C++: Calling the virtual function of the derived class

Question

Suppose I have a class with a virtual function and a derived class that implements the virtual function in a different way. Suppose I also have a vector of the base class u

Answer 1

You can't. The objects in the vector will have been sliced -- any derived-class instance data will have been chopped off, so calling the method would be a super-bad idea.

If, on the other hand, you have a vector of pointers to base, then you simply call the virtual method, and the derived-class version will be invoked.