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From: Caffeine Junky <nospam@hotmail.com>
Subject: Unchecked Deallocation?
Newsgroups: comp.lang.ada
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Xref: archiver1.google.com comp.lang.ada:27028
Date: 2002-07-12T22:44:10+00:00
List-Id: <comp.lang.ada>

I think I'm getting a handle on using Unchecked Deallocation. However
I'm running into a problem. In my spec file I have these types defined...

with unchecked_deallocation;

generic
  type Item is private;

package genstack is 	-- A basic generic stack. Singlely linked list. --

        type stack is limited private;
        ...  -- Some other procedures defined here. --       
	function Clear_Stack(S : in Stack) return Boolean;


private

	type Cell;
	type stack is access Cell;
	type Cell is record
	   Value: Item;
	   Next: Stack;
	end record;

end genstack;


Now in the body of the package I have this...

package body genstack is

	procedure Free is new Unchecked_Deallocation(Cell, Stack);

	.....	-- Some other procedures here  --

	function Clear_Stack(S : in Stack) return Boolean is -- Return True or
													 -- or False if the
													 -- operation succeeded. --

	begin

	  Free(Stack);
	  return True;	   -- No reason for it not to return True in this --
				   -- instance, as far as I can tell.             --
        end Clear_Stack;

end genstack;

Now, when attempting to compile the code, I get this...

	41. Free(Stack)
	         |
        >>> Invalid use of subtype mark in expression or call
        >>> actual for 'X' must be a variable

I'm using Gnat 3.14p (Debian)


Now, as far as I can tell, I cannot give it an actual variable because
Cell is defined in the private type and is only accessible via the
'Stack' pointer variable. I dont think even Aliases can get past this
rule.
Also, the Unchecked_Deallocation documentation in the LRM that came with
GNAT (found it using GNU Info) shows Unchecked_Deallocation being
instantiated just the way I did it here.(Assuming I understood the docs.)
As does my "Programming in Ada95" book by John Barnes.
I suspect that it needs access to the Cell variable declared in the
private section, but I'm not sure how to do that safely.

Any pointers would be appreciated.

St4pL3