class with virtual functions takes more space

Question

There is such code:

#include 

class A{
    int a;
    int fun(){}
};

class B{
    int a;
    virtual int fun(){}
};

int main()
{
    std::         


        

Answer 1

Any class with a virtual function needs a pointer to the vtable for that class. Therefore, there is a hidden member that's the size of the pointer.

http://en.wikipedia.org/wiki/Virtual_method_table