Meta Class Variable

In DelphiLanguage, a variable of a type defined with a class of declaration.

This variable is assignment compatible with the ClassType? of the classes inherited from the class declared in variable type.

---

 unit Dummy;

 interface

 type
   IMyObject = interface
   ['{DDF6CE40-55D1-11D5-B57E-00AA00ACFD08}']
      procedure AMethod;
   end;

   IMyObjectClass = interface
   ['{DDF6CE41-55D1-11D5-B57E-00AA00ACFD08}']
      function CreateInstance: IMyObject;
   end;

 type
   TMyObjectBase = class(TInterfacedObject)
   public
      class function CreateInstance: IMyObject; virtual; abstract;
   end;


   //Declaration of a class of type

   TMyObjectBaseClass = class of TMyObjectBase;

   TMyDelegator = class(TInterfacedObject,IMyObjectClass)
   private
      FMyObjectClass: TMyObjectBaseClass;
      function CreateInstance: IMyObject;
      constructor Create(AMyObjectClass: TMyObjectBaseClass);
   public
      class function CreateInterfaced?(AMyObjectClass: TMyObjectBaseClass): IMyObjectClass;
   end;

 type

   TFooObject = class(TMyObjectBase,IMyObject)
   private
      procedure AMethod;
   public
      class function CreateInstance: IMyObject; override;
   end;

   TBarObject = class(TMyObjectBase,IMyObject)
   private
      procedure AMethod;
   public
      class function CreateInstance: IMyObject; override;
   end;

 procedure Test;

 implementation

 { TMyDelegator }

 class function TMyDelegator.CreateInterfaced?(
  AMyObjectClass: TMyObjectBaseClass): IMyObjectClass;
 begin
   result := Create(AMyObjectClass);  //hides Free method ...
 end;

 constructor TMyDelegator.Create(AMyObjectClass: TMyObjectBaseClass);
 begin
   inherited Create;
   FMyObjectClass := AMyObjectClass;  //a MetaClassVariable
 end;

 function TMyDelegator.CreateInstance: IMyObject;
 begin
   result := FMyObjectClass.CreateInstance;
 end;

 { TFooObject }

 procedure TFooObject.AMethod;
 begin
   WriteLn(ClassName,'.AMethod');
 end;

 class function TFooObject.CreateInstance: IMyObject;
 begin
   result := Create;
 end;

 { TBarObject }

 procedure TBarObject.AMethod;
 begin
   WriteLn(ClassName,'.AMethod');
 end;

 class function TBarObject.CreateInstance: IMyObject;
 begin
   result := Create;
 end;

 ///////////////////////////////////////////////////////////////////////////////

 { Test }

 procedure Test;
 var
   a: IMyObjectClass;
   b: IMyObjectClass;
   x: IMyObject;
   y: IMyObject;
 begin

   a := TMyDelegator.CreateInterfaced?(TFooObject);
   b := TMyDelegator.CreateInterfaced?(TBarObject);

   x := a.CreateInstance;
   y := b.CreateInstance;

   x.AMethod;
   y.AMethod;

 end;

 end.

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