Is an array of Derived a kind-of array of Base?
Nope. This is a corollary of the previous FAQ. Unfortunately this one can get you into a lot of hot water. Consider this: class Base { public: virtual void f(); // 1 }; class Derived : public Base { public: // … private: int i_; // 2 }; void userCode(Base* arrayOfBase) { arrayOfBase[1].f(); // 3 } main() { Derived arrayOfDerived[10]; // 4 userCode(arrayOfDerived); // 5 } The compiler thinks this is perfectly type-safe. Line 5 converts a Derived* to a Base*. But in reality it is horrendously evil: since Derived is larger than Base, the pointer arithmetic done on line 3 is incorrect: the compiler uses sizeof(Base) when computing the address for arrayOfBase[1], yet the array is an array of Derived, which means the address computed on line 3 (and the subsequent invocation of member function f()) isn’t even at the beginning of any object! It’s smack in the middle of a Derived object. Assuming your compiler uses the usual approach to virtual functions, this will reinterpret the int i_ of t
