Question

I have a small problem with interfaces. Here it is in Pseudo code :

type   Interface1 = interface   end;    Interface2 = interface   end;    TParentClass = class(TInterfacedObject, Interface1)   private     fChild : Interface2;   public     procedure AddChild(aChild : Interface2);   end;    TChildClass = class(TInterfacedObject, Interface2)   private     fParent : Interface2;   public     constructor Create(aPArent : Interface1);   end; 

Can anyone see the flaw? I need the child to have a reference to it’s parent, but the reference counting doesn’t work in this situation. If I create a ParentClass instance, and add a child, then the parent class is never released. I can see why. How do I get round it?

Answer 1

A reference-counted reference has two semantics: it acts as a share of ownership as well as a means of navigating the object graph.

Typically, you don’t need both of these semantics on all links in a cycle in the graph of references. Perhaps only parents own children, and not the other way around? If that is the case, you can make the child references to the parent weak links, by storing them as pointers, like this:

TChildClass = class(TInterfacedObject, Interface2) private   fParent : Pointer;   function GetParent: Interface1; public   constructor Create(aPArent : Interface1);   property Parent: Interface1 read GetParent; end;  function TChildClass.GetParent: Interface1; begin   Result := Interface1(fParent); end;  constructor TChildClass.Create(AParent: Interface1); begin   fParent := Pointer(AParent); end; 

This is safe if the root of the tree of instances is guaranteed to be kept alive somewhere, i.e. you are not relying on only keeping a reference to a branch of the tree and still being able to navigate the whole of it.