[Lazarus] IUnknown and reference counting

[Kostas Michalopoulos]

Yes, that is what i mean with "topmost" type: if you look at the
inheritance as a tree, TInterfacedObject is the topmost (or root, if you
think of it as an upside-down tree) of the type hierarchy. In this code

  type
    IBlah = interface ... end;
    TBlah = class(TInterfacedObject, IBlah) ... end;

TBlah is an IBlah, so i'd expect it to behave like IBlah.



On Thu, Mar 21, 2013 at 2:10 PM, Sven Barth <pascaldragon at googlemail.com>wrote:

> Am 21.03.2013 13:55, schrieb Kostas Michalopoulos:
>
>  I haven't used (COM) interfaces so far since i didn't found any use for
>> them in my code, but i didn't knew about their reference counting
>> properties. That could save a lot of headaches i have with my 3D world
>> editor's lightmap generation (currently there is some wrong memory
>> deallocation somewhere that crashes the editor in partial lightmap
>> recalculations). I plan to redesign it at some point soon and i was
>> thinking how to handle this. Since there is native refcounting support in
>> FPC it makes things much easier.
>>
>> However i did a small test and i noticed something that, to me (as
>> someone who hasn't used COM at all) looks a bit weird:
>>
>> If i do a declaration like
>>
>> type
>>   IResource = interface  end;
>>   TResource = class(TInterfacedObject, IResource) ... stuff ... end;
>>
>>   TSomething = class
>>     ...
>>     Resources: array of TResource;
>>     ...
>>   end;
>>
>> then reference counting doesn't work. However if i change Resources to
>>
>>     Resources: array of IResource;
>>
>> then it works. It seems that the compiler checks only the "topmost" type
>> and doesn't check if TResource implements the IResource and thus doesn't do
>> any reference counting. Is this a bug or it is supposed to work this way?
>>
>> If the latter, is there a way to make the compiler do reference counting
>> on a variable that has a class type which implements an interface instead
>> of a type that is an interface itself? Or is there any other form of
>> reference counted classes?
>>
>>  The compiler does not check for any topmost type. It uses reference
> counting only for variables that are of an interface type not for a class
> type.
>
> E.g.:
>
> === example begin ===
>
> var
>   i: IInterface;
>   t: TInterfacedObject;
> begin
>   t := TInterfacedObject.Create;
>   t.Free;
>   // no reference counting is done here
>
>   i := TInterfacedObject.Create;
>   i := Nil;
>   // reference counting is done here
> end;
>
> === example end ===
>
> Regards,
> Sven
>
>
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