Re: [Boost-users] inappropriate ADL

Subject: Re: [Boost-users] inappropriate ADL
From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2010-04-01 13:02:08

Hi!

That one works with g++. It uses Barton-Nachman trick to make a find a
better match by introducing a non-template function parameter Derived.

#include <deque>
#include <boost/algorithm/string/find.hpp>
#include <boost/operators.hpp>

template<class Derived>
struct helper : boost::equality_comparable<Derived>
{
  template<class Rng>
  friend typename boost::range_iterator<Rng>::type find(Rng& rng, Derived
const& d)
  {
    return std::find(boost::begin(rng), boost::end(rng), d);
  }
};

struct STest : helper<STest>
{
   bool operator==(STest const& test) const { return true; }

};

int main() {
   std::deque<STest> deq;
   find( deq, STest() );
}

Good Luck,
Ovanes

On Thu, Apr 1, 2010 at 6:12 PM, Volker Schöch <vschoech_at_[hidden]>wrote:

>  Hi all,
>
> given this code...
>
>
>
> #include <deque>
>
> #include <boost/algorithm/string/find.hpp>
>
> #include <boost/operators.hpp>
>
>
>
> template< class Rng, class T >
>
> typename boost::range_iterator<Rng>::type find( Rng& rng, T const& t ) {
>
>       return std::find( boost::begin(rng), boost::end(rng), t );
>
> }
>
>
>
> struct STest : boost::equality_comparable<STest>
>
> {
>
>       bool operator==(STest const& test) const { return true; }
>
> };
>
>
>
> void main() {
>
>       std::deque<STest> deq;
>
>       find( deq, STest() );
>
> }
>
>
>
> ...the VS9 compiler fails with “error C2668: 'find' : ambiguous call to
> overloaded function”. This is due to the fact that STest inherits from a
> type that is defined in boost namespace which triggers the compiler to try
> ADL which finds boost::algorithm::find(RangeT& Input, const FinderT&
> Finder).
>
>
>
> An obvious solution is to prefix the call to find(…) with ::, but this
> feels wrong: It does not make sense that the compiler does ADL in this case.
> There is nothing special about boost::equality_comparable<…> that would make
> it a more natural argument for boost::algorithm::find(RangeT& Input, const
> FinderT& Finder) than any other type, and there is nothing special about
> boost::algorithm::find(RangeT& Input, const FinderT& Finder) that would make
> it a more natural find implementation for boost::equality_comparable<…> than
> my find(Rng& rng, T const& t). Maybe the function and the type should not be
> in the same namespace in the first place. Currently, the mere presence of
> the specific implementation boost::algorithm::find(RangeT& Input, const
> FinderT& Finder) disallows the implementation of the canonical find(Rng&
> rng, T const& t).
>
>
>
> Opinions? Suggestions?
>
> Thanks a lot,
>
>    Volker
>
> --
> Volker Schöch (vschoech_at_[hidden])
> Senior Software Engineer
>  think-cell Software GmbH http://www.think-cell.com Chausseestr. 8/E phone +49
> 30 666473-10 10115 Berlin, Germany toll-free (US) [phone] Directors:
> Dr. Markus Hannebauer, Dr. Arno Schoedl Amtsgericht Berlin-Charlottenburg,
> HRB 85229
>
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