a return type of a member function pointer

Question

Is there a way to determine a return type of a member function pointer?

Code sample:

///// my library
void my_func(auto mptr) { // have to use `auto`
  // some logic based on a return type of mptr: int, string, A, etc.
}

///// client code
struct A {
  int foo();
  std::string bar(int);
};

class B{
public:
  A func(int, double);
};
// ... and many other classes

my_func(&A::foo);
my_func(&A::bar);
my_func(&B::func);
// ... many other calls of my_func()

I need to "fill in" my_func().

Edit: I can't use std::result_of/std::invoke_result as I don't know the full list of parameters of mptr. It's not important with which params a method is supposed to be called as I'm not calling it. I would like to avoid creating an object of base class of mptr even if I'm able to determine it (using declval is ok).

Answer 1

You can use partial template specialization to determine the return type of mptr:

template <typename T>
struct ReturnType;

template <typename Object, typename Return, typename... Args>
struct ReturnType<Return (Object::*)(Args...)>
{
    using Type = Return;
};

void my_func(auto mptr) {
  typename ReturnType<decltype(mptr)>::Type obj;
}

Live Demo