Class operators

Class operators are slightly different from the operators above in the sense that they can only be used in class expressions which return a class. There are only 2 class operators, as can be seen in table (12.8).

Table 12.8: Class operators

Operator	Action

is	Checks class type
as	Conditional typecast

An expression containing the is operator results in a boolean type. The is operator can only be used with a class reference or a class instance. The usage of this operator is as follows:

Object.InheritsFrom(Class)

Var
  A : TObject;
  B : TClass;

begin
  if A is TComponent then ;
  If A is B then;
end;

The as operator performs a conditional typecast. It results in an expression that has the type of the class:

  If Object=Nil then
    Result:=Nil
  else if Object is Class then
    Result:=Class(Object)
  else
    Raise Exception.Create(SErrInvalidTypeCast);

Var
  C : TComponent;
  O : TObject;

begin
  (C as TEdit).Text:=’Some text’;
  C:=O as TComponent;
end;

The as and is operators also work on COM interfaces. They can be used to check whether an interface also implements another interface as in the following example:

{$mode objfpc}

uses
  SysUtils;

type
  IMyInterface1 = interface
    [’{DD70E7BB-51E8-45C3-8CE8-5F5188E19255}’]
    procedure Bar;
  end;

  IMyInterface2 = interface
    [’{7E5B86C4-4BC5-40E6-A0DF-D27DBF77BCA0}’]
    procedure Foo;
  end;

  TMyObject = class(TInterfacedObject, IMyInterface1, IMyInterface2)
    procedure Bar;
    procedure Foo;
  end;

procedure TMyObject.Bar;
begin

end;

procedure TMyObject.Foo;
begin

end;

var
  i: IMyInterface1;
begin
  i := TMyObject.Create;
  i.Bar;
  Writeln(BoolToStr(i is IMyInterface2, True)); // prints true
  Writeln(BoolToStr(i is IDispatch, True)); // prints false
  (i as IMyInterface2).Foo;
end.

Additionally, the is operator can be used to check if a class implements an interface, and the varas operator can be used to typecast an interface back to the class:

{$mode objfpc}
var
  i: IMyInterface;
begin
  i := TMyObject.Create;
  Writeln(BoolToStr(i is TMyObject,True)); // prints true
  Writeln(BoolToStr(i is TObject,True)); // prints true
  Writeln(BoolToStr(i is TAggregatedObject,True)); // prints false
  (i as TMyObject).Foo;
end.

Although the interfaces must be COM interfaces, the typecast back to a class will only work if the interface comes from an Object Pascal class. It will not work on interfaces obtained from the system by COM.

12.8.7 Class operators