Re: [boost] Formal review: Optional library

From: Joel de Guzman (djowel_at_[hidden])
Date: 2002-12-10 17:04:45

----- Original Message ----- 
From: "David Abrahams" <dave_at_[hidden]>

> "Joel de Guzman" <djowel_at_[hidden]> writes:
> 
> > ----- Original Message ----- 
> > From: "Fernando Cacciola" <fernando_cacciola_at_[hidden]>
> >
> >
> >
> >> optional<> is trying to model using C++ a concept that
> >> it is not really covered by the language, that of uninitialzed values.
> >> It uses pointer semantics *just* because pointers are the only sort of
> >> C++ objects which has a clear uninitialized state.
> >
> > Hi,
> >
> > Probably a dumb question but allow me to ask anyway:
> >
> > Wouldn't a more generic variant<T0, T1...TN> class do what the 
> > optional is trying to do? I feel that optional<T> is just a variant<T, nil_t>
> > in disguise. Correct me if I'm wrong.
> 
> Someone once said that if a theorem in Physics involves numbers other
> than zero, 1, or infinity, it's lacking in purity.

:-) Cool! 
 
> optional captures the zero/one distinction, which, as its analogy to
> pointers makes clear, is a very useful one.  A type which can contain
> any number of different types would have a much more complicated
> interface.  Maybe it's just a different library?

Or perhaps, optional is just a limited add-on API over the variant?
The low-level  implementation mechanism in place seems to be very
similar and having 2 separate libraries will be redundant, I think.

Joel de Guzman
joel_at_[hidden]
http://www.boost-consulting.com