Class reference from Generic parameter class

4
votes

I have a persistence framework, and I am trying to use generics so I don't have to keep creating new list classes for each type of object I want to store in a type safe way.

I have a method that returns the class of the contained object in the list class (so I know which queries to run and which object to create.

As an example, it looks something like this:

type

  TMyObject = class

  end;
  TMyObjectClass = class of TMyObject;


  TMyObjectList = class
  public
    function ListClass: TMyObjectClass; virtual; abstract;

  end;

  TMyObjectList<T: TMyObject, constructor> = class(TMyObjectList)
  public
    function ListClass: TMyObjectClass; override;

  end;

implementation

{ TMyObjectList<T> }

function TMyObjectList<T>.ListClass: TMyObjectClass;
begin
  result := T;  //  <==== this wont compile
end;

end.

Is there a way of returning the class of the generic parameter in this case?

Thanks

N@ (using Delphi 2009)

2
delphigenericsdelphi-2009

2 Answers

6
votes

This is a known issue in Delphi 2009. It's been fixed in 2010. I just tested it and your code compiles just fine there.

0
votes

T is not an instance of an object.

In your specific example, you should write something like:

result := self;

I think you're looking the wrong way...