Visitor Pattern implemenation

[Ethan Yager 0]

I can't seem to implement the visitor pattern in Delphi without circular references. Does anyone know of a way to implement the visitor pattern without having all of the classes in one big unit? For instance, if I have a complex tree structure (psuedo code so that I can demonstrate concisely):

 

Unit Base

type

TBase = class

end;

-------------------------------------------------------

Unit Bolt

uses Base;

TBolt = class(TBase)

  procedure Tighten;

  procedure Untighten;

 

  Visit(visitor: TAutomobileVisitor); <-- Requires circular reference to Automobile unit 

end;

-------------------------------------------------------

Unit Door

uses Base, Bolt;

type

TDoor = class(TBase)

  Bolts: array of TBolt;

  procedure Open;

  procedure Close;

 

  Visit(visitor: TAutomobileVisitor);  <-- Requires circular reference to Automobile unit

end;

-------------------------------------------------------

Unit Seat

uses Base, Bolt;

type

TSeat = class(TBase)

  Bolts: array of TBolt;

  procedure Heat(on: boolean);

 

  Visit(visitor: TAutomobileVisitor);  <-- Requires circular reference to Automobile unit

end;

-------------------------------------------------------

Unit Automobile

uses Base, Door, Seat, Bolt;

type

TAutomobile = class(TBase)

  FOwnerName: string;

  Doors: array of TDoor;

  Seats: array of TSeat;

 

  Visit(visitor: TAutomobileVisitor); 

end;

 

TAutomobileVisitor = class

  procedure Visit(o: TAutomobile); overload; virtual;

  procedure Visit(o: TBolt); overload; virtual;

  procedure Visit(o: TDoor); overload; virtual;

  procedure Visit(o: TSeat); overload; virtual;

end;

-------------------------------------------------------

 

I have tried moving things around and using interfaces but can't seem to avoid circular references.

 

[Remy Lebeau 1394]

See Uwe Raabe's blog articles about the Visitor Pattern in Delphi:

 

Part 1

Part 2

Part 3

Part 4

 

He shows how to work around the circular referencing issue using interfaces and multiple units.

Edited June 19, 2019 by Remy Lebeau

[A.M. Hoornweg 144]

Use interfaces & tInterfacedObject.

 

Put the interfaces like iVisitor, iDoor, iAutomobile etc in a common unit that contains no implementation code at all.  The implementations+class factories should be in entirely different units that don't reference each other at all. 

 

Oh and instead of writing a big bunch of overloaded Visit() methods in a class, you could consider using a single method and use the "is" or the "supports" keyword if you really need to know the type of visitor.