Re: Unconstrained Arrays

[Jean-Pierre Rosen <rosen@adalog.fr>]

sjw a �crit :
> There is a deep language-lawyerly reason (which I don't understand)
> why an array like your My_Array can't be aliased (at any rate in
> Ada95); you have to use the initialize-with-aggregate approach.
> Perhaps that's what leads to the initialize-with-aggregate style.
Here is an example:

procedure essai is
   type Acc is access all String;
   V : Acc;
   S1 : aliased String := "abcd";
   S2 : aliased String (1..4);
begin
   V := S1'Access;
   V := S2'Access;
end;

And this is what GNAT says:

essai.adb:5:04: warning: aliased object has explicit bounds
essai.adb:5:04: warning: declare without bounds (and with explicit
initialization)
essai.adb:5:04: warning: for use with unconstrained access
essai.adb:8:09: object subtype must statically match designated subtype

The message is pretty explicit. The underlying reason has to do with
dope vectors (i.e. how the bounds of an array are attached to the
array). It might be different, depending on whether the array is
/declared/ constrained or not (S1 is declared unconstrained, although
constrained by initialization).

For example, in the constrained case, the array might include a pointer
to a structure that describes the subtype, while in the unconstrained
case, the array might keep the bounds with the data. Therefore, a
pointer to a constrained array is incompatible with a pointer to an
unconstrained array.
-- 
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           J-P. Rosen (rosen@adalog.fr)
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