How to define class with self-referencing generics constraint in Delphi?

Question

As shown in this article, one can define in C#:

public class MyTreeNode<T> where T: MyTreeNode<T>
{

}

In Delphi, however, the following code does not compile and complains "E2003 Undeclared identifier: 'MyTreeNode<>'":

type
    TMyTreeNode<T: TMyTreeNode<T>> = class
    end;

The formal terminology seems to be "self-referencing generics constraint". I wonder how to do this in Delphi ?

PS:

Another useful article as David pointed out.

Relevent SO post regarding Delphi and covariance / contravariance.

Wikipedia page of co/contra-variance.

You just cannot do that with Delphi generic constraints

David Heffernan
– David Heffernan

2014-04-23 13:37:05 +00:00
Commented Apr 23, 2014 at 13:37 — David Heffernan
– David Heffernan, Commented Apr 23, 2014 at 13:37

David Heffernan · Accepted Answer · 2014-04-23 13:54:27Z

2

There is no way in Delphi to express such a generic constraint. Furthermore, I believe that there is no typesafe way to achieve what can be achieved in C# with such a constraint.

answered Apr 23, 2014 at 13:54

David Heffernan

616k46 gold badges1.1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

SOUser Over a year ago

Thank you for your comment ! Could you help to explain more about "there is no typesafe way to achieve what can be achieved in C# with such a constraint" ?

David Heffernan Over a year ago

You are using generics because you want type safety. You can implement the container without type safety by abandoning generics.

SOUser Over a year ago

Thank you for your comment ! Could you help to show an example regarding "You can implement the container without type safety by abandoning generics." ?

David Heffernan Over a year ago

I suppose that you simply remove the constraint and instead add runtime type checking whenever an element is added to the container.

Collectives™ on Stack Overflow

How to define class with self-referencing generics constraint in Delphi?

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related