Re: [boost] [optional] std::tr2::optional

Subject: Re: [boost] [optional] std::tr2::optional
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2011-11-19 07:03:45

> Do we even want the third variant? That is, do we want to support
tr2::optional of something that is neither copyable nor movable?
> My gut instinct says no, that's unintuitive.

boost::optional can be used for many purposes. One of these (and I had to
use optional like that) is to make classes that provide RAII-like interface
(noncopyable, nonmoveable) usable in contexts where I need to use two-phase
initialization or to release the resource before the end of the scope
(subject to some runtime condition) -- all that in order to have a resource
acquired for as short span of time as possible.
I would be forced to do that for std::lock_guard if the library did not
provide a second template std::unique_lock. But not every library provides
two interfaces for almost the same thing, and this is arguably an
unnecessary redundancy for an interface. optional<RAII> as described above
is a generic tool for transforming types that implement 'scoped ownership'
into types that implement 'unique ownership' of a resource.

Given this optional<RAII> pattern I can already re-set my object to a
different resource:

  template< typename ...Args >
  void reset( boost::optional<RAII> & o, Args &&... args )
  {
    o = boost::none;
    o = in_place( std::forward<Args>(args)... );
  }

Or perhaps boost::optional already implements this 'strange' assignment. I
didn't have a need to reset a handle to a different resource. But you can
see how (arguably) natural it becomes for optional<RAII>.

Regards,
&rzej