Question

I’m not sure if this is possible in Delphi. I’ve looked around and can’t seem to find an answer (Example or inidaction that it’s not possible):

I have a generic list class, and I want to create an instance of the generic type. For example:

type
  TTypeA = class(TObject);

procedure Test;
var
  MyList: TobjectList<TTypeA>;
  NewListObject: TTypeA;
begin
  MyList := TObjectList<TTypeA>.Create;
  NewListObject := MyList.xxx //what to put on the xxx
end;

Is it possible to create a function xxx that creates a new object of the type TTypeA?

@jeroen: thanks for the answer below. However, I forgot an important detail in my question:

I would like this code to work for any other type as well, so without prior knowledge about the type T for TObjectList. I might create the following lists:

MyList: TObjectList<TCar>;
MyList: TObjectList<TBike>;

Without knowing if MyList contains TCar or TBike (both derived from the same base class and equal constructors) I want to add a new item to MyList.

And with the suggestion from Uwe Raabe I run into the next problem:

I modified my class to

TMyObjectList<T:class, constructor> = class(TMyBaseObjectList<T>)

where TMyBaseObjectList is defined as

TMyBaseObjectList<T:TMyBaseObject> = class(TObjectList)

Now I get an error:
Type parameter ‘T’ is not compatible with type ‘T:TMyBaseObject’

Answer 1

As you know the type, why don’t you just write

NewListObject := TTypeA.Create;
MyList.Add(NewListObject);

It would be the natural way to do here. (I’m just notified by Jeroen’s similar answer)

If you want the container create the object, you have to make a descendant class that knows a little bit more about the generic type to create an instance. The constructor constraint might help here.

type
  TMyObjectList<T:class, constructor> = class(TObjectList<T>)
  public
    function NewObject: T;
  end;

function TMyObjectList<T>.NewObject: T;
begin
  result := T.Create;
end;

Note: This will only work when the actual type has a parameterless constructor named Create.

Update: This will help

  TMyObjectList<T:constructor, TMyBaseObject> = class(TObjectList<T>)
  public
    function NewObj: T;
  end;