Larry Evans wrote:
http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/users_g...
contains:
we want all expressions that involve the calculator placeholders to be calculators
which means, IIUC, that calulator expressions are built bottom up. IOW, to build a calculator expressions, you must start with a proto terminal calculator expression. However, I would like to start from a non-terminal expression (e.g. any binary or unary expression) and convert all the nodes to calculator expressions. What's the simplest way to do this? I've tried understanding trans::construct and other trans templates; however, I can't figure it out so far :(
TIA for any clues.
If you have an expression tree, and you want to wrap each node in some wrapper, the construct transform is what you want. For instance: #include <boost/xpressive/proto/proto.hpp> #include <boost/xpressive/proto/transform/construct.hpp> using namespace boost; using namespace proto; template<typename Expr> struct wrap { explicit wrap(Expr const &expr) {} }; struct WrapItUp : transform::construct< or_< terminal<_> , nary_expr<_, vararg<WrapItUp> > > , wrap<_>(_) > {}; terminal<int>::type const _1 = {1}; terminal<int>::type const _2 = {2}; int main() { int i = 0; WrapItUp::call( _1 + _2, i, i ); return 0; } This little example works, but if the wrap struct inherits from proto::extends, it doesn't compile because of a bug in construct<> which causes the eager instantiation of wrap<_>. You could write your own as_wrap transform which doesn't have that problem and use that instead of construct. HTH, -- Eric Niebler Boost Consulting www.boost-consulting.com