Re: controlled initialization

[Simon Wright <simon@pushface.org>]

Hyman Rosen <hyrosen@mail.com> writes:

> Interesting. So (remembering that I don't know Ada) the following
> would be illegal as well? If it is, is there a way to do what the
> code obviously intends? Do I understand the issue correctly, that
> is that MakeT could use O?
> 
> package P is
>    type T is private;
>    O : constant T;
> private
>    type T is new Controlled with null record;
>    function MakeT return T;
>    O : constant T := MakeT;
> end P;

One possibility is to make O a function (inlined if you need to).

A related difficulty is where T is indefinite: from the under-work
Booch Components,

1   type Unconstrained_Map
2     (Number_Of_Buckets : Positive) is new Abstract_Map with private;
3
4   subtype Map is Unconstrained_Map (Number_Of_Buckets => Buckets);
5
6   function Null_Container return Unconstrained_Map;
7   --  Note, this function has to be provided but the object returned
8   --  is in fact a Map (ie, it is constrained).

Buckets in line 4 is a generic parameter, retained for consistency
with previous releases, specifying the default number of hash
buckets. Null_Container at line 6 is required because it's abstract in
the parent. Still, folk relying on Null_Container can use it with the
previous semantics.