Larry Evans wrote:
Hi Eric,
I need a context with something like the following methods:
template< typename Tag, typename Left, typename Right > result_type operator()(expr<Tag, Left, Right> const &expr) const { std::cout<<std::setw(indent) <<""<<"binary_expr:"<<expr.get_instance()<<"\n"; indent+=2; proto::eval(left, *this); proto::eval(right, *this); indent-=2; }
where the expression tree node argument remains "packaged":
(expr<Tag, Left, Right> const &expr)
in contrast to the unpacked version:
(Tag, Left const &left, Right const &right)
Sure. Only context types derived from callable_context get unpacked nodes like that. If you don't inherit from callable_context, you'll get the whole node without any modification. The interface is a bit different, though. You'll need something like: struct my_context { template<typename Expr> struct eval { typedef ... result_type; result_type operator()(Expr &expr, my_context &ctx) const { return ...; } }; }; If you want to handle binary nodes specially, you would define the eval member template as: template<typename Expr, long Arity = Expr::arity::value> struct eval and then partially specialize for binary nodes: template<typename Expr> struct eval<Expr, 2> HTH, -- Eric Niebler Boost Consulting www.boost-consulting.com