On Wed, 17 Jun 2009 12:07:33 -0500, Joel de Guzman <djowel@gmail.com> wrote:
Mike Tegtmeyer wrote:
I am hoping someone can point me the in right direction regarding this code snippit failing to compile. My suspicion is that it is a header problem but any help would be appreciated. In short, I seem to be missing why the binary form of mpl::transform won't work with fusion::vector. (unary form works just fine). The code below just attempts to make a std::pair<> from the contents of two sequences (this is just an example to illustrate the transform problem). If the two sequences are mpl::vectors, everything is fine. If they are fusion::vectors then 'lots of errors'. I'm fairly new to both mpl and fusion so I am not ruling out the fact that I have done something stupid.
Trying the code, it seems mpl::(binary)transform is looking for Seq::tag which is not a requirement for MPL::Forward Sequence which mpl::transform requires (http://tinyurl.com/l6kcb4). fusion::vector adheres fully to an MPL::Forward Sequence.
No, the error is caused by the following code in MPL's pair_view (which is the underlying implementation mechanism for the binary transform): typedef typename min< typename iterator_category<iter1_>::type , typename iterator_category<iter2_>::type >::type category_; That is, as per iterator_category's requirements [1], pair_view assumes that the iterator_category's result would be model of MPL's Integral Constant, which does not seem to be the case with Fusion categories. [1] http://www.boost.org/doc/libs/1_39_0/libs/mpl/doc/refmanual/iterator-categor... HTH, -- Aleksey Gurtovoy MetaCommunications Engineering