Limited_Controlled and out parameters

[Maciej Sobczak <see.my.homepage@gmail.com>]

I have found a conceptual problem with the rules for Limited_Controlled types when they are used as out parameters in subprograms.

Consider:

   type Int_Ptr is access Integer;
   type Int_Holder is new Limited_Controlled with record
      I : Int_Ptr;
   end record;

The intent is that the Int_Holder can optionally hold an integer value. Optionally means that it might as well be empty (null);

Then:

   procedure Set (H : out Holder; Value : in Integer) is
   begin
      H.I := new Integer'(Value);
   end Set;

All is fine and the conveniently overriding procedure Finalize cleans the mess at the end.
The problem is that the Set procedure can be called many times in a row:

   Set (H, 7);
   Set (H, 8);
   -- ...

Each time it is called on the same H (Holder) object, a new Integer is allocated - this is a memory leak.
A possible solution is to check if the holder already contains some value and deallocate it before planting a new one. The problem is - this is an *out* parameter and notionally it is uninitialized. Well, controlled types are passed by reference, so it *would* work fine, but that's just cheating (and a more strict data flow would reject it as a clear violation).

What's the proper solution?

-- 
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com