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

[Michael Van Canneyt]

On Wed, 6 Sep 2017, Graeme Geldenhuys wrote:

> Hi,
>
> Playing with this small sample application to answer another question in 
> this mailing list, I noticed the sample application has a memory leak. 
> For the life of me I can't see why or how to resolve it.
>
> I tested with FPC 2.6.4, 3.0.2 and 3.0.4-rc1 under 64-bit FreeBSD.
>
> =======================[ project1.pas ]============================
> program project1;
>
> {$mode objfpc}{$H+}
> {$interfaces COM}
>
> type
>   IHook = interface
>     ['{4BCAEDD8-92D8-11E7-88D3-C86000E37EB0}']
>     procedure DoIt;
>   end;
>
> type
>   THook = class(TInterfacedObject, IHook)
>   private
>     procedure DoIt;
>   end;
>
>   procedure THook.DoIt;
>   begin
>     writeln(ClassName + ' did it');
>   end;
>
> type
>   TBaseClass = class(TInterfacedObject, IHook)
>   private
>     FHook: IHook;
>     property Hook: IHook read FHook implements IHook;
>   public
>     constructor Create;
>     destructor Destroy; override;
>   end;
>
>   constructor TBaseClass.Create;
>   begin
>     FHook := THook.Create;  // FPC 2.6.4 reports a memory leak here
>   end;
>
>   destructor TBaseClass.Destroy;
>   begin
>     // nothing to do here
>   end;

You must free FHook here, because you are keeping a reference to the object,
not the interface. And you must call inherited. 
You must always call inherited in the destructor.

Michael.