Object slicing happens when a derived class object is assigned to a base class object, additional attributes of a derived class object are sliced off to form the base class object.
class Base { int x, y; };
class Derived : public Base { int z, w; };
int main()
{
Derived d;
Base b = d; // Object Slicing, z and w of d are sliced off
}
http://www.geeksforgeeks.org/object-slicing-in-c/