[Boost-users] Re: [Foreach]TroubleiteratingoverBoost.MultiIndexcontainers

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-03 21:04:47

"Thorsten Ottosen" <nesotto_at_[hidden]> writes:

> "Arkadiy Vertleyb" <vertleyb_at_[hidden]> wrote in message 
> news:d05g68$qqu$1_at_sea.gmane.org...
> | "Eric Niebler" <eric_at_[hidden]> wrote
> |
> | > Jonathan Turkanis wrote:
> | > Yes, Boost.Range is designed that way. It is an extensible design -- you
> | > can range-ify your own type by defining your own begin() and end() and
> | > letting ADL find them. So it seems there is no simple work-around for
> | > GCC's bizarre ADL rules in this case. We'll need to think of something
> | else.
> |
> | Well, then I hate to say this, but IMHO this is a problem in the Boost.Range
> | design :-(
>
> It is deliberate design. 

It might still be a design mistake.

> All generic code that uses boost.range should use unqualified calls
> to enable ADL.
>
> <aside>
> It is possible to imagine alternative designs where
> all calls to end(c) can be made qualified and still enable
> ADL; however, it requires the unqulified call of another function
> inside boost::end(c) , say adl_end(c), to enable ADL of that function.
> </aside>

ADL might not be the best customization solution.  Problems with ADL
are well-known, and there are other mechanisms; in this case static
member functions of a class template specialization might have been
better... especially since you can't stick overloads in std. For
example:

namespace boost { namespace ranges {

template <class Range>
struct traits;

template <class Range>
typename traits<Range>::iterator begin(Range& r)
{
    return traits<Range>::begin(r);
};

// ... etc...
}}

> | I am not really that familiar with this library, but I assume it has to do
> | with containers, correct?  Assume the following usage (pseudocode)
> |
> | Range(std::vector<MyNamespace::SomeTemplate<boost::multi-index<boost::mpl::v
> | ector> > > )
> |
> | Now ADL will use std, boost, MyNamespace, boost::multi_index, and boost::mpl
> | to find Range.  There is absolutely no guarantee that it won't find
> | conflicting functions :-(
>
> I don't get this. Surely one of the functions would be a better match than
> the others and hence called.

The problem is that controlling which is the best match is sometimes
very difficult.  Sometimes the best match doesn't get called.
Sometimes there is ambiguity.  ADL is a very blunt instrument.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com