c++ - virtual function default arguments behaviour

Question

I have a strange situation over the following code. Please help me to get it clarified.

class B
{
       public:
            B();
            virtual void print(int data=10)
            {
                  cout << endl << "B--data=" << data;
            }
};
class D:public B
{
       public:
            D();
            void print(int data=20)
            {
                  cout << endl << "D--data=" << data;
            }
};

int main()
{
     B *bp = new D();
     bp->print();
return 0;
}

Regarding the output I expected

[ D--data=20 ]

But in practical it is

[ D--data=10 ]

Please help. It may seem obvious for you but I am not aware of the internal mechanism.

Answer 1

The standard says (8.3.6.10):

A virtual function call (10.3) uses the default arguments in the declaration of the virtual function determined by the static type of the pointer or reference denoting the object. An overriding function in a derived class does not acquire default arguments from the function it overrides.

This means, since you are calling print through a pointer of type B, it uses the default argument of B::print.