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Subject: Re: [boost] [Phoenix V3] for_each name conflict
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2011-02-27 11:51:18


On Sunday, February 27, 2011 04:29:11 PM Mathieu - wrote:
> Hi everyone,
>
>
> While reviewing phoenix V3 I encountered a bug while using mpl::for_each
> with a phoenix lambda, because of a name conflict (lazy phoenix for_each
> with mpl::for_each).
> The following code reproduces the bug :
>
> #include <boost/mpl/for_each.hpp>
> #include <boost/mpl/vector_c.hpp>
> #include <boost/phoenix/phoenix.hpp>
> #include <iostream>
>
> using boost::phoenix::arg_names::arg1;
>
> int main(int argc, const char *argv[])
> {
> typedef boost::mpl::vector_c<int,0,1,2,3> vec;
>
> boost::mpl::for_each<vec>(std::cout << arg1);
>
> return 0;
> }

Interesting, which compiler do you use?
I tested this snippet with gcc 4.2, 4.5 and clang 2.8 and all three compiled
it without complaining a bit.

> which fails to compile with the error :
>
> In file included from main.cpp:1:
> dev/phoenix3/boost/phoenix/stl/algorithm/iteration.hpp: In function
> ‘void boost::mpl::for_each <snip irrelevant long mpl / phoenix types>
> main.cpp:25: instantiated from here
> dev/phoenix3/boost/phoenix/stl/algorithm/iteration.hpp:91: error:
> ‘boost::phoenix::for_each’ is not a function,
> /usr/include/boost/mpl/for_each.hpp:109: error: conflict with
> ‘template<class Sequence, class F> void boost::mpl::for_each(F,
> Sequence*)’
> /usr/include/boost/mpl/for_each.hpp:111: error: in call to ‘for_each’
>
>
> The mpl code does an unqualified call to for_each here :
>
> void for_each(F f, Sequence* = 0)
> {
> for_each<Sequence, identity<> >(f); // this is line 111
> }
>
> and phoenix provides :
>
> function<impl::for_each> const for_each = impl::for_each();
>
> The (silly) change to mpl::for_each<Sequence, identity<> >(f); obviously
> unbreak the build.
>
> The problem seems to be that ADL kicks in and as f (in the mpl call of
> for_each) is a phoenix type.
>
> I don't have a patch (yet) as I don't really know the best strategy
> here, nor I'm sure that my diagnostic is accurate (the ADL part that
> is).
>
> Mathieu-
>
> :wq


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