Question

I run into what seems to be a very classical problem: An item and a collection class, both referencing each other, that require a forward declaration. I’m using Delphi 2010 with update 5.

This works well with non generic classes but I can’t workaround the E2086 error with generic types:

type
  // Forward declarations
  TMyElement = class; // E2086: Type 'TMyElement' is not yet completely defined

  TMyCollection<T:TMyElement> = class
    //
  end;

  TMyElement = class
    FParent: TMyCollection<TMyElement>;
  end;

The same issue happens when switching the class declaration order.

I didn’t found any reference to this issue here or in QualityCentral (other issues with E2086 were found, but not related to this use case)

The only workaround I have for now is to declare the parent as TObject, and cast it to the collection generic type when required (not a clean solution…)

How did you workaround this issue, or forward-declare your generic classes?

Thanks,

[Edit Oct 22, 2011] Follow up on QualityCentral:
I reported this bug in quality central here

This has been closed recently by EMB with the following resolution status:
Resolution: As designed
Resolved in build: 16.0.4152

I only have Delphi 2010. Could someone confirm that it has been fixed in Delphe XE2 Update1, or does it mean that it works ‘as expected’?

[Edit Oct 23, 2011] Final answer from EMB:
EMB confirmed today that using forward declaration of a generic type is not supported by the actual Delphi compiler. You can see their answer in QC, with the link provided above.

Answer 1

You can work around it by declaring an ancestor class:

type
  TBaseElement = class
  end;

  TMyCollection<T: TBaseElement> = class
  end;

  TMyElement = class(TBaseElement)
  private
    FParent: TMyCollection<TBaseElement>;
  end;