Am 12.11.2016 04:46 schrieb "Ryan Joseph" <r...@thealchemistguild.com>: > > > > On Nov 12, 2016, at 3:22 AM, Jonas Maebe <jonas.ma...@elis.ugent.be> wrote: > > > > You're passing a class, not an object. Passing a class that implements an interface to a method that expects that interface should work fine. Converting a class instance to an interface that it implements is trivial and always works, it's doing the opposite that is hard and that requires specific support in the interface. > > Here’s a better example of where this breaks. Yes casting does work to an extent but breaks eventually down the chain when I lose the reference. Is there anyway to get around this? Maybe I could store the reference from Supports inside TMyObjectClass so I don’t need to call Supports again? > > type > IMyInterface = interface > procedure DoSomething; > function GetObject: TObject; > end; > > type > TMyObjectClass = class(IMyInterface) > procedure DoSomething; > function GetObject: TObject; > end; > > var Obj: TMyObjectClass > ObjInt: IMyInterface; > > Obj := TMyObject.Create; > ObjInt := Obj; // this works fine, casting to an interface > > // Unit A only knows about IMyInterface so we pass > // the class cast as an interface (not sure what to call this) > DomeSomethingInUnitA(ObjInt); > > // Unit A > var Obj: TObject; > Arr: TArray; // collection class the stores TObject > > Arr := TArray.Create; > Obj := ObjInt.GetObject; > Arr.AddObject(Obj);
*Here* is your problem. You should use a container class that handles your interface type to store your interface reference. Especially generics come to mind here (e.g. fgl's TFPGList<> or Generics.Containers' TList<>). Also it's a bad design that you work with an interface and then assume that it's baked by a TObject instance. E.g. with a COM interface it might come from some C++ code. Then what? > [do some stuff] > > // here the problems start because TArray always returns TObject > // and I can’t cast TObject to IMyInterface even though the object > // stored in the array does in fact implement IMyInterface > > ObjInt := IMyInterface(Arr.GetObject(0)); // error! but I need to get a reference to IMyInterface from the array > > // I have to use “as” or Supports here and it defeats the purpose > ObjInt := Arr.GetObject(0) as IMyInterface; Avoid this casting by storing the interface reference. Will make your life much easier and your code more maintainable. Regards, Sven
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