Re: [Boost-users] [fusion] deduce type of resulting vector from a transformation

10 Oct 2010


      Am 10.10.2010 20:48, schrieb alfC:
...
On Oct 10, 3:17 am, Christopher Schmidt 
wrote:
...
Am 10.10.2010 12:02, schrieb alfC:
...
Hi,
...
(I keep struggling with Boos.Fusion.)
...
How do I deduce the type of the vector resulting from the
transformation of another vector?
...
I have a source vector of different types, e.g.
vectorsi::length, quantitysi::mass,    quantitysi::time
...
and I have a transformation that is well defined and returns another
type depending on the original type.
The question is how do I generate a vector type that is the result of
the given transformation. I though that
result_of::transform was the function to obtain the result but it
turns not because it returns a type transform_view instead of a
vector.
...
vector< ... > v1;
T? v2(transform(v1, transformation()));
...
i.e. what is the type of T as a fusion::vector as deduced from the
type of v1 and the transformation type? -- Thank you,    Alfredo--
You can generate a vector from an arbitrary fusion sequence using the
conversion metafunction:
http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/cont...
fusion::result_of::as_vector<
        fusion::result_of::transform<
                v1,
                transformation
        >::type
...
::type v2(fusion::transform(v1, transformation()));
Hi, thank you. I tried something like this but I keep getting an error
from the compiler.
/usr/include/boost/utility/result_of.hpp:68: error: invalid use of
incomplete type ‘struct divide::result
what is the problem with the following transformation? that works well
as a function but not as metafunction as called from result_of
struct divide{
  template <typename SignatureT> struct result;
  template<class Q>
  struct result{
      typedef typename divide_typeof_helpersi::time >::type
type;
  };
  template<typename Q>
  typename result::type
      operator()(Q const& q) const
  {
      return q/quantitysi::time(1.*si::second);
  }
};
It works well as a function but not as a metafunction in the following
typedef
typedef
      boost::fusion::result_of::as_vector<
      	boost::fusion::result_of::transform<
      		source_type,
      		divide
      	>::type
      >::type
      result_type
  ;
Try const-qualifying source_type in fusion::result_of::transform<...> or
add a specialization for a non-const reference argument type in
divide::result.
(fusion::transform just accepts a reference to a const-qualified
sequence whereas fusion::result_of::transform does not care about
const-qualification at all and passes the template parameter types right
down to the fusion::transform_view .)

-Christopher