[boost] Re: Re: Formal review or Variant Library (Ed B.)

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2003-02-19 12:21:31

"David Abrahams" <dave_at_[hidden]> wrote in message
news:uwujw9tkt.fsf_at_boost-consulting.com...
> "Fernando Cacciola" <fernando_cacciola_at_[hidden]> writes:
>
> > "Peter Dimov" <pdimov_at_[hidden]> wrote in message
> > news:00d901c2d820$bd2225c0$1d00a8c0_at_pdimov2...
> >> David Abrahams wrote:
> >> > "Peter Dimov" <pdimov_at_[hidden]> writes:
> >> >
> >> >> Joel de Guzman wrote:
> >> >>> David Abrahams wrote:
> >> >>>
> >> >>>> BTW, I just realized that a conversion from variant<T> to
> >> >>>> optional<T> could be used to do extraction as well.  Maybe it
> >>
> >> -------------^
> >>
> >> >>>> would be better to ditch extract altogether and just use optional?
> >> >>>
> >> >>> I think this makes sense. The disadvantage is the overhead of
> >> >>> optional just to do "extract"ion.
> >> >>
> >> >> That means an extra copy
> >> >
> >> > Really? You can't convert to an optional<T&>?
> >>
> >> You said "optional<T>" above.
> >>
> >> It may be possible to use optional<T&> (is it supported?)
> >
> > No, it isn't.
> > And I don't think it ever will.
> > optional<X> intends to represent a value of type X wich is possiblly
> > uninitialized.
> > But you can't have X=T& since you can't have an uninitialized reference.
>
> Sorry, that's a broken analogy.  You also can't have an uninitialized
> object of non-POD class type U, but you allow optional<U>.
>
OK, I see the point.
I was looking at the wrong level of indirection :-)
optional<T&> is not a reference to an optional object, it's an optional
reference to an object.

All right. Now that we've settled on the conceptual validity of
optional<T&>...
Can we have it? Any idea about what to do with reference to reference
problem?

I have to look at reference_wrapper() yet...

What was the idea of:  optional< exactly<T&> > ?

--
Fernando Cacciola