On 10/23/2005 12:12 PM, Korcan Hussein wrote: [snip]
I'll get back to you on that one, for the time being i have uploaded the example i have been writing about, it is in the vault here: http://www.boost-consulting.com/vault/index.php?action=downloadfile&filename=expr_test.cpp&directory=Template%20Metaprogramming&
In the example program you should see something similar, take note however
I ran it and tried to understand it. AFAICT, the key part of the code is: template < typename Tp > struct var { ... struct basic_eval : boost::static_visitor<float> { size_type curr_indx; basic_eval(size_type i = 0): curr_indx(i) {} float operator()(const Tp& val) const { return val[curr_indx]; } template < typename Expression > float operator()(const Expression& exp) const { using boost::apply_visitor; typedef typename Expression::operation_type Oper; float lhs_val=apply_visitor(*this, exp.get_lhs()); float rhs_val=apply_visitor(*this, exp.get_rhs()); float my_val=Oper::apply(lhs_val,rhs_val); return my_val; } }; ... }; I could see the Expression::operation_type is one of: addf_tag minusf_tag The apply_visitor is from: boost/variant/detail/apply_visitor_unary.hpp and the body of this apply_visitor is: apply_visitor(const Visitor& visitor, Visitable& visitable) { return visitable.apply_visitor(visitor); } Since there's no apply_visitor in the Expression<...> , I added a few variables (*_val) to store intermediate results to enable using gdb to see what was going on. After tracing several levels of calls in gdb in an effort to find how: apply_visitor(*this, exp.get_lhs()) get's back to var<Tp>::basic_eval::operator(...), I finally just decided it does, somehow, and since all variants of the var<Tp>::expr have a corresponding exact match amoung the basic_eval::operator(...)'s, not dynamic dispatching is done. HTH any other reviewers of expr_test.cpp.