You could take advantage of polymorphism. Define an abstract method in the abstract base class which returns an instance of the abstract base class. Then define a method in each concrete descendant which instantiates a new instance of that type & copies all pertinent elements to the new instance which is then returned.
Example: function TAbstractClass.Clone : TAbstractClass; abstract; virtual; function TConcreteClass1.Clone : TAbstractClass; override; begin // TConcreteClass1 is aware of all interesting elements // and responsible for copying itself. Result := TConcreteClass1.Create; Result.ElementA := self.ElementA; Result.ElementB := self.ElementB; Result.ElementC := self.ElementC; end; function TConcreteClass2.Clone : TAbstractClass; override; begin // TConcreteClass2 is aware of all interesting elements // and responsible for copying itself. Result := TConcreteClass2.Create; Result.ElementA := self.ElementA; Result.ElementE := self.ElementE; Result.ElementG := self.ElementG; end; Called like: // Compiler will know correct type. NewInstance := InstanceToCopy.Clone; I hope I didn't make a stupid error, I typed it off the top of my head. Hope this helps, Roger C. Morgan II --- In [email protected], Threetwosevensixseven <[EMAIL PROTECTED]> wrote: > I have a set of concrete classes which all inherit from an abstract > class. The client code which references them only knows them as > instances of the abstract class (I have a parameterised constructor > method, which is working fine). > > The instances all contain elements which can contain primitive types > or else contain other elements. This follows the Composite design > pattern. The instances hide all details of these elements from the > client code. > > The client code needs make a deep copy of an instance, which will > involve copying all the elements and their child elements, even > though the client doesn't know about these. (Ultimately, it also will > also need to 'add' two instances together to form a third instance, > which will involve 'adding' all the child elements together, but this > would follow the same principle as the deep copy). > > To achieve this, I though it would be straightforward to pass the > instance to be copied to a class method in the abstract class. This > would determine the actual class and create a new instance of that > class. It should then be possible to pass the new instance to a > method in the passed instance. This method would copy its child > elements, and tell each of its children to do the same. These copy > methods could all be protected methods of the abstract classes, > meaning they would be visible to the cloning class method and the > instances themselves, but not to the client code, which would only > need to call the public cloning class method. > > I started to write it, as the code example below. Although the class > type is correctly detected, and the object is apparently created, the > constructors the abstract class or the concrete class are not called. > I have code in both of these, and both are being called if I create > the object in the client code using the parameterised constructor > method, or 'normally' using TConcreteClass.Create. > > What am I doing wrong? > > Thanks, Robin > > ------------------------ > > interface > > class function Clone(ConcreteInstance: TAbstractClass): TAbstractClass; > > implementation > > class function TAbstractClass.Clone(ConcreteInstance: > TAbstractClass): TAbstractClass; > var > ConcreteClass: TClass; > begin > > //Determine class of passed object > ConcreteClass := ConcreteInstance.ClassType; > > //Create a new object of the appropriate class - > //WHY are NEITHER constructors of concrete class > //NOR abstract class being called?! > Result := ConcreteClass.Create as TAbstractClass; > > //Tell passed object to deep copy its attributes to the > //newly created object, which will tell its children > //to do the same, and so on > ConcreteInstance.DeepCopy(Result); > > end; ----------------------------------------------------- Home page: http://groups.yahoo.com/group/delphi-en/ To unsubscribe: [EMAIL PROTECTED] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/delphi-en/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
