19.15.txt

ordinary function pointers can point to nonmember functions, conversion from function to pointer happens automatically

pointer to member functions can point to member functions, conversion from member function to pointer to member functions don't happen automatically, need to include the address-of operator (&)
these pointers can be called on objects, but they can exist without binding an object to the member function
we need to include the const qualifier of l/r-value reference qualifier on the pointer to member function declaration