"Fernando Cacciola" <[EMAIL PROTECTED]> writes: > "Peter Dimov" <[EMAIL PROTECTED]> wrote in message > 00d901c2d820$bd2225c0$1d00a8c0@pdimov2">news:00d901c2d820$bd2225c0$1d00a8c0@pdimov2... >> David Abrahams wrote: >> > "Peter Dimov" <[EMAIL PROTECTED]> 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>. > Of course, optional<T&> could have a special meaning, but I can't > see what would it be. What's the meaning of an optional reference? > I think there cannot be such a thing. The same as an optional std::string. Either you have one, or you don't. >> or optional< >> reference_wrapper<T> > but this looks like an "obfuscated C++" entry to me >> compared to T*. What's wrong with it? What does optional<T&> add? >> > Exactly. An optional reference is almost like a possibly null pointer, > except that references > must be bounded. > If given a particular design you would need optional<T&>, then you > definitely need T* instead. I've my doubts. -- Dave Abrahams Boost Consulting www.boost-consulting.com _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
