type
  TPerson = class(TObject, ISwimmer)
    ...
  end;

...

var
  swimmer : ISwimmer;
  person : TPerson;
begin
  swimmer := TPerson.Create;
  person := swimmer as TPerson;
  person := TPerson(swimmer);

• Previously this was done either through hackery of the Hallvard Vassbotn kind or by adding a function that returned the object type to the interface definition.
• I find this very useful when unit testing code that uses interfaces.