Re: [boost] More on the optional class.

From: Peter Dimov (pdimov_at_[hidden])
Date: 2001-09-14 11:09:52

From: "Fernando Cacciola" <fcacciola_at_[hidden]>
> From: Peter Dimov <pdimov_at_[hidden]>
> > I apologize if this has already been discussed, but did you consider the
> > idiom (often used with dynamic_cast)
> >
> > optional<T> opt;
> >
> > if(T * pt = opt.get())
> > {
> > // initialized, use pt
> > }
> > else
> > {
> > // not initialized, no pt in scope
> > }
> >
> This is already supported.
> I've added .get() precisely in order to support this idiom, but I've been
> kind of reluctant to talk about it.
> It usage is *barely* mentioned in the documentation included in the
header.
> I'm not sure if I would encourage this idiom since it uses a member
> function, and I'm not very fond of using member functions for general
> classes -which could be replaced for something similar-

Well, the emphasis is not on the 'get' member function. You could easily
provide a free function instead.

The important thing about the idiom is that it checks for initialized
optional<T> _and_ retrieves a pointer to T at the same time, so - assuming
that you follow the idiom - you can't access an uninitialized optional by
mistake (since the pointer is not in scope in the 'else' clause.)

--
Peter Dimov
Multi Media Ltd.