Re: [Boost-users] [Iterators] Change in transform_iterator from1.31 to 1.32?

From: Nat Goodspeed (ngoodspeed_at_[hidden])
Date: 2005-07-27 08:44:22

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> bounces_at_[hidden]] On Behalf Of David Abrahams
> Sent: Tuesday, July 26, 2005 11:01 PM
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] [Iterators] Change in transform_iterator
> from 1.31 to 1.32?
> 
> "Nat Goodspeed" <ngoodspeed_at_[hidden]> writes:
> 
> > [Nat] boost::bind() is a runtime expression. I need to write a
> > typedef for const_iterator before we get anywhere near that point.
How
> > can I use a type defined on the bind object for the typedef I need?
> 
> If the bind documentation doesn't tell you the type of that object
> (does it?), then officially you can't.  

[Nat] This page:
http://www.boost.org/libs/bind/bind.html#Interface 
uses 'unspecified-N' as the type returned by boost::bind().

> Unofficially, you could pass
> the result of the bind expression to a function template that does
> something illegal on it, like
> 
>           template <class F>
>           void show_me_the_type(F const& f) { f += "hello"; }
> 
> and your compiler will probably show it to you.

[Nat] Heh, a useful trick to remember. I do need a reference, so I'm
using Peter Dimov's suggested bind<R>() syntax. With this construct:

show_me_the_type(boost::bind<const MapType::mapped_type&>
                            (&MapType::value_type::second, _1));

The type described in the compiler output translates to:

boost::_bi::bind_t<
    const MapType::mapped_type&,
    (ugly pointer-to-data-member syntax),
    boost::_bi::list1<boost::_bi::list_av_1<boost::arg<1> >::B1>
>

Forgive me, but that seems a bit too implementation-dependent to capture
in my own code. I do want to be able to migrate forward to Boost 1.33
and beyond...

As far as I'm aware, the only documented way to capture an object
created by boost::bind() is to use boost::function. I understand your
point about the inefficiency of using boost::function in my
const_iterator type, but right now I know no better way to express it.

Perhaps it would be useful to augment bind.hpp with a companion template
that could reliably describe the type of the object produced by a
particular call to boost::bind()?