Class reference from Generic parameter class

Question

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)

Mason Wheeler · Accepted Answer · 2009-09-15 05:22:19Z

6

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.

answered Sep 15, 2009 at 5:22

Mason Wheeler

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2 Comments

Nat Over a year ago

You. Are. A. Legend. Thanks! Can't wait to get 2010 (should be tomorrow, yay!).

Mason Wheeler Over a year ago

Cool! The enhanced RTTI will make persistence a lot easier, BTW. :)

vIceBerg · Accepted Answer · 2009-09-15 05:16:06Z

0

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...

answered Sep 15, 2009 at 5:16

vIceBerg

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2 Comments

Nat Over a year ago

I would like to return a reference to the class not a reference to an instantiated object.

Rob Kennedy Over a year ago

No, T is not an instance of an object. T is a class. You're confused because the constraint "T: TMyObject" looks like a variable declaration where T would be a TMyObject reference. Generics mean that classes now how type parameters in the same way that functions have value parameters.

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