Hi to all proto experts, I have a grammar whose only goal is to build a type: template <class Expr> struct ActionBuilder with an operator() which simply calls: proto::eval(Expr(),my_context); Unfortunately, I have an expression like: boost::proto::exprns_::expr<boost::proto::tag::plus,boost::proto::argsns_::list2<const A&,const B&> > And the compiler is unhappy about finding no appropriate default constructor (I suppose because of the references). deep_copy is probably not usable inside a grammar definition, so is there an easy way of getting rid of the references (if they are really the problem) or do I have to go and get my MPL canon? I am really just interested in having the recreated expression use a new instance of A and B, not the original ones. Thanks, Christophe _________________________________________________________________ Drag n’ drop—Get easy photo sharing with Windows Live™ Photos. http://www.microsoft.com/windows/windowslive/products/photos.aspx