[boost] Proto expresion construction question

From: Cédric Venet (cedric.venet_at_[hidden])
Date: 2008-03-10 17:10:49

Hi,

I decided to try to write a small vector library with proto to test it.

The following worked without problem (I will use extend further on but at
the moment the as_expr are needed)

  proto::display_expr( proto::as_expr(x)+proto::as_expr(y)*proto::as_expr(z)
, std::cout );

I then added

struct fastvec_reduction_tag {};
struct sum_ {};
template<typename A>
typename proto::result_of::make_expr<
    fastvec_reduction_tag
//  , MixedDomain
  , sum_ const
  , A const &
>::type sum(A const &a) {
    return proto::make_expr<fastvec_reduction_tag/*, MixedDomain*/>(sum_(),
boost::ref(a));
}

Which work well

  proto::display_expr( sum(x) , std::cout );

And now, I would like to create a "function" dot(x,y) which expand to
sum(x*y).
I tried:

template<typename A, typename B>
typename proto::result_of::make_expr<
    fastvec_reduction_tag
//  , MixedDomain
  , sum_ const
  , BOOST_TYPEOF(boost::ref(proto::as_expr(A())*proto::as_expr(B())))
> const &
>::type dot(A const &a, B const &b) {
    return proto::make_expr<fastvec_reduction_tag/*, MixedDomain*/>(sum_(),
boost::ref(
      proto::as_expr(a)*proto::as_expr(b)
      ));
}

Which compile but fail at runtime when I do

  proto::display_expr( dot(x,y) , std::cout );

(it call << on an invalid vector (type of x and y)).

Is there a ways to create the tree for sum(x*y) directly or should I use a
transform (create a tree with a dot tag and then transform it with proto)?

The page 'users_guide\expression_construction\construction_utils.html' seems
very complete, but hard to understand how to put it in practice, perhaps you
could link it to some use in the example.

Regards,

-- 
Cédric