Re: generic question

["Matthew Heaney" <mheaney@on2.com>]

Pascal Obry wrote:
>
> You are saying that for every anonymous access type you need to declare
> a named access type from which allocate/deallocate.

Yes, to implement the factory function and destructor for a (tagged)
type.  But that's internal to the package that declares the tagged
type.  The factory function (called by a user of the package to create
instances of this specific type) should return an anonymous access type
as its return type.

If this is a non-tagged type, then anonymous access types are less
necessary, since there's no need for type conversions (as would be the
case for tagged types in a class).


> One point of the
> anonymous access type was to avoid proliferation of named access type...

No.  When you're doing object-oriented programming with different types
in a tagged type hierarchy, you need to be able to easily convert among
specific types and class-wide types towards the root of the hierarchy.
This is simpler with anonymous access types since the conversions are
implicit.

> I find anonymous access type less useful this way... Why even bother
> with them then ?

So that conversions towards the root of a tagged type hierachy are
implicit.  If you were to use named access types to convert towards the
root then this is crying wolf.  You want to reserve explicit
conversions for those times when you want the reader to take notice
that something special is going on, which is the case when performing a
down-cast, away from the root.