C++ virtual function table memory cost

Question

Consider:

class A
{
    public:
        virtual void update() = 0;
}

class B : public A
{
    public:
        void update() { /* stuff goes in here... */ }
         


        

Answer 1

As others already said, in a typical popular implementation approach, once a class becomes polymorphic, each instance grows by a size of an ordinary data pointer. It doesn't matter how many virtual functions you have in your class. On a 64-bit platform the size would increase by 8 bytes. If you observed 8-byte growth on a 32-bit platform, it could have been caused by padding added to 4-byte pointer for alignment (if your class has 8-byte alignment requirement).

Additionally, it is probably worth noting that virtual inheritance can inject extra data pointers into class instances (virtual base pointers). I'm only familiar with a few implementations and in at least one the number of virtual base pointers was the same as the number of virtual bases in the class, meaning that virtual inheritance can potentially add multiple internal data pointers to each instance.