BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Re: Interface reference vs. Object reference Goto page 1, 2, 3  Next          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message DPi Guest Posted: Thu Oct 16, 2003 7:26 am    Post subject: Re: Interface reference vs. Object reference When You have several different classes implementing some Interface. You can pass different objects to generic functions. i.e. Move(IMovable). To the functions which are specific for some kind of objects You use object reference. Animate(TAnimation) "Petter Holmström" <petter.holmstrom (AT) JUNK-BUSTERparnet (DOT) fi> wrote Quote: Hello everybody, Let's say I have an interface declaration like this: IMyInterface = interface [...] function MyMethod(MyVar: string): Boolean; end; My object declaration looks like this; TMyObject = class(TInterfacedObject, IMyInterface) public function MyMethod(MyVar: string): Boolean; end; My question is: When should I access the object via an Interface reference, like this: function ProcessObject(AObject: IMyInterface): Boolean and when should I use an object reference such as: function ProcessObject(AObject: TMyObject): Boolean Also, are there any situations where one should not use interfaces? Thanks in advance, -Petter- Back to top Joanna Carter Guest Posted: Thu Oct 16, 2003 7:30 am    Post subject: Re: Interface reference vs. Object reference Petter Holmström wrote: Quote: My question is: When should I access the object via an Interface reference, like this: function ProcessObject(AObject: IMyInterface): Boolean and when should I use an object reference such as: function ProcessObject(AObject: TMyObject): Boolean If youo want to retain your sanity and not have to search for memory leaks or access violations, then you should never mix interface and object references in Delphi. There are lifetime management issues that play serious havoc with one's sense of well-being :-) Also you should always use a const modifier for interface parameters to avoid triggering reference counting on the way in to and out of the method that takes the parameter. Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Ignacio Vazquez Guest Posted: Thu Oct 16, 2003 9:33 am    Post subject: Re: Interface reference vs. Object reference Hello, Petter! You wrote on Thu, 16 Oct 2003 11:59:39 +0300: Quote: In other words it is either-or but not both??? Correct, unless you're extremely careful and know exactly what you're doing. Quote: Is it also true that I never have to call Free on an interfaced object (because everytime I try I get an Invalid Pointer Operation exception...)? .._Release is called whenever an interface reference goes out of scope, and ..Free is called whenever the reference count reaches zero, so never call either of these (or ._AddRef as well) directly. Cheers. Ignacio Back to top Joanna Carter Guest Posted: Thu Oct 16, 2003 10:09 am    Post subject: Re: Interface reference vs. Object reference Petter Holmström wrote: Quote: | In other words it is either-or but not both??? Yes Quote: | Is it also true that I never have to call Free on an interfaced object Yes Quote: One more question: In an OPF, does this mean I have to create a different interface for each business object, like this: Yes, but you might like to consider delegating IPersistable to an implementing class and aggregating that into business classes. IPersistable = interface [..] function GetState: TMyState; function GetObjectID: Longword; function GetTypeID: Longword; end; ICustomer = interface [..] function GetCustomer: string; procedure SetCustomer(Value: string); function GetCustNo: integer; procedure SetCustNo(Value: string); end; TPersistable = class(TInterfacedObject, IPersistable) private .... fController: Pointer; protected function QueryInterface(const IID: TGUID; out Obj): HResult; override; stdcall; public constructor Create(const Controller: IInterface); end; constructor TPersistable.Create(const Controller: IInterface); begin inherited Create; fController := Pointer(Controller); end; function TPersistable.QueryInterface(const IID: TGUID; out Obj): HResult; begin if IsEqualGUID(IID, IPersistable) then begin IPersistable(Obj) := self; Result := S_OK; end else Result := IInterface(fController).QueryInterface(IID, Obj); end; TCustomer = class(TInterfacedObject, ICustomer, IPersistable) private // ICustomer stuff // IPersistable fPersistable: IPersistable; property Persistable: IPersistable read fPersistable implements IPersistable; public constructor Create; end; constructor TCustomer.Create; begin inherited Create; fPersistable := TPersistable.Create; end; This way, you will only write the IPersistable implementation once. The delegation will mean you have a ICustomar reference that can be cast to a IPersistable and the 'controller' means that you cast the other way. Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top DPi Guest Posted: Thu Oct 16, 2003 12:48 pm    Post subject: Re: Interface reference vs. Object reference Oh. We should mention here that Interfaces can eiter use or not reference counting. In the case WITH reference counting You are right. But in other case interfaces have a little different meaning for object oriented design. That was what i wrote about in my earlier post. Also W/O reference counting You can use both referencing types, and have great object oriented design in your apps (just use conventional Freeing). Darius Lithuania "Joanna Carter" <joannac (AT) btinternet (DOT) com> wrote Quote: If youo want to retain your sanity and not have to search for memory leaks or access violations, then you should never mix interface and object references in Delphi. Back to top John Elrick Guest Posted: Thu Oct 16, 2003 1:41 pm    Post subject: Re: Interface reference vs. Object reference "DPi" <DPi (AT) TAkasLT (DOT) LT> wrote Quote: Oh. We should mention here that Interfaces can eiter use or not reference counting. In the case WITH reference counting You are right. But in other case interfaces have a little different meaning for object oriented design. That was what i wrote about in my earlier post. Also W/O reference counting You can use both referencing types, and have great object oriented design in your apps (just use conventional Freeing). Not that easy. You can get very hard to detect random errors by mixing Interfaces and Objects...even if they are not reference counted. The problem is that _Release is called on the interface reference when it is nilled or goes out of scope...even if the underlying object has already been destroyed. The short answer is that, if you mix them ALWAYS nil the interface reference yourself, never let the compiler handle it, or it may do so on an invalid address that has already been destroyed. John Back to top DPi Guest Posted: Thu Oct 16, 2003 2:16 pm    Post subject: Re: Interface reference vs. Object reference Do I need to Nil them if I'm using approach as said in Help "you can implement the IInterface _AddRef and _Release methods as empty functions to bypass the interface reference counting mechanism"? I think - no. "John Elrick" <jelrick (AT) adelphia (DOT) net> wrote Quote: The short answer is that, if you mix them ALWAYS nil the interface reference yourself, never let the compiler handle it, or it may do so on an invalid address that has already been destroyed. Back to top Franz-Leo Chomse Guest Posted: Thu Oct 16, 2003 3:02 pm    Post subject: Re: Interface reference vs. Object reference Quote: I think - no. The answer is wrong. The problem is that you can't control the position in a method were the compiler adds its cleanup code. If it is run AFTER you have freed the implementing object by your code you are in trouble. Regards from Germany Franz-Leo Back to top John Elrick Guest Posted: Thu Oct 16, 2003 3:44 pm    Post subject: Re: Interface reference vs. Object reference "DPi" <DPi (AT) TAkasLT (DOT) LT> wrote Quote: Do I need to Nil them if I'm using approach as said in Help "you can implement the IInterface _AddRef and _Release methods as empty functions to bypass the interface reference counting mechanism"? I think - no. You think...incorrectly. The problem is that the interface left to its own devices, may go out of scope _after_ the object is destroyed. Going out of scope calls a _Release which will cause an Invalid Pointer Operation if the object has been destroyed. John Back to top mike Guest Posted: Thu Oct 16, 2003 6:29 pm    Post subject: Re: Interface reference vs. Object reference Joanna Carter wrote: Quote: TPersistable = class(TInterfacedObject, IPersistable) private ... fController: Pointer; protected function QueryInterface(const IID: TGUID; out Obj): HResult; override; stdcall; public constructor Create(const Controller: IInterface); end; QueryInterface function (and neither _AddRef nor _Release, for that matter) is not declared as virtual in TInterfacedObject, so you can't override it. At least, in D7 Pro. Mike. Back to top Joanna Carter Guest Posted: Thu Oct 16, 2003 7:36 pm    Post subject: Re: Interface reference vs. Object reference mike wrote: Quote: QueryInterface function (and neither _AddRef nor _Release, for that matter) is not declared as virtual in TInterfacedObject, so you can't override it. At least, in D7 Pro. Sorry, I just copied TInterfacedPersistentfrom Classes.pas and changed the virtual to override; not something I would normally do, but I didn't take long enough to check. :-) Joanna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top DPi Guest Posted: Fri Oct 17, 2003 6:25 am    Post subject: Re: Interface reference vs. Object reference I don't understand You guys... I have'nt it tested specialy in Delphi, but I think it works like in C++: you can call functions (methods) of objects because they are not in the same place as (deleted) object. There is only one instance of each function in different place. And You'll get exception ONLY if you try to access objects internal data, which woul'd be deleted. In case I rewrite _Release as empty function, I won't get any exceptions. Simply try it : to call function on deleted object like function TFooClass.Add(a: Integer; b: Integer): integer; begin Result := a + b; end; I'll try it too. "John Elrick" <jelrick (AT) adelphia (DOT) net> wrote Quote: You think...incorrectly. The problem is that the interface left to its own devices, may go out of scope _after_ the object is destroyed. Going out of scope calls a _Release which will cause an Invalid Pointer Operation if the object has been destroyed. John Back to top Joanna Carter Guest Posted: Fri Oct 17, 2003 7:13 am    Post subject: Re: Interface reference vs. Object reference DPi wrote: Quote: I don't understand You guys... I have'nt it tested specialy in Delphi, but I think it works like in C++: Delphi is not the same as C++ when it comes to object lifetime management, if you haven't tried it, then don't give advice for Delphi based on C++ knowledge. C++ has garbage collection, Delphi does not. Joannna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top DPi Guest Posted: Fri Oct 17, 2003 8:12 am    Post subject: Re: Interface reference vs. Object reference Oh my god. Don't You really have any idea about memory management? Not even in Delphi? At first: C++ does'nt have garbage collection - same as Delphi. Second: (same as in C++) You can call functions of freed objects unless they are accessing internal object data. See my earlier example. Although that is not a proper techniqe and may lead to unreadable code. As I mentioned earlier (why do I need to teach You again?) functions are addressed differently than objects and are located not depending of any particular object address. So feel safe when calling empty functions (like _Release) on deleted objects. Darius, M 24 Lithuania "Joanna Carter" <joannac (AT) btinternet (DOT) com> wrote Quote: C++ has garbage collection, Delphi does not. Joannna -- Joanna Carter Consultant Software Engineer TeamBUG support for UK-BUG TeamMM support for ModelMaker Back to top Joanna Carter Guest Posted: Fri Oct 17, 2003 9:44 am    Post subject: Re: Interface reference vs. Object reference DPi wrote: Quote: Don't You really have any idea about memory management? More than you seem to assume. We have a saying here: 'Please don't tell you grandmother how to suck eggs' :-) Quote: At first: C++ does'nt have garbage collection - same as Delphi. Then you had better tell Bjarne Stroustroup, the creator of C++, because he states that certain implementations have GC in his book on C++. Quote: Second: (same as in C++) You can call functions of freed objects unless they are accessing internal object data. See my earlier example. Although that is not a proper techniqe and may lead to unreadable code. That has been discussed many times in these groups and is always advised against. Such practices will not port to .NET Quote: As I mentioned earlier (why do I need to teach You again?) functions are addressed differently than objects and are located not depending of any particular object address. So feel safe when calling empty functions (like _Release) on deleted objects. If you leave empty methods for _AddRef and _Release in Delphi and only instantiate the object into an interface reference, Destroy will never be called and the object will never be returned to the heap, thus creating a memory leak. If you do not create an object reference before obtaining an interface reference, there is no way to free the allocation because you can't cast an interface to an object to circumvent the lack of assignment of the original allocation to an object reference. Then you have the sceanario where C# has and Delphi for.NET will have garbage collection and now you have to go through all your non-standard code and conditionally compile all your _AddRef and _Release methods because: 1. they are not needed, and 2. stdcall methods are not allowed in .NET languages. 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