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Newsgroups: comp.lang.ada
Subject: Re: Ada 0Y plans for garbage collection?
References: <MPG.19c7f581f148da0798975a@News.CIS.DFN.DE>
From: Matthew Heaney <matthewjheaney@earthlink.net>
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Date: Sun, 14 Sep 2003 13:19:28 GMT
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Date: 2003-09-14T13:19:28+00:00
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Jano <nono@celes.unizar.es> writes:

> I'm very happy using Ada but when one needs to use heap-allocated 
> objects and unchecked deallocation it becomes tiresome rapidly, and that 
> leaving aside memory leaks.

Why don't you simply encapsulate the allocation and deallocation behind
an abstraction?

For example, if I have a container like this:

procedure Op (C : in out CT; E : ET) is
begin
   Insert (C, E);
   ...
   Clear (C);

The memory necessary to store element E in container C is allocated
behind the scenes in the implementation of Insert.  That same memory is
deallocated when the container is cleared.

The container itself is controlled, so that when the scope of the
container ends, all the memory it owns is deallocation automatically.

If you must manipulte pointers --say, you're doing class-wide
programming-- then you can still hide the allocation and deallocation
like this:

package P is
   type T is abstract tagged null record;
   type T_Class_Access is access all T'Class;
   procedure Free (X : in out T_Class_Access);
   ...
end P;

package P.C is
   type NT (<>) is new T with private;
   function New_NT return T_Class_Access;
...
private
   type NT is new NT with null record;
end;


By declaring the partial view of the type as indefinite, then you force
clients to use your contructor function:

declare
  O : T_Class_Access := New_NT;
begin
  ...
  Free (O);
end;


That hides explicit calls, and allows each type in the class to allocate
objects in whatever manner is most appropriate.

If you want to automate calls to Free, you can use something like the
auto_ptr in C++, which transfers ownership of an access object across
assignments.  In the Charles library, there is similar abstraction,
allowing you to do this:

declare
   O, O2 : Pointer_Type;
begin
   Initialize (O, New_NT);  --O owns ptr
   Op (+O);
   Assign (O2, O);  --now O2 owns ptr
   Op (+O2);
end; --no Free: O2 does it automatically

Here, when the Pointer_Type object is finalized, it calls the Free
operation automatically.

See Charles.Access_Control for the full implementation.

<http://home.earthlink.net/~matthewjheaney/charles/index.html>

If you need to have pointer sharing, then you can use reference
counting, something like:

package P is
   type T is abstract tagged null record;
   type T_Class_Access is access all T'Class;

   type Handle_Type is private;  --aka "smart pointer"
...
end P;

package P.C is
   type NT is new T with private;
   function New_NT return Handle_Type;
end;

During assignment, the reference count is bumped:

declare
   O1 : Handle_Type := New_NT;  --cnt is 1
begin
   Op (+O1);

   declare
      O2 : Handle_Type := O1;  --inc cnt
   begin
      Op (+O2);
   end;  --dec cnt
end; --dec cnt; cnt = 0; call Free


The point is that the language already provides many abstraction
mechanims to make manipulation of memory easy and safe, and demonstrated
above.

GNAT also provides a debug storage pool, that can help find errors in
heap memory use.  They'll probably add a garbage-collected pool if
there's interest.