Re: Access to tagged type parameters

[tmoran@acm.org]

>I'd like to be able to use access values to a tagged type in a subprogram
>which has the type itself as a parameter rather than an access type. That
>way, when the tagged type is inherited, the subprograms will go with it.
>Subprograms which only have access types to the tagged type do not get
>inhereted by the derived type it seems.
  "A subprogram declared ... is a primitive operation of that type if
it has a parameter of type T, an access parameter pointing to objects
of type T, ..."  Ada as a language, bottom of p. 504

>non-local pointer cannot point to local object
  Consider:

   type A_Type is tagged record ...
   type A_Access_type is access all A_Type'Class;

   Outer_B : A_Access_Type;

   procedure A_Test (B:in out A_Type'Class) is
      B_Access: A_Access_Type;
   begin
      B_Access:=B'Access;
      Outer_B := B_Access;
      Ada.Text_IO.Put_Line("Done.");
   end A_Test;

   procedure Test is
     X : A_Type;
   begin
     A_Test(X);
   end Test;
begin
   Test;
   Outer_B.all := ....     -- what does this point to???

A local object, X, disappears while a non-local pointer, Outer_B, is
still pointing to it.  You need to make "type A_Access_Type" more
local so any such pointer will be guaranteed to disappear no later
than anything it points to.