This was discussed many times, even recently enhancement request was created in bugzilla. Try to search in archives (in briefly - no, it is a bad idea, which has opposition, me included).
- how to define opCast outside class Timothee Cour
- Re: how to define opCast outside class Jonathan M Davis
- Re: how to define opCast outside class Timothee Cour
- Re: how to define opCast outside class Maxim Fomin
- Re: how to define opCast outside class Maxim Fomin