BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Interface and inheritance problem          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message R. Vermeij Guest Posted: Mon Nov 22, 2004 2:17 pm    Post subject: Interface and inheritance problem I have this hierarchy: IBaseConfigurationFile = interface IBaseNamedFile = interface(IBaseConfigurationFile) IMachineTypeFile = interface(IBaseNamedFile) (all three interfaces have a GUID) TBaseConfigurationFile = class(TInterfacedObject, IBaseConfigurationFile) TBaseNamedFile = class(TBaseConfigurationFile) TMachineTypeFile = class(TBaseNamedFile, IMachineTypeFile) In a method I pass: (AFile: IBaseConfigurationFile) Why does: Supports(AFile, IBaseNamedFile, BaseNamedFile) return False? Supports(AFile, IMachineTypeFile, MachineTypeFile) return True? Supports(AFile, IBaseConfigurationFile, BaseConfigurationFile) return True? (I am quite new to interfaces, so I guess I misunderstand something...) Back to top Angel Guest Posted: Mon Nov 22, 2004 5:57 pm    Post subject: Re: Interface and inheritance problem Delphi object model required that in class declaration you have to mention all interfaces this class implements even if this interface has been inherited from other interface than this class implements: TMachineTypeFile = class(TBaseNamedFile, IMachineTypeFile, IBaseConfigurationFile) Regards Angel "R. Vermeij" <rSv (AT) veroPmaAMtic (DOT) nl> wrote Quote: I have this hierarchy: IBaseConfigurationFile = interface IBaseNamedFile = interface(IBaseConfigurationFile) IMachineTypeFile = interface(IBaseNamedFile) (all three interfaces have a GUID) TBaseConfigurationFile = class(TInterfacedObject, IBaseConfigurationFile) TBaseNamedFile = class(TBaseConfigurationFile) TMachineTypeFile = class(TBaseNamedFile, IMachineTypeFile) In a method I pass: (AFile: IBaseConfigurationFile) Why does: Supports(AFile, IBaseNamedFile, BaseNamedFile) return False? Supports(AFile, IMachineTypeFile, MachineTypeFile) return True? Supports(AFile, IBaseConfigurationFile, BaseConfigurationFile) return True? (I am quite new to interfaces, so I guess I misunderstand something...) Back to top Joanna Carter (TeamB) Guest Posted: Mon Nov 22, 2004 6:52 pm    Post subject: Re: Interface and inheritance problem "Angel" <afilev (AT) pcreducator (DOT) com> a écrit dans le message de news: 41a22870$1 (AT) newsgroups (DOT) borland.com... Quote: TMachineTypeFile = class(TBaseNamedFile, IMachineTypeFile, IBaseConfigurationFile) or TBaseNamedFile = class(TBaseConfigurationFile, IMachineTypeFile) :-) Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Joanna Carter (TeamB) Guest Posted: Mon Nov 22, 2004 8:26 pm    Post subject: Re: Interface and inheritance problem "Joanna Carter (TeamB)" <joannac (AT) btinternetxx (DOT) com> a écrit dans le message de news: 41a23587$1 (AT) newsgroups (DOT) borland.com... Quote: TBaseNamedFile = class(TBaseConfigurationFile, IMachineTypeFile) Sorry, should have been : Quote: TBaseNamedFile = class(TBaseConfigurationFile, IBaseNamedFile) Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top R. Vermeij Guest Posted: Tue Nov 23, 2004 6:17 am    Post subject: Re: Interface and inheritance problem I suspected that already :-( But why then does this return True? Supports(AFile, IBaseConfigurationFile, BaseConfigurationFile) where: AFile: IBaseConfigurationFile TMachineTypeFile = class(TBaseNamedFile, IMachineTypeFile) AFile is filled with a TMachineTypeFile Is this because the interface variable itself is of type IBaseConfigurationFile? Or should my declaration be: TMachineTypeFile = class(TBaseNamedFile, IBaseConfigurationFile, IBaseNamedFile, IMachineTypeFile) (In other words, what is the best way to do this?) Ruud Back to top R. Vermeij Guest Posted: Tue Nov 23, 2004 6:33 am    Post subject: Re: Interface and inheritance problem o-: **** AHA-ERLEBNISS **** :-o Now I see my error. I forgot to implement IBaseNamedFile on TBaseNamedFile! Quote: TBaseNamedFile = class(TBaseConfigurationFile) should have been: TBaseNamedFile = class(TBaseConfigurationFile, IBaseNamedFile) Well, many thanks to Angel and Joanna! Ruud Back to top Roman Kaßebaum Guest Posted: Tue Nov 23, 2004 8:25 am    Post subject: Re: Interface and inheritance problem R. Vermeij schrieb: Quote: I have this hierarchy: IBaseConfigurationFile = interface IBaseNamedFile = interface(IBaseConfigurationFile) IMachineTypeFile = interface(IBaseNamedFile) (all three interfaces have a GUID) Interfaces should habe only few methods, thats's why in many cases it would be better not to use inheritance in interfaces. The delphi-objects, that provide the interfaces, should use inheritance. -- Roman Back to top Joanna Carter (TeamB) Guest Posted: Tue Nov 23, 2004 9:37 am    Post subject: Re: Interface and inheritance problem "Roman Kaßebaum" <roman.kassebaumnospam (AT) kdv-dt (DOT) de> a écrit dans le message de news: [email]41a2f3e2 (AT) newsgroups (DOT) borland.com[/email]... Quote: Interfaces should habe only few methods, thats's why in many cases it would be better not to use inheritance in interfaces. The delphi-objects, that provide the interfaces, should use inheritance. If I may explain further... When designing Interfaces, you are declaring behaviours that may or may not inherit from each other. e.g. You could do something like this : IConfiguration IName IMachine TConfigurationFile = class(TInterfacedObject, IConfiguration) TNamedFile = class(TInterfacedObject, IConfiguration, IName) TMachineFile = class(TInterfacedObject, IConfiguration, IName, IMachine) OTOH, you could also implement them like this : TConfigurationFile = class(TInterfacedObject, IConfiguration) TNamedFile = class(TConfigurationFile, IName) TMachineFile = class(TNamedFile, IMachine) or even possibly TMachineFile = class(TConfigurationFile, IName, IMachine) I am not saying that you should never inherit interfaces, just to check whether one interface is really an extension of the behaviour of another; and regard the inheritance of interfaces differently to the inheritance of classes. Joanna -- Joanna Carter (TeamB) Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Roman Kaßebaum Guest Posted: Tue Nov 23, 2004 9:43 am    Post subject: Re: Interface and inheritance problem Joanna Carter (TeamB) schrieb: Quote: I am not saying that you should never inherit interfaces, just to check whether one interface is really an extension of the behaviour of another; and regard the inheritance of interfaces differently to the inheritance of classes. Absolutely correct, that's I wanted to say. -- Roman Back to top R. Vermeij Guest Posted: Tue Nov 23, 2004 10:24 am    Post subject: Re: Interface and inheritance problem Quote: Interfaces should habe only few methods, thats's why in many cases it would be better not to use inheritance in interfaces. The delphi-objects, that provide the interfaces, should use inheritance. The reason I use interfaces here, is the garbage collection functionality that comes with the reference counting. It is convenient for me to have the class inheritance tree be parallel to the interface (inheritance) tree. But I understand what you are saying (and I'm learning)! 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