[boost] Re: Proposal: boost::pool fast pointer

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