Default constructor parameters in inheritant class

I need to use default parameters for an inherited class's constructor.

class A
{
public:
    A(int data)
    {
        a_data = data;
    }
    ~A(){}

    int a_data; 
};


class B : public A
{
public:
    B(int in, int num = DATA) : A(num)
    {
        b_data = in;
    }
    const int DATA = 9;
    int b_data;
};

int main()
{
    B b(3);
    int x = b.a_data;

    return 0;
}

I've found here that I can use a default parameters for a child's class constructor. But it is said that we should use a global parameter.

But I use a data from class "B" so the A constructor is called before this data is released. In the result I get some garbage in "x" but not 9.

! So, the problem was extracted from another, more particular one. I have a basic class "Object". And some children like "Mob" or "Player". Object has a physical (Box2D) object inside. Certanly, all the actions creating that physical body are the same, except of some parametres. So, I store algorythm in "Object" construtor, and put all the needed data in child classes. But, I can't reach this data, before creating an "Object" which requires that data.

Here is an example of the solution for one dependent class.

I would appreciate any help.