Benedetto Proietti schrieb:

Greetings.

I am kind of newbie to Boost.Fusion.

I have a sequence of "Field"s:

typedef fusion::vector< Field<int, 3, Check1, ..>, Field<float, 8, Check2, ..>, Field<MyString, 2, Check3, ..>, ...

...

MySequence;

Each "Field" knows how to (de)serialize, has some checking policy and a few more little stuff. "Field" also has a "int" property "TypeSize":

static const int TypeSize = somevalue_known_at_compile_time;

I'd like to traverse the type "MySequence" and get the sum of all the sizes at compile time. Like: Field<int,3,Check1>::TypeSize + Field<float, 8, Check2>::TypeSize + Field<MyString,2,Check3>::TypeSize + ... ;

I tried playing with fusion::accumulate and mpl<int_> but couldn't do it. There's no much documentation/examples about fusion (correct me if I am wrong please). How can I get the accumulated sum of Field<...>::TypeSize of all the members in "MySequence"? [snip]

An implementation based upon MPL's fold is easier than one based upon Fusion's equivalent. That way you do not need to deal with ref-qualified arguments. #include <boost/fusion/mpl.hpp> #include <boost/fusion/container/vector/vector.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/fold.hpp> #include <boost/mpl/plus.hpp> #include <boost/mpl/placeholders.hpp> template<int Size> struct my_type { typedef boost::mpl::int_<Size> size; }; template<typename T> struct get_size:T::size{}; typedef boost::fusion::vector<my_type<1>,my_type<2>,my_type<3> > vec; typedef boost::mpl::fold< vec, boost::mpl::int_<0>, boost::mpl::plus<boost::mpl::_1, get_size<boost::mpl::_2> >

::type sum;

char is_correct[sum::value - 6 ? 0 : 1]; A fusion-only implementation probably looks like this: #include <boost/fusion/container/vector/vector.hpp> #include <boost/fusion/algorithm/iteration/fold.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/plus.hpp> #include <boost/type_traits/remove_reference.hpp> template<int Size> struct my_type { typedef boost::mpl::int_<Size> size; }; struct accumulator { template<typename Sig> struct result; template<typename Self, typename State, typename T> struct result<Self(State, T)>: boost::mpl::plus< typename boost::remove_reference<State>::type, typename boost::remove_reference<T>::type::size > {}; template<typename State, typename T> typename result<accumulator const&(State,T)>::type operator()(State,T)const; }; typedef boost::fusion::vector<my_type<1>,my_type<2>,my_type<3> > vec; typedef boost::fusion::result_of::fold< vec, boost::mpl::int_<0>, accumulator

::type sum;

char is_correct[sum::value - 6 ? 0 : 1]; Both snippets are untested! -Christopher