Re: [Boost-users] [fusion] sequence conversion

From: Istvan Buki (buki.istvan_at_[hidden])
Date: 2008-07-18 16:17:28

Steven,

your solution is exactly what I needed.
My only dream is to be able to manipulate these concepts as well as you do.

Thank you very much for your help!

On Fri, Jul 18, 2008 at 1:37 AM, Steven Watanabe <watanabesj_at_[hidden]>
wrote:

> AMDG
>
> Istvan Buki wrote:
>
>> Dear Boost experts,
>>
>> I'm trying to copy data between different types of fusion sequences with
>> limited success.
>> The example below shows what I'm trying to achieve.
>>
>> <snip>
>> Does fusion provide a way to achieve what I'm trying to do?
>>
>
> The elements of the map are fusion pair.  You need to
> extract the second element.
>
> #include <boost/fusion/include/vector.hpp>
> #include <boost/fusion/include/map.hpp>
> #include <boost/fusion/include/pair.hpp>
> #include <boost/fusion/include/transform.hpp>
>
> namespace fields {
>   struct i;
>   struct l;
>   struct d;
> }
>
> template<class F>
> struct result_of_second;
>
> template<class Self, class Arg1>
> struct result_of_second<Self(Arg1)> {
>   typedef const typename Arg1::second_type& type;
> };
> template<class Self, class Arg1>
> struct result_of_second<Self(Arg1&)> {
>   typedef typename Arg1::second_type& type;
> };
> template<class Self, class Arg1>
> struct result_of_second<Self(const Arg1&)> {
>   typedef const typename Arg1::second_type& type;
> };
>
> struct second {
>   template<class F>
>   struct result {
>       typedef typename result_of_second<F>::type type;
>   };
>   template<class T>
>   typename T::second_type& operator()(T& t) const { return(t.second); }
>   template<class T>
>   const typename T::second_type& operator()(const T& t) const {
> return(t.second); }
> };
>
> int main() {
>   using namespace boost::fusion ;
>
>   typedef map<
>       pair<fields::i, int>,
>       pair<fields::l, long>,
>       pair<fields::d, double>
>   > map_type;
>
>   typedef vector<int, long, double> vect_t;
>
>   vect_t tup1(1, 2, 3.0);
>   map_type m = tup1;
>   vect_t tup2 = transform(m, second());
>
>   // expected result: tup1 == tup2
> }
>
>
>  As a side question, I was wondering whether it is possible to synthetize
>> the vect_t type from map_type (with MPL?)
>>
>
> Indeed it is possible,
>
> typedef result_of::as_vector<
>   mpl::transform<
>       map_type,
>       result_of::second<mpl::_1>,
>       mpl::back_inserter<mpl::vector0<> >
>   >::type
> >::type vect_t;
>
> In Christ,
> Steven Watanabe
>
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