Call a member function pointer

Question

How to call a member function pointer which is assigned to a member function? I am learning callbacks and this is just for learning purpose. How I can invoke m_ptr function ?

class Test{
   public:
       void (Test::*m_ptr)(void) = nullptr;
       void foo()
       {
           std::cout << "Hello foo" << std::endl;
        }

   };

void (Test::*f_ptr)(void) = nullptr;

int main()
{
     Test t;
     f_ptr = &Test::foo;
     (t.*f_ptr)();     
     t.m_ptr =  &Test::foo;
    //  t.Test::m_ptr();   //Does not work
    //  t.m_ptr();         //Does not work
    //  (t.*m_ptr)();      //Does not work
     return 0;
}

Answer 1

You are almost there. Recall that m_ptr is a data member itself, so in order to access it you need to tell the compiler to resolve that relationship to the instance holding the pointer to member. Call it like this:

 (t.*t.m_ptr)();
 //  ^^ this was missing

Answer 2

如果您有C ++ 17，也可以

std::invoke(t.m_ptr, t);

Answer 3

Operator .* requires expression that return a pointer to member of class-type on the right side and expression which object of that type on the left side (ie t in your case). But your pointer is a member, so right expression should be t.m_ptr