oops · medium

Object Slicing

When a Derived object is copied into a Base by value, only the Base subobject is copied — the extra Derived members are 'sliced off', and Base's copy constructor sets the vptr to Base's vtable. Virtual calls on the slice therefore dispatch to Base, silently losing polymorphism. Binding a Base& or Base* to the object copies nothing and keeps the Derived vptr, so virtual dispatch still reaches Derived. Rule: handle polymorphic objects through references or pointers, not by value.

The code

struct Base {
    int id;
    virtual void speak();
};
struct Derived : Base {
    double extra;
    void speak() override;
};

Derived d;
Base b = d; // BY VALUE: copies only the Base subobject
b.speak();  // -> Base::speak  (Derived part is gone)

Base& r = d; // BY REFERENCE: no copy, full object
r.speak();   // -> Derived::speak  (polymorphic)

What this lesson walks through

1. 01
2. 02
3. 03
4. 04
5. 05
6. 06