Size of virtual pointer-C++

Question

What is the size of virtual pointer(VPTR) for a virtual table in C++? Also this is not a homework question...just a question that came to my mind while I was reading a C++ b

Answer 1

That depends on your implementation, but it's easy to find out. For this program

#include 

struct virtual_base {
    int data;
    virtual_base() {}
    virtual ~virtual_base() {}
};

struct non_virtual_base {
    int data;
    non_virtual_base() {}
    ~non_virtual_base() {}
};

int main() {
    std::cout << sizeof( virtual_base ) - sizeof( non_virtual_base ) << '\n';
    return 0;
}

mine (VC 2008) will print 4, so the cost of polymorphism is, in this case, 4 byte.