Hi, The default implementation of boost::fusion::pair<first,second> only stores an instance of second. I might need a fusion::pair which does not store that instance either, maybe that will cause trouble in several parts of fusion. At first I would like to explain what I am doing right now. At the moment I do work on an expression template framework, similiar to Eric Nieblers boost::proto, which was used to build boost::xpressive. I do extend that library by adding a rule system which enables the user to define the allowed operations of a certain domain. To simplify and generalize the rule writing process, every node in the expression tree has a fusion::map<> which is supposed to hold some meta information. E.g. in the matrix vector domain, that attribute container of a binary node returned by binary plus, might look like that: map< pair<domain_tag, linear_algebra_domain> , pair<operator_tag, plus_tag> , pair<category_tag, column_vector_tag> , pair<dimension_tag, dynamic::dimension> , pair<element_type_tag, float>
There are entries in the map that do not require runtime data at all, for these entries all information is coverd by the second template parameter of pair. And others like the dimension_tag which require an instance of the second type stored in the container. How can I acchieve that with fusion? I thougt about a rather ugly solution: template <typename T> struct ct{ typedef T type }; map< pair<domain_tag, ct<linear_algebra_domain> > , pair<operator_tag, ct<plus_tag> > , pair<category_tag, ct<column_vector_tag> > , pair<dimension_tag, dynamic::dimension> , pair<element_type_tag, ct<float> >
Then only an empty ct is stored. The main reason for all this is the element_type_tag. It should be possible to define a matrix with some matrix type as element_type. Regards, Andreas Pokorny