Problem when casting with a generic type

[Basile B.]

Hello, I'm thinking about refactoring a program using the standard observer pattern, based on generics and simple non-COM interfaces. The problem is that FPC (either 2.6.4 or latest trunk) doesn't seem to accept the parameterized type to cast:

I've attached the full source as it's a simple console program test, however the subject class declaration looks like this, (ITF is the observer interface):

Code: [Select]

  generic TGenericEmitter<ITF> = class(IInfoEmitter)
  protected
    fReceivers: TObjectList;
    function acceptReceiver(aObject: TObject): boolean; virtual; // test for ITF
    function getReceiver(index: Integer): ITF;
  public
    constructor create;
    destructor destroy; override;
    //
    procedure updateReceivers; virtual;
    procedure addReceiver(aReceiver: TObject);
    procedure removeReceiver(aReceiver: TObject);
    //
    property receiver[index: integer]: ITF read getReceiver;
  end;

and the implementation:

Code: [Select]

constructor TGenericEmitter.create;
begin
  fReceivers := TObjectList.create(false);
end;

destructor TGenericEmitter.destroy;
begin
  fReceivers.Free;
  inherited;
end;

function TGenericEmitter.getReceiver(index: Integer): ITF;
begin
  // ouch 1:Error: Class or interface type expected, but got "<undefined type>"
  result := (fReceivers.Items[index] as ITF);
end;

function TGenericEmitter.acceptReceiver(aObject: TObject): boolean;
begin
  // ouch 2:Error: Class or interface type expected, but got "<undefined type>"
  result := (aObject as ITF) <> nil;
end;

procedure TGenericEmitter.addReceiver(aReceiver: TObject);
begin
  if fReceivers.IndexOf(aReceiver) <> -1 then
    exit;
  if not acceptReceiver(aReceiver) then
    exit;
  fReceivers.Add(aReceiver);
end;

procedure TGenericEmitter.updateReceivers;
begin
end;

procedure TGenericEmitter.removeReceiver(aReceiver: TObject);
begin
  fReceivers.Remove(aReceiver);
end;

What's the right syntax to get rid of those errors ?

[taazz]

I have never used generics so this is a bit trial and error but can you try to declare like this

Code: [Select]

generic TGenericEmitter<T:ITF> = class(IInfoEmitter)
...

[Basile B.]

alas this does not do the trick.

[guest58172]

I have never used generics so this is a bit trial and error but can you try to declare like this

Code: [Select]

generic TGenericEmitter<T:ITF> = class(IInfoEmitter)
...

It works finally:

Code: Pascal  [Select][+][-]

1. program Project1;
2. {$Mode objfpc}
3. {$Interfaces CORBA}
4.  
5. type
6.   IDummy = interface
7.   end;
8.  
9.   ISomething = interface(IDummy)
10.     function test(value: TObject): boolean;
11.   end;
12.  
13.   generic TTest<T:IDummy> = class
14.     function test(value: TObject): boolean;
15.   end;
16.  
17.   TSomething = specialize TTest<ISomething>;
18.  
19. function TTest.test(value: TObject): boolean;
20. begin
21.   result := value is T;
22. end;
23.  
24. begin
25. end.  
26.  

Thx...