The type of your specialised class is set at compile time. However you can have multiple specialisations for a single generic class.
One of the obvious uses for generics is lists and container classes. E.g. in the Generics.Collections unit in D2009 there are a bunch of classes such as TList<T> = class(TEnumerable<T>) TQueue<T> = class(TEnumerable<T>) Etc You can create a specialised list(s) simply by going: var MyList: TList<TMyClass>; MyOtherList: TList<TMyOtherClass>; MyYetOtherList: TList<TMyYetOtherClass>; MyList now has all the methods of a list, specialised to TMyClass. Ie Add will only take a TMyClass, Items will only take and return a TMyClass, enumerators will use TMyClass ... All this happens without any required typecasting. I use this with the tiOPF object persistence framework. In tiopf, a complete container class takes about 100 loc for basic functionality. This can be replaced with a single line of code using generics. I have 13 such containers in my project. Regards Sean Cross CIO Catalyst Risk Management -----Original Message----- From: delphi-boun...@elists.org [mailto:delphi-boun...@elists.org] On Behalf Of Robert Meek Sent: Sunday, 26 April 2009 4:44 p.m. To: delphi@elists.org Subject: Generics in D2009 Even though I had done some limited work using Oxygene, ( now Delphi Prism ), I never got into or even tried to make use of Generics. There was too much to learn in .NET already so I was saving them for after I became comfortable with it. Now however, and since Delphi has gone to a lot of trouble to include them, I figured it was time to take a good look at them. Once again though, Cantu's D2009 handbook and his theoretical approach to Generics has left me a little cold. It's easy enough to create and instantiate a Generic Class all right, but at the same time, the mere fact that once a Generic Type has been set for that class you cannot alter it leaves me wondering exactly when and why I would put this language feature to practical use...if of course, that is truly how Generics work! In this example: type TSampleClass <T> = class private data: T; public function GetDataSize: Integer; function GetDataName: string; end; function TSampleClass<T>.GetDataSize: Integer; begin Result := SizeOf (T); end; function TSampleClass<T>.GetDataName: string; begin Result := GetTypeName (TypeInfo (T)); end; end; Am I correct in assuming that 'data : T;' cannot be altered to a different type once the class is created and a value/Type is given to 'T', because that is how it came across to me despite it not seeming to make much sense! If such is the case why would I even bother using a Generic Class? If on the other hand, I created an instance of the class and gave 'T' a value of '0' or 'nil', depending upon the type of data I wish to pass it, then I could write a procedure that first sets the Value/Type of 'T' to any I want to at the moment, and then call the three class functions from within, it would be possible for me to reset 'T' to a different Value/Type each time I call this procedure! THAT I can see as being very useful! Which of these is true? I'm hoping I missed something along the way and the latter description of a Generic Class's use is correct or at least closer to the truth! from "Robert Meek" dba "Tangentals Design" Creative Concepts Programming for Windows Vista E-mail: ffo...@comcast.net "Most people would sooner die than think; in fact, they do so!" [Bertrand Russell 1872-1970] _______________________________________________ Delphi mailing list -> Delphi@elists.org http://lists.elists.org/cgi-bin/mailman/listinfo/delphi _______________________________________________ Delphi mailing list -> Delphi@elists.org http://lists.elists.org/cgi-bin/mailman/listinfo/delphi
