Re: [boost] New Library Proposal: dual_state

From: Rob Stewart (stewart_at_[hidden])
Date: 2005-07-20 09:50:05

From: "Jost, Andrew" <Andrew_Jost_at_[hidden]>
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Rob Stewart
> > From: "Jost, Andrew" <Andrew_Jost_at_[hidden]>
> > > From: boost-bounces_at_[hidden]
> 
> > > // METHOD #2b: use an adaptor-like predicate template< 
> > class T > class 
> > > adaptor_substitute : public std::unary_function< 
> > boost::optional<T>, T 
> > > > {
> > >     T Def;
> > > public:
> > >     _adaptor_() : Def( T() ) {}
> > >     _adaptor_(T def) : Def( def ) {}
> > >     T operator()(const argument_type& x ) {
> > >         return ( x ? x.get() : Def );
> > >     }
> > > };
> > > max = *std::max_element(
> > >     boost::make_transform_iterator( v.begin, 
> > > adaptor_substitute<int>(-1) ),
> > >     boost::make_transform_iterator( v.end,   
> > adaptor_substitute<int>(-1)
> > > )
> > > );
> > 
> > This isn't really an adapter substitute.  It is an adapter, 
> > adapted (pun intended) to be a predicate.
> 
> But a predicate must return bool, not T.  Did I miss something?

Well, you called it a predicate at the top and I didn't really
look that closely to see that it wasn't a predicate.  I took your
word for it and added confusion with my comment.

The point I was trying to make was that you did, for all intents
and purposes, write the adapter class.

> > > // METHOD #3: use an adaptor
> > > max = *std::max_element(
> > >     boost::make_transform_iterator( v.begin,
> > > boost::optional<int>::adaptor(-1) ),   // NOTE:
> > > boost::optional<int>::adaptor does not currently exist
> > >     boost::make_transform_iterator( v.end,
> > > boost::optional<int>::adaptor(-1) )
> > > );
> > 
> > That is clearly the most straightforward of the bunch, but 
> > I'd like to see a proper Boost.Lambda or Boost.Function 
> > variant for comparison.
> 
> I would prefer to use Boost.Lambda, but I am currently stuck on VC6 for
> Win32 development.  It is becoming a problem...

I'm sorry for your pain. ;)

> > BTW, why have you shown the adapter as a nested type of 
> > boost::optional?  Do you not think it is applicable to 
> > anything else?  I can envision adapting pointers or even 
> > floating point types (where NaN is the "empty" value), to 
> > name just two more types adapter might support.
> 
> I guess I started with Boost.Optional in mind and never considered
> anything else.  Also, depending on the implementation the adapter may
> need to interact with Boost.Optional.  Can you provide examples of some
> ways you might use an adapted pointer or float?

The real justification for your adapter idea is that the creator
of the data doesn't know what default the consumer may need, or
that there may be multiple consumers each with a different
default.  Thus, the producer uses a suitable sentinel value to
indicate "no data" and the consumer uses an adapter to convert
the sentinel value into a suitable default.

A null pointer means no data, so the producer records null
pointers and the consumer provides a default in lieu of null
pointers.  Likewise, NaN is an appropriate "no data" value, and
the consumer uses an adapter to provide the desired default.

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;