Re: How to declare a generic formal type "covered" by another?

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 05/01/2014 12:33 AM, Natasha Kerensikova wrote:
>
> Well maybe the problem here is my limited vocabulary. I used the word
> "constrained" there only to mean "Something := new S" or "Variable : S;"
> are both valid, i.e. objects of that type can be created without
> further information. There must be some way to express that in generics.

As Rosen has pointed out, this is the definite/indefinite idea, expressed in 
generic formals as lacking/having unknown discriminants ["(<>)"].

Most definite subtypes are constrained, and most indefinite, unconstrained, but 
that's not always the case. Consider

subtype VSI is Integer range 0 .. 255;

type VS (Length : VSI := 0) is record
    Value : String (1 .. Length);
end record;

VS is definite, because you can say

S : VS;

but it's also unconstrained, because you can change the discriminant:

S := (Length => 1, Value => "A");
S := (Length => 2, Value => "Hi");

For your uses, perhaps you should consider something like

generic -- GP
    type T (<>) is limited private;
package GP is
    type T_Access is access all T;

    type Ref is private;

    generic -- F
       with function Allocate return T_Access;
    function Allocate return Ref;

    ...
end GP;

Most smart-pointer packages expose the access type; see, for example,

http://www.oopweb.com/Ada/Documents/AdaLinux/Volume/18.html
http://www.adacore.com/adaanswers/gems/gem-97-reference-counting-in-ada-part-1/

HTH.

-- 
Jeff Carter
"C++ is like jamming a helicopter inside a Miata
and expecting some sort of improvement."
Drew Olbrich
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