The Wiert Corner – irregular stream of stuff

Generics and constraints in Delphi is still a bit of pain.

A while ago, I needed a factory for threads. It was more convoluted than I meant it to be, so I had a short chat with Stefan Glienke.

He came up with an example similar to the one below. I tore it apart so you can on each line see what the compiler dislikes.

This fails, but was kind of how I wanted it to be:

type
   TFactory = record
     class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
   end;

class function Manufacture<T: TBaseClass, constructor>(parameter definitions): T;
begin
  Result := T.Create(parameter values);
end;

Which means I need to update Delphi Constraints in Generics – RAD Studio XE documentation wiki

The factory:

type
  TComponentFactory = class
    class function CreateComponent<T: TComponent>(AOwner: TComponent): T; static;
  end;

class function TComponentFactory.CreateComponent<T>(AOwner: TComponent): T;
var
  ComponentClass: TComponentClass;
  Component: TComponent;
begin
  ComponentClass := T;
  Component := ComponentClass.Create(AOwner); // you can't do `T.Create(AOwner)`
  Result := T(Component); // you can't do `Result := ComponentClass.Create(AOwner);`
end;

The usage:

var
  Component: TButton;
  Owner: TComponent;
begin
  Owner := Application.MainForm;
  Component := TComponentFactory<TButton>.CreateComponent(Owner);
  Component.Parent := Owner;
end;

Full source example is at: After a discussion with Stefan Glienke: a pattern for a factory, this one for components, but can be uses for anything having a base class like a TThread descendant.

In the mean time, Spring4D has added the [Archive.is] Spring4D: TActivator record with many CreateInstance methods that allow you to create instances of classes for which you just have type information.

–jeroen