BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  A virtual class method was overriden? Goto page 1, 2  Next          BorlandTalk.com Forum Index -> Delphi OO design View previous topic :: View next topic   Author Message Joao Morais Guest Posted: Fri Feb 09, 2007 6:41 pm    Post subject: A virtual class method was overriden? Hello, I need to verify if a virtual method was overriden, something like this: TBaseClassClass = class of TBaseClass; TBaseClass = class class function Metadata: string; virtual; end; TImplementClass = class(TBaseClass) class function Metadata: string; override; end; TNonimplementClass = class(TBaseClass) end; .... method(AClass: TBaseClassClass); begin if not (AClass has your _own_ Metadata implementation) then ... A virtual class that returns an empty string isn't enough because: TBaseClass = class Metadata <<- returns '' TSomeClass = class(TBaseClass) Metadata <<- returns 'some metadata' TProblematicClass = class(TSomeClass) no metadata implementation, so TSomeClass.Metadata would be called. If I'd implement the method in every class I'd solve the problem, I am just trying to check if such method was really implemented. Hints? Thanks, -- Joao Morais Back to top Ivo Bauer Guest Posted: Fri Feb 09, 2007 7:08 pm    Post subject: Re: A virtual class method was overriden? Hi Joao, Joao Morais napsal(a): Quote: A virtual class that returns an empty string isn't enough because: TBaseClass = class Metadata <<- returns '' TSomeClass = class(TBaseClass) Metadata <<- returns 'some metadata' TProblematicClass = class(TSomeClass) no metadata implementation, so TSomeClass.Metadata would be called. If I'd implement the method in every class I'd solve the problem, I am just trying to check if such method was really implemented. Well, I'm sure you already knew that, but one can force *every* descendant of a given ancestor class to implement a virtual method if it's declared as abstract. Given your example class hierarchy listed above, if TSomeClass overrides TBaseClass.GetMetadata abstract method, there is however no way (at least that I know of) to force TProblematicClass to provide its own implementation of this method. If you want to be (for any reason) sure that TProblematicClass provides its own implementation of GetMetadata method, make it descend directly from TBaseClass. -- Ivo Bauer [OZM Research] Back to top Ivo Bauer Guest Posted: Fri Feb 09, 2007 7:21 pm    Post subject: Re: A virtual class method was overriden? Ivo Bauer napsal(a): Quote: If you want to be (for any reason) sure that TProblematicClass provides its own implementation of GetMetadata method, make it descend directly from TBaseClass. But if it makes more sense to descend TProblematicClass from TSomeClass rather than from TBaseClass, then I can't see anything inherently wrong with the fact that the inherited implementation of TSomeClass.GetMetadata method would be actually called if this method is not overridden in TProblematicClass. It's just the way how it works, or am I missing anything? -- Ivo Bauer [OZM Research] Back to top Joao Morais Guest Posted: Fri Feb 09, 2007 8:06 pm    Post subject: Re: A virtual class method was overriden? Ivo Bauer wrote: Quote: but one can force *every* descendant of a given ancestor class to implement a virtual method if it's declared as abstract. But you will be able to compile. Anyway this is valid only to the first descendant class, not to the descendant of the descendant. Quote: If you want to be (for any reason) sure that TProblematicClass provides its own implementation of GetMetadata method, make it descend directly from TBaseClass. I cannot do that. This was a sample, the real TProblematicClass need to inherits the TSomeClass stuff. Quote: But if it makes more sense to descend TProblematicClass from TSomeClass rather than from TBaseClass, then I can't see anything inherently wrong with the fact that the inherited implementation of TSomeClass.GetMetadata method would be actually called if this method is not overridden in TProblematicClass. It's just the way how it works, Right, my implementation is an exception. Quote: or am I missing anything? Any metadata string is related with only one class. No parents, no descendants, only that class. Something like RTTI. The metadata method describes what that class implements, if it implements something. If not, it should return '' instead of the parent's metadata. This is part of a framework, I am just trying to work around some users' omissions. Thanks, -- Joao Morais Back to top Ivo Bauer Guest Posted: Fri Feb 09, 2007 9:00 pm    Post subject: Re: A virtual class method was overriden? Joao Morais napsal(a): Quote: Ivo Bauer wrote: but one can force *every* descendant of a given ancestor class to implement a virtual method if it's declared as abstract. But you will be able to compile. Sure, it would not cause compiler to emit an error but as soon as the runtime code calls that method on instance of class that does not provide actual implementation, EAbstractError exception will be raised. Quote: Anyway this is valid only to the first descendant class, not to the descendant of the descendant. Actually this is valid only for the first descendant class that provides its own implementation of that method (even empty). It does not necessarily be the first/direct descendant. But I see your point. I should have expressed myself more clearly. Quote: Any metadata string is related with only one class. No parents, no descendants, only that class. Something like RTTI. The metadata method describes what that class implements, if it implements something. If not, it should return '' instead of the parent's metadata. This is part of a framework, I am just trying to work around some users' omissions. OK. What about stripping this metadata stuff off your TBaseClass altogether and implement some kind of (global) metadata registration system: type TBaseClass = class of TMyBaseObject; procedure RegisterClassMetadata(AClass: TBaseClass; const AMetaData: string); procedure UnregisterClassMetadata(AClass: TBaseClass); function FindClassMetadata(AClass: TBaseClass): string; There could be a global list of TClassMetadataItem instances holding information about all registered classes, but this list will not be accessible outside the unit where it would be declared. Instead it will be manipulated by the above mentioned helper methods (RegisterClassMetadata, ...): type TClassMetadataItem = class public ClassRef: TBaseClass; Metadata: string; end; Every new (direct or indirect, it doesn't matter) descendant of TBaseClass should be registered into this system. In your code you can then easily check if a given class has been registered by calling FindClassMetadata function which would return EmptyStr if it could not find requested information. If that happens, simply raise an exception indicating that the metadata info was not registered for given class and it will be the responsibility of the one who developed such class to take care of the registration/unregistration. You can even make these global routines a class methods of TBaseClass: TBaseClass = class public class procedure RegisterClassMetadata(const AMetaData: string); class procedure UnregisterClassMetadata; class function FindClassMetadata(AClass: TBaseClass): string; end; initialization TMyProblematicClass.RegisterClassMetadata('my metadata'); finalization TMyProblematicClass.UnregisterClassMetadata; What do you think? -- Ivo Bauer [OZM Research] Back to top Peter Morris Guest Posted: Fri Feb 09, 2007 10:02 pm    Post subject: Re: A virtual class method was overriden? Quote: Sure, it would not cause compiler to emit an error When you create an instance of a class containing abstract methods the compiler does emit a warning though. -- Pete Blessed are the geek, for they shall public class GeekEarth : Earth {} ==== Audio compression components, DIB graphics controls, ECO extensions, FastStrings : http://www.droopyeyes.com My blog : http://mrpmorris.blogspot.com Back to top Ivo Bauer Guest Posted: Fri Feb 09, 2007 10:12 pm    Post subject: Re: A virtual class method was overriden? Peter Morris napsal(a): Quote: Sure, it would not cause compiler to emit an error When you create an instance of a class containing abstract methods the compiler does emit a warning though. Correct, but IINM the compiler cannot detect cases when such class instance is created via class reference, e.g.: type TMyBaseClass = class of TMyBaseObject; TMyBaseClass = class end; TMyConcreteClass = class(TMyBaseClass); function CreateMyClassInstance(AClass: TMyBaseClass): TMyBaseClass; begin Result := AClass.Create; end; -- Ivo Bauer [OZM Research] Back to top Joao Morais Guest Posted: Fri Feb 09, 2007 10:21 pm    Post subject: Re: A virtual class method was overriden? Ivo Bauer wrote: Quote: Anyway this is valid only to the first descendant class, not to the descendant of the descendant. Actually this is valid only for the first descendant class that provides its own implementation of that method (even empty). It does not necessarily be the first/direct descendant. But I see your point. I should have expressed myself more clearly. Yup. It really wasn't what I said but it was what I meant. Quote: Any metadata string is related with only one class. No parents, no descendants, only that class. Something like RTTI. The metadata method describes what that class implements, if it implements something. If not, it should return '' instead of the parent's metadata. This is part of a framework, I am just trying to work around some users' omissions. OK. What about stripping this metadata stuff off your TBaseClass altogether and implement some kind of (global) metadata registration system: /snip/ What do you think? This is exactly what my first approach does and it is available yet. The problem is that the metadata string is implemented far from the rest of the class stuff -- ie a bit more difficult code to maintain when you prefer to don't use the expert. Because of that I implemented the virtual class approach as well. It is the perfect option for me, provided that the metadata method is implemented in *all* classes. -- Joao Morais Back to top Peter Morris Guest Posted: Fri Feb 09, 2007 10:41 pm    Post subject: Re: A virtual class method was overriden? Quote: Correct, but IINM the compiler cannot detect cases when such class instance is created via class reference, e.g.: type TMyBaseClass = class of TMyBaseObject; TMyBaseClass = class end; TMyConcreteClass = class(TMyBaseClass); function CreateMyClassInstance(AClass: TMyBaseClass): TMyBaseClass; begin Result := AClass.Create; end; Probably not, no. If that is what he is doing then that'll teach me for not bothering to read the post properly Back to top Les Pawelczyk Guest Posted: Sat Feb 10, 2007 12:57 am    Post subject: Re: A virtual class method was overriden? Quote: method(AClass: TBaseClassClass); begin if not (AClass has your _own_ Metadata implementation) then procedure Test(AClass: TBaseClassClass); var lfvM: function: string of object; begin lfvM := AClass.Metadata; if TMethod(lfvM).Code = Addr(TBaseClass.Metadata) then ShowMessage('not implemented') end; Les. Back to top Peter Below (TeamB) Guest Posted: Sat Feb 10, 2007 1:38 am    Post subject: Re: A virtual class method was overriden? Joao Morais wrote: Quote: Hello, I need to verify if a virtual method was overriden, something like this: TBaseClassClass = class of TBaseClass; TBaseClass = class class function Metadata: string; virtual; end; TImplementClass = class(TBaseClass) class function Metadata: string; override; end; TNonimplementClass = class(TBaseClass) end; ... method(AClass: TBaseClassClass); begin if not (AClass has your own Metadata implementation) then ... A virtual class that returns an empty string isn't enough because: TBaseClass = class Metadata <<- returns '' TSomeClass = class(TBaseClass) Metadata <<- returns 'some metadata' TProblematicClass = class(TSomeClass) no metadata implementation, so TSomeClass.Metadata would be called. If I'd implement the method in every class I'd solve the problem, I am just trying to check if such method was really implemented. Display('TBaseclass.Metadata: %p', [@TBaseclass.Metadata]); Display('TImplementclass.Metadata: %p', [@TImplementClass.Metadata]); Display('TNonImplementclass.Metadata: %p', [@TNonImplementClass.Metadata]); Shows: TBaseclass.Metadata: 0045EC34 TImplementclass.Metadata: 0045EC44 TNonImplementclass.Metadata: 0045EC34 The problem is that this scheme only works with explicite classnames, since the compiler needs to be able to resolve the address at compile time. If you have a class reference in a variable typed as the base classes class the compiler will refuse to evaluate an expression of the the form @classvar.Metadata. But you can work around that using a method pointer type. type TBaseClass = class class function Metadata: string; virtual; end; TBaseCLassCLass = class of tbaseclass; TProc = function : string of object; TImplementClass = class(TBaseClass) class function Metadata: string; override; end; TNonimplementClass = class(TBaseClass) end; procedure TForm1.Button1Click(Sender: TObject); var bcc: TBaseclassclass; proc: TProc; begin Display('TBaseclass.Metadata: %p', [@TBaseclass.Metadata]); Display('TImplementclass.Metadata: %p', [@TImplementClass.Metadata]); Display('TNonImplementclass.Metadata: %p', [@TNonImplementClass.Metadata]); bcc := TImplementclass; proc := bcc.Metadata; Display('bcc.Metadata: %p', [@proc]); end; That gives TBaseclass.Metadata: 0045ED38 TImplementclass.Metadata: 0045ED48 TNonImplementclass.Metadata: 0045ED38 bcc.Metadata: 0045ED48 So, the solution to your problem is to construct a method pointer for TBaseclass.Metadata, another for the class you want to examines (even if it is held in a class reference var), and then compare the two pointers. If they are equal the class at hand does not override the method, if they are different the method was overriden. -- Peter Below (TeamB) Don't be a vampire (http://slash7.com/pages/vampires), use the newsgroup archives : http://www.tamaracka.com/search.htm http://groups.google.com http://www.prolix.be Back to top Joao Morais Guest Posted: Sat Feb 10, 2007 2:28 am    Post subject: Re: A virtual class method was overriden? Les Pawelczyk wrote: Quote: method(AClass: TBaseClassClass); begin if not (AClass has your _own_ Metadata implementation) then procedure Test(AClass: TBaseClassClass); var lfvM: function: string of object; begin lfvM := AClass.Metadata; if TMethod(lfvM).Code = Addr(TBaseClass.Metadata) then ShowMessage('not implemented') end; Perhaps this another piece of code will give what I need: function ImplementsMetadataMethod(AClass: TBaseClassClass): Boolean; var vm1, vm2: function: string of object; begin if AClass <> TBaseClass then begin vm1 := AClass.Metadata; vm2 := TBaseClassClass(AClass.ClassParent).Metadata; Result := TMethod(vm1).Code <> TMethod(vm2).Code; end else Result := True; end; You and Peter Below solved the trick, thank you very much! -- Joao Morais Back to top Joao Morais Guest Posted: Sat Feb 10, 2007 2:30 am    Post subject: Re: A virtual class method was overriden? Peter Below (TeamB) wrote: Quote: So, the solution to your problem is to construct a method pointer for TBaseclass.Metadata, another for the class you want to examines (even if it is held in a class reference var), and then compare the two pointers. If they are equal the class at hand does not override the method, if they are different the method was overriden. Your approach is very close to the Les' one, comments are there. Thank you very much for your comments and tests. -- Joao Morais Back to top David M Guest Posted: Sat Feb 10, 2007 9:13 am    Post subject: Re: A virtual class method was overriden? Hi Joao Quote: TBaseClassClass = class of TBaseClass; TBaseClass = class class function Metadata: string; virtual; end; TImplementClass = class(TBaseClass) class function Metadata: string; override; end; TNonimplementClass = class(TBaseClass) end; If your function is published and your class emits RTTI (e.g. compiled with M+ or descends from TPersistent) then you will have a published MethodTable in the class's VMT at the vmtMethodTable offset. You can scan through this to see if the method is included or not. Take a look at the GetMethodInfo to see how to proceed. All other static and virtual methods (whether overridden or not) will appear in the class's VMT method listing so you can't check this to determine if it has been overridden. Another option is to declare the base method as dynamic and then check the dynamic method table. Beware of the performance implications of dispatching dynamic methods though. HTH D David Moorhouse Moorhouse Works ltd www.moorhouse.co.nz Back to top Ivo Bauer Guest Posted: Sat Feb 10, 2007 8:43 pm    Post subject: Re: A virtual class method was overriden? Joao Morais napsal(a): Quote: This is exactly what my first approach does and it is available yet. The problem is that the metadata string is implemented far from the rest of the class stuff -- ie a bit more difficult code to maintain when you prefer to don't use the expert. Because of that I implemented the virtual class approach as well. It is the perfect option for me, provided that the metadata method is implemented in *all* classes. I know that you already have the solution you were asking for but I'm still in the wondering mode myself. Could you please reveal some more details about those metadata and what they're used for in your framework? I don't know for sure, but it seems to me that from the design point of view they have no relation to the base class (and their descendants) themselves but rather to the way how these classes are treated and managed by your framework. If this is the case then it might be a good idea to separate the metadata handling from the class. But I may be wrong, of course. :-) Any ideas/comments? -- Ivo Bauer [OZM Research] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         BorlandTalk.com Forum Index -> Delphi OO design All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C++ Builder Newsgroups----------------C++ BuilderC++ Builder (ActiveX)C++ Builder (Announcements)C++ Builder (Command Line Tools)C++ Builder DatabasesC++ Builder Databases (ADO)C++ Builder Databases (Desktop)C++ Builder Databases (InterBase Express)C++ Builder Databases (Multi-Tier)C++ Builder Databases (SQL Servers)C++ Builder (Graphics)C++ Builder (IDE)C++ Builder (Internet)C++ Builder (Internet Socket)C++ Builder (Internet Web)C++ Builder (Jobs)C++ Builder (Language)C++ Builder (Language C++)C++ Builder (Compatibility with MFC)C++ Builder (Native API)C++ Builder (Non-Technical)C++ Builder (Students)C++ Builder (Third Party Tools)C++ Builder (Upgrade)C++ Builder (VCL)C++ Builder (VCL Components Usage)C++ Builder (VCL Components Development)C++ Builder (VCL Usage)C++ Builder (WinAPI)C++ Builder Installation Delphi Newsgroups----------------Delphi (General)Delphi ActiveX Controls UsageDelphi ActiveX Controls DevelopmentDelphi As400Delphi DatabasesDelphi Databases (ADO)Delphi Databases (dbExpress)Delphi Databases (Desktop)Delphi Databases (InterBase Express)Delphi Databases (Multi-Tier)Delphi Databases (SQL Servers)Delphi DeploymentDelphi GraphicsDelphi IDEDelphi InternationalizationDelphi InternetDelphi Internet Isapi-WebBrokerDelphi Internet WebSnapDelphi Internet WinsockDelphi JobsDelphi Language BASMDelphi Language ObjectPascalDelphi Model Driven Architecture (General)Delphi Model Driven Architecture (Third Party)Delphi MultimediaDelphi Native APIDelphi Non-TechnicalDelphi ObjectPascalDelphi OLE AutomationDelphi OO designDelphi OpenToolsAPIDelphi Reporting-ChartingDelphi RTLDelphi StudentsDelphi TeamSourceDelphi Thirdparty ToolsDelphi Thirdparty Tools (General)Delphi Thirdparty Tools (IntraWeb)Delphi Thirdparty Tools (RaveReports)Delphi UpgradeDelphi VCLDelphi VCL ComponentsDelphi VCL Components UsingDelphi VCL Components WritingDelphi WebServices SOAPDelphi WinAPIDelphi XMLDelphi Installationcomp.lang.pascal.delphi.components.misccomp.lang.pascal.delphi.components.usagecomp.lang.pascal.delphi.components.writingcomp.lang.pascal.delphi.databasescomp.lang.pascal.delphi.misccomp.sources.delphi JBuilder Newsgroups----------------JBuilderJBuilder AnnouncementsJBuilder Applet IssuesJBuilder As400JBuilder CompilerJBuilder CORBA/RMIJBuilder DatabasesJBuilder DB SwingJBuilder DebuggerJBuilder DeploymentJBuilder DocumentationJBuilder EnterpriseJBuilder HandheldJBuilder IDEJBuilder JavaJBuilder JavaAPIJBuilder Java LanguageJBuilder Java SwingJBuilder Java BeansJBuilder Java Beans UsageJBuilder Java Beans DevelopmentJBuilder JBCLJBuilder JDataStoreJBuilder JobsJBuilder LinuxJBuilder MacintoshJBuilder & MobileSetJBuilder MultiLingual AppsJBuilder Non-TechnicalJBuilder OpenToolsJBuilder Servlets (JSP)JBuilder StudentsJBuilder Team DevelopmentJBuilder Thirdparty ToolsJBuilder UpgradeJBuilder Installation InterBase Newsgroups----------------InterBaseInterBase Bug DiscussionInterBase GeneralInterBase IBConsoleInterBase OpenSourceInterBase ReplicationInterBase SQLInterBase ThirdpartyInterBase Installation Miscellaneous Borland Newsgroups----------------AppCenterAppServerEnteraVisiBroker  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.