Earlier in the chapter, you saw that if you use a base class pointer to a derived object only the base class members can be safely accessed. The other members are still there, but they can only be accessed through an appropriate derived class pointer.
However, if you cast a derived class object to a base class object, something else happens: you create a new object, and that object is the base class object, just the base class object. The variable that you have cast to will only have the memory for the base class object, so the result is only the base class object part of the derived object:
struct base { /*members*/ }; struct derived : base { /*members*/ }; derived d; base b1 = d; // slicing through the copy constructor ...
