Re: [Boost-users] [lambda / multi_index] compilation problem

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-03-13 09:55:23

Filip Konvi?ka ha escrito:

> Hi,
>
> getting lambda work with multi_index seems a bit tough for me....
> the following compiles fine with MSVC++ 2003, but not when I #include
> sequenced_index.hpp or ordered_index.hpp.
>
> The error message says:
> error C2872: '_1' : ambiguous symbol
>          could be
> 'c:\Boost\include\boost-1_33_1\boost\lambda\core.hpp(69) : const
> boost::lambda::placeholder1_type &boost::lambda::`anonymous-namespace'::_1'
>          or
> 'c:\Boost\include\boost-1_33_1\boost\bind\placeholders.hpp(42) :
> boost::arg<I> `anonymous-namespace'::_1'
>          with
>          [
>              I=1
>          ]

Hello Filip, the problem you describe is only incidentally related with
Boost.MultiIndex (anyway, thank you for using the lib!): actually, it's a
manifestation of a symbol clash between Boost.Lambda and Boost.Bind:

#include <boost/config.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/bind.hpp>
#include <list>

using namespace boost;
using namespace boost::lambda;

int main(int argc, char* argv[])
{
     std::list<int> v(10);
     for_each(v.begin(), v.end(), _1 = 1);
     return 0;
}

You're having the error under the conditions you describe just because
boost/multi_index/sequenced_index.hpp and
boost/multi_index/ordered_index.hpp internally include boost/bind.hpp.

> This can of course be fixed by using boost::lambda::_1, but I doubt that
> it is the expected behavior.

I guess the using declaration  is a perfectly good solution if you don't
use Boost.Bind in the offending translation unit: This is a well known
problem with Boost.Bind defining its placeholders in an unnamed
space (::_1,::_2,...), which causes ambiguity problems when combined
with the "using namespace boost::lambda" directive. You might want to
read the following thread for further info on the issue:

http://lists.boost.org/Archives/boost/2004/11/76822.php

AFAIK this is not currently being addressed in any way.

> Can anyone please explain what I'm doing wrong?

HTH,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo