Re: [Boost-users] trac ticket 4697 submitted (was Re: [Fusion] How to join more than two sequences

On 10/06/10 10:53, Christopher Schmidt wrote: [snip]

You can fix your example by removing the references to Lhs and Rhs before passing down to Fusion:

namespace fold_join{struct join_ftor { template<typename Sig> struct result;

template struct result { typedef boost::fusion::joint_view< typename boost::remove_reference< Lhs >::type, typename boost::remove_reference< Rhs >::type > type; };

template typename result::type operator()(Lhs& lhs, Rhs& rhs)const { return boost::fusion::joint_view< Lhs, Rhs >(lhs,rhs); } };}

Thanks. I made the changes you suggest above; however, it still fails to compile without variadic fusion. The changed source is attached.

A few remarks:

You cannot use the function(!) fusion::join as it only accepts const-qualified sequences. See

https://svn.boost.org/trac/boost/ticket/3954

for more information. The port has this 'fixed'. In your code you should use fusion::joint_view directly.

Thanks.

It is definitely not a good idea to specialize boost::result_of with the signature of your functor. You should stick to the official protocol, that is defining a nested type 'result_type' or a nested template type 'result'.

The fusion html reference mentioned the in source comments after @brief the result_of specialization explain why I thought I had to specialize boost::result_of. IOW, the Table 1.37 here: http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/algorithm/i... lead me to this conclusion. If that's wrong, then maybe the fold.html should provide further clarification.

You should consider that the functor may be const- and/or even reference-qualified (in c++11) when passed to the result metafunction. See FCD 20.7.6.6 for more information.

template typename boost::result_of::type operator()(LhSequence& lhs, RhSequence& rhs)const

is not a good idea as well, as LhSequence and RhSequence in join_ftor(LhSequence,RhSequence) loose cv-qualification.

-Christopher

Thanks very much for the explanation Christopher. -Larry