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

[AdaMagica <christ-usch.grein@t-online.de>]

> 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/

But exposing the access type is bad since pointes may easily outlive the object they point to.

Let Get return the access value of the smart pointer P and let P be the only pointer to the object accessed. Let Q be another smart pointer.

  declare
    Obj: access Client_Data'Class := Get (P);
  begin
    My_Data (Obj.all).I := 2012;  -- No problem.
    P := Q;                       -- Frees the original object,
                                  -- because the reference count
                                  -- is zero after this operation.
    My_Data (Obj.all).I := 95;    -- Oops - writing freed memory!
  end;

See also:

http://www.adacore.com/adaanswers/gems/gem-107-preventing-deallocation-for-reference-counted-types/