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

Given all the answers that are already here, I think I must be crazy, but this seems right to me so I'm posting it anyways. When I first saw your code example, I thought you were slicing the instances of B and C, but then I looked a little closer. I'm now reasonably sure your example won't compile at all, but I don't have a compiler on this box to test.

A * array = new A[1000];
array[0] = new B();
array[1] = new C();

To me, this looks like the first line allocates an array of 1000 A. The subsequent two lines operate on the first and second elements of that array, respectively, which are instances of A, not pointers to A. Thus you cannot assign a pointer to A to those elements (and new B() returns such a pointer). The types are not the same, thus it should fail at compile time (unless A has an assignment operator that takes an A*, in which case it will do whatever you told it to do).

So, am I entirely off base? I look forward to finding out what I missed.