While working with Alexander on paxCompiler for Windows, Linux and Mac OS X, a common problem is implementing a true garbage collector (This problem exists in a wide range of script engines, even popuplar ones like PHP.), I came across an example using IUNKNOWN Interface. The trick is when an interface goes out of scope the Release() method is automatically called, if it contains a pointer to the actual object it can call that object.free for you automatically.

IUnknown Interface

      Enables clients to get pointers to other interfaces on a given object through the QueryInterface method, and manage the existence of the object through the AddRef and Release methods. All other COM interfaces are inherited, directly or indirectly, from IUnknown. Therefore, the three methods in IUnknown are the first entries in the VTable for every interface.

      About IUnknown, it works on Windows 2000 and later, its IID_IUnknown is defined as 00000000-0000-0000-C000-000000000046, and this code has been ported and test with FPC on Windows, Linux and Mac OS X.
    Download garbagecollector.pp Source for Free Pascal Compiler

program test;

{$MODE DELPHI}{$H+}
uses garbagecollector;

type
   TTest = class(TObject)
   public
      constructor Create;
      destructor Destroy; override;
      procedure Test;
   end;

constructor TTest.Create;
begin
   WriteLn('Create()');
end;

destructor TTest.Destroy;
begin
   WriteLn('Destroy()');
   inherited;
end;

procedure TTest.Test;
begin
   WriteLn('Test()');
end;

procedure TestProc;
var
   SafeGuard : ISafeGuard;
   obj : TTest;

begin
   obj := TTest(Guard(TTest.Create, SafeGuard)); // Create!
   obj.Test;
end;

begin
   TestProc;
end. 

      Now when compiled and executed, you will see your output state Create(), Test() and Destroy(), even though you have obviously not called obj.Free; in your code. After the call to obj.Test your "obj"ect goes out of scope and can be destroyed. Due to the design of most script engines they do not track a reference counter to their variables, nor are they doing a stack tracker (another solution I developed for my JavaScript Interpreter) and eventually a long script (one which runs non-stop for days will eventually run out of RAM and potentially bring the host operating system down.

G.E. Ozz Nixon Jr.

Brian Ellixson
FreePascal User