Larry Evans schrieb:
On 09/29/10 11:45, Larry Evans wrote: [snip]
Using Christopher Schmidt's variadic fusion: http://svn.boost.org/svn/boost/sandbox/SOC/2009/fusion/ solves problem. Submitted bug:
I am not sure whether this is an actual bug. Your problem boils down to fusion::joint_view not supporting reference template type arguments. My port changed that, but that's an entirely new feature that was introduced to differentiate semantics for l- and rvalue sequences. All inbuilt views of Fusion implicitly use references to underlying *non-view* sequences. For example, in typedef fusion::vector<...> t1; typedef fusion::transform_view<t1, ...> t2; typedef fusion::joint_view<t1, t2> t3; t2 stores a reference to an instance of t1 and t3 stores a reference to an instance to t1, but stores the underlying instance of t2 by value. 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<typename Self, typename Lhs, typename Rhs> struct result<Self(Lhs,Rhs)> { typedef boost::fusion::joint_view< typename boost::remove_reference< Lhs >::type, typename boost::remove_reference< Rhs >::type > type; }; template<typename Lhs, typename Rhs> typename result<join_ftor const(Lhs&,Rhs&)>::type operator()(Lhs& lhs, Rhs& rhs)const { return boost::fusion::joint_view< Lhs, Rhs >(lhs,rhs); } };} 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. 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'. 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 LhSequence, typename RhSequence> typename boost::result_of<join_ftor(LhSequence,RhSequence)>::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