Introducing interfaces into an existing class hierarchy in Delphi

Question

Are there any side effects to changing a class hierarchy's ancestor from TObject to TInterfacedObject so that I can implement interfaces further down the inheritance chain?

I've programmed in Delphi for several years but never encountered interfaces. I became accustomed to using them in other languages. Now that I'm involved in a Delphi project again I'd like to start taking advantage of them but I know they work a bit differently than in Java or C#.

Answer 1

If you already have existing code using the class you will probably have to modify a lot of it to keep references to interfaces instead of object instances. Interfaces are reference counted and released automatically, as a result, any reference to the implementor instance will become an invalid pointer.

Answer 2

This will work fine as long as you inherit from the class below at the top (bottom?) of your hierarchy. This code ensures that your new classes dont free themselves - as is the default behaviour of TInterfaceObject - you are presumably already freeing them yourself and want to preserve this. This activity is actually exactly what TComponent in the VCL does - it supports interfaces but is not reference counted.

type


  TYourAncestor = class( TInterfacedObject )
    function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
    function _AddRef: Integer; stdcall;
    function _Release: Integer; stdcall;

  end;



implementation



function TYourAncestor.QueryInterface(const IID: TGUID; out Obj): HResult;
const
  E_NOINTERFACE = HResult($80004002);
begin
  if GetInterface(IID, Obj) then Result := 0 else Result := E_NOINTERFACE;
end;


function TYourAncestor._AddRef: Integer;
begin
  Result := -1   // -1 indicates no reference counting is taking place
end;

function TYourAncestor._Release: Integer;
begin
  Result := -1   // -1 indicates no reference counting is taking place
end;