From: Philippe A. Bouchard (philippeb_at_[hidden])
Date: 2002-07-30 15:04:31
> class X { int x; };
> class Y { int y; };
> class Derived: public X, public Y {};
[...]
> A plain old Y object, we have a ptr<Y>, containing some pointer P. One
> reasonable implementation is:
> +----------------
> | Ref count
> P -> +----------------
> | Y::y
> +----------------
>
> Now, for a Derived object, which we now make a ptr<Y> for. Where to
> place the pointer P for this, A or B?
> +----------------
> | Ref count
> A -> +----------------
> | X::x
> B -> +----------------
> | Y::y
> +----------------
>
[...]
> If you claim you have a solution that keeps sizeof(ptr<X>) == sizeof(X
> *), you'll have to demonstrate it more convincingly.
What about a second template parameter which will restrict operator new ()
allocations:
template <typename _T, class _C = _T>
struct ptr
{
template <typename _U>
ptr(_U * a_p) : m_ptr(a_p)
{
// typeid(_C) == typeid(_U) compile-time assertion here
}
};
ptr<Derived> pD = new Derived;
ptr<Derived, X> pX = new X;
ptr<Derived, Y> pY = new Y;
pX = pD; // Won't work
pY = pD; // Won't work
... or maybe there is another compile-time way to assert the pointer is
always at the same offset also ...
Philippe A. Bouchard