Question

I am trying to implement clear in the following example code in Delphi 2009.

interface
...
  TFoo<T : IInterface> = class(TObject)
    FField : T;
    procedure Clear;
  end;
...
implementation
...
procedure TFoo<T>.Clear;
begin
  // Line Below Results In
  //  E2010 Incompatible types: 'T' and 'Pointer' 
  FField := nil;
end;
...

I could understand the complie time error if “T” was not constrained.
But since “T” must be an Interface, I would have thought this syntax would have
worked.

Is there away to set FField to NIL, so the interface can be released?

Answer 1

Instead of nil you must use the new Default(T) which returns the default value for the generic parameter type. And for interfaces it is nil

procedure TFoo<T>.Clear;
begin
  FField := Default(T);
end;