Forward record declaration

Forward declaration is not really a new concept. It was already present in the original Wirth Pascal where it allowed the programmer to do one thing and one thing only – call procedure A from procedure B and procedure B from procedure A. Remember – in ye olde times of good old Pascal we had no interfaces, no classes, no units … just procedures and functions.

Because the code tells more than thousand words, here’s an example (still perfectly valid in modern Object Pascal).

procedure ProcA; forward;

procedure ProcB;
begin
  ProcA;
end;

procedure ProcA;
begin
  ProcB;
end;

A more familiar concept in modern times is forward declaration for classes and interfaces. Again, a code tells more than a long explanation …

type
  TClassA = class;

  TClassB = class
    ObjA: TClassA;
  end;

  TClassA = class
    ObjB: TClassB;
  end;

  IIntfA = interface;

  IIntfB = interface
    function Other: IIntfA;
  end;

  IIntfA = interface
    function Other: IIntfB;
  end;

Object Pascal, however, does not understand the concept of a forward record declaration. IOW, this will not compile.

type
  TRecA = record;

  TRecB = record
    function Other: TRecA;
  end;

  TRecA = record
    function Other: TRecB;
  end;

There’s however a trick that allows us to achieve the same functionality with a different syntactic sugar – record helpers. We can remove the TRecB.Other declaration from TRecB and reintroduce it later via a TRecB helper.

type
  TRecB = record
  end;

  TRecA = record
    function Other: TRecB;
  end;

  TRecBHelper = record helper for TRecB
    function Other: TRecA;
  end;

Just keep in mind that this solution is a bit less stable as other code can hide away the functionality from TRecBHelper by introducing their own helper for TRecB. (A nasty “feature” of the language which really should be fixed loooong ago.)