delphi - How to declare two inter-linked classes?

Question

I have a question similar to this, but in delphi.

type
  TThreadPopulator = class(TThread)
  private
    _owner:TASyncPopulator; //Undeclared identifier
  end;

type
  TAsyncPopulator = class
  private
    _updater: TThreadPopulator;
  end;

Solution of mentioned question is not applicable to delphi

Answer 1

See Forward Declarations and Mutually Dependent Classes documentation.

type (* start type section - one unified section "to rule them all" *)
  TAsyncPopulator = class; (* forward declaration *)

  TThreadPopulator = class(TThread)
  private
    _owner:TASyncPopulator;
  end;

  TAsyncPopulator = class (* final declaration - WITHIN that very section where forward declaration was made *)
  private
    _updater: TThreadPopulator;
  end;

Use the source, Luke! Your Delphi installation has full VCL and RTL sources for you to read and watch and learn. And it uses this template a lot. Every time when you ask yourself "how I could do it", just think along "how did Borland do it" and pretty chance that you can already get a ready-made example in Delphi-provided sources.