[fpc-pascal] Where and Why is there a memory leak?

[Graeme Geldenhuys]

On 2017-09-06 10:41, Sven Barth via fpc-pascal wrote:
> I think THook needs to derive from TAggregatedObject, cause that couples
> the reference counting to that of the controlling instance.

That seems to be heading in the right direction, but such a change on 
its own doesn't seem to solve the two memory leaks either. I'll test 
under Delphi XE which can report memory leaks to see what it does with 
that same code. So this might end up being a bug in FPC somewhere.

Either way, modifying the example to use a getter method AND 
TAggregatedObject, I managed to get rid of both memory leaks. Here is 
the working [memory leak free] code now:

=======================[ project1.pas ]=======================
program project1;

{$mode objfpc}{$H+}
{$interfaces COM}

type
   IHook = interface
     ['{4BCAEDD8-92D8-11E7-88D3-C86000E37EB0}']
     procedure DoIt;
   end;

type
   THook = class(TAggregatedObject, IHook)
   private
     procedure DoIt;
   end;

   procedure THook.DoIt;
   begin
     writeln(ClassName + ' did it');
   end;

type
   TBaseClass = class(TInterfacedObject, IHook)
   private
     FHookInstance: THook;
     function GetHook: IHook;
     property Hook: IHook read GetHook implements IHook;
   public
     destructor Destroy; override;
   end;

   function TBaseClass.GetHook: IHook;
   begin
     if FHookInstance = nil then
       FHookInstance := THook.Create(self);
     Result := FHookInstance;
   end;

   destructor TBaseClass.Destroy;
   begin
     FHookInstance.Free;
     inherited Destroy;
   end;


var
   base: IHook;

begin
   base := TBaseClass.Create;
   base.DoIt;
   base := nil; // just to see if it helped with the memory leak - it 
doesn't

end.
========================[ end ]===============================

And the program output:

[t1]$ ./project1
THook did it
Heap dump by heaptrc unit
4 memory blocks allocated : 107/112
4 memory blocks freed     : 107/112
0 unfreed memory blocks : 0
True heap size : 1114112 (32 used in System startup)
True free heap : 1114080


I can't remember ever [while using Delphi] being forced to implement a 
getter method in the "property ... implements..." line, but it seems 
that is the only way it remove memory leaks under FPC. Weird. :-/


Regards,
   Graeme

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