class rigidBody3D {
public:
    void setPosition(float x, float y, float z);
    rigidBody* rb;
};

class box3D : public rigidBody3D {
public:
    box3D(float w float h, float l);
    ..other box functions..
};

class rigidBody2D {
public:
    rigidBody2D();
    void setPosition(float x, float y);
    rigidBody3D body;
};

class box2D : public rigidBody2D {
public:
    box2D(float w, float h);
    box3D box;
};

void rigidBody2D::setPosition(float x, float y)
{
    body.setPosition(x,y,0);
}