Steven Watanabe wrote:
[ skiped ] Finally, here is an example of how I am defining operations: [ skiped ]
I also use CRTP trick but I pass a signature of an operation as well: struct add_int_constant : function<add_int_constant, anyT (anyT const&, int)> { // ... }; Here anyT is a placeholder for any<OperationList>. It lets me distinguish between any<OperationList> and regular arguments. Base class has operator() that deduces OperationList from a first anyT argument. So a user can just call the add_int_constant functor: any< mpl::vector<add_int_constant> > a(9); a = add_int_constant()(a, 11); assert(extract<int>(a) == 20); There is no stable interface for operation implementation yet. It should be simple for very generic operation like to_ostream and for an opposite case of different functions for every combination of types. For the latter I'm thinking of the following approach: struct plus : function<plus, anyT (anyT const&, anyT const&)> { int entry(id<1>, int, int) const; double entry(id<2>, double, int) const; // ... std::string entry(id<100>, std::string const&, char) const; // ... }; Basically, it's multimethods where all entries belong to one class. Using id<N> agruments, it's possible to view entries as MPL immutable sequence of signatures (typeof is required). Obviously, such entries can't be templates. If OperationList is needed, a user could resort to a nested "definition" template (pay attention to entry 200): struct plus : function<plus, anyT (anyT const&, anyT const&)> { template<class L> // L is OperationList struct definition { int entry(id<1>, int, int) const; double entry(id<2>, double, int) const; // ... any<L> entry(id<200>, any<L> const&, int) const; }; }; I believe that SFINAE can detect a presence of "definition". Operations with only one anyT agrument can be implemented without using id<N> agruments: struct to_ostream : function<to_ostream, ostream& (ostream&, anyT const&)> { template<class T> ostream& entry(ostream& o , T const& t) const { return o << t; } ostream& entry(ostream& o, emptyT) const { return o; } }; Similarly, "definition" can be used if required. -- Alexander Nasonov Project Manager at Akmosoft ( http://www.akmosoft.com ) Blog: http://nasonov.blogspot.com Email: $(FirstName) dot $(LastName) at gmail dot com