Re: Ada-Singleton-Why does it work like this?

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

On Mar 25, 3:10 pm, "patrick.gu...@googlemail.com"
<patrick.gu...@googlemail.com> wrote:
> > If you want to declare an access type, you do not need an instance;
> > you only need a type, which may be unconstrained:
>
> > type T (<>) is limited private;
> > type Access_T is access T; -- OK
>
> > If you want an access *value*, then you do need an object; either one
> > from a storage pool, or a pre-existing one:
>
> > Access_Value : Access_T := new T'(...);
> > -- storage pool. Possible only in the package declaring T, since T is
> > private.
>
> > Other_Access_Value : Access_T := Access_Value; -- possible anywhere
>
> > Neither Access_Value nor Other_Access_Value allow you to see the
> > components of T because T is private.
>
> I´m currently trying to formulate some rules for using Ada myself,
> trying to explain the concepts which are used. I got one last question
> concerning the box-operator (<>) in this particular context.
> It declares a type to be unconstrained, which means, that it needs to
> be initialized. I only found this construction together with generic
> types, for example here:http://en.wikibooks.org/wiki/Ada_Programming/Generics
> When I try to define a sort-routine I can use this generic form to
> state, that discrete types may be used as input for such a routine.
> But as I don´t use this construct together with the keyword generic, I
> simply don´t get the meaning of (<>) in my Singleton-context. Do I say
> that the type I declare is a discrete type? And what would I have to
> do to use this type if it wasn´t limited and private?
>
> Thank you!

Please re-read the wikibook chapter on generics; there is a difference
between:

generic
   type T1 is <>; -- any discrete type

and

generic
   type T2 (<>) is private; -- any type with or without discriminants

Formally, T2 is declared to have "unknown discriminants".  This is
also the case of the singleton type. Because, to the outside world, it
has unknown discriminants, it is unconstrained. The full view of the
type (in the private part of the package) may or may not specify
discriminants.

--
Ludovic Brenta.