Re: [boost] [phoenix] not playing nice with other libs

Subject: Re: [boost] [phoenix] not playing nice with other libs
From: Vicente BOTET (vicente.botet_at_[hidden])
Date: 2011-05-02 09:54:50

> Message du 02/05/11 13:19
> De : "Thomas Heller" 
> A : "Eric Niebler" 
> Copie à : "Boost mailing list" 
> Objet : Re: [boost] [phoenix] not playing nice with other libs
> 
> On Mon, May 2, 2011 at 12:54 PM, Eric Niebler  wrote:
> > The following trivial program fails to compile:
> >
> > Â #include 
> > Â #include 
> >
> > It generates the following:
> >
> > In file included from
> > ../../../../branches/release/boost/numeric/conversion/detail/is_subranged.hpp:17,
> > Â  Â  Â  Â  Â  Â  Â  Â  from
> > ../../../../branches/release/boost/numeric/conversion/detail/conversion_traits.hpp:21,
> > Â  Â  Â  Â  Â  Â  Â  Â  from
> > ../../../../branches/release/boost/numeric/conversion/conversion_traits.hpp:13,
> > Â  Â  Â  Â  Â  Â  Â  Â  from
> > ../../../../branches/release/boost/numeric/conversion/converter.hpp:13,
> > Â  Â  Â  Â  Â  Â  Â  Â  from main.cpp:2:
> > ../../../../branches/release/boost/mpl/multiplies.hpp:38: error: wrong
> > number of template arguments (12, should be 5)
> > ../../../../branches/release/boost/mpl/aux_/preprocessed/gcc/times.hpp:68:
> > error: provided for ‘temp
> > late struct
> > boost::mpl::times’
> >
> > Phoenix is changing the following fundamental constants:
> >
> > Â BOOST_PROTO_MAX_ARITY
> > Â BOOST_MPL_LIMIT_METAFUNCTION_ARITY
> > Â BOOST_PROTO_MAX_LOGICAL_ARITY
> > Â BOOST_RESULT_OF_NUM_ARGS
> >
> > IMO, Phoenix shouldn't be touching these. It should work as best it can
> > with the default values. Users who are so inclined can change them.
> 
> Eric,
> This problem is well known. As of now I have no clue how to fix it properly.
> 
> Let me sketch why i changed these constants:
> 1) Phoenix V2 has a composite limit of 10:
> This is equivalent to the number of child expressions a expression can hold.
> This is controlled by BOOST_PROTO_MAX_ARITY for the number of
> template arguments for proto::expr and proto::basic_expr
> 2) Boost.Bind can take up to 10 parameters in the call to boost::bind
> 
> The default BOOST_PROTO_MAX_ARITY is 5.
> 
> The BOOST_RESULT_OF_NUM_ARGS constant needed to be changed because i
> needed to provide 11 arguments in a "call" to boost::result_of. But i
> guess a workaround
> can be found in this specific case.
> 
> I wonder what qualifies as "User". Phoenix is certainly a user of mpl,
> result_of and proto.
> Spirit is a user of proto and phoenix. Spirit needs an arity of 7 (IIRC).

I agree. A library using another library having a max number of parameters must redefine it if the value is not enough big to its needs.

> Anybody got any ideas?

Is there a way to avoid using the preprocessed MPL files (BOOST_MPL_PREPROCESSING_MODE?)

Best,
Vicente