boost::proto::functional::display_expr — 漂亮地打印一个 Proto 表达式树。
// In header: <boost/proto/debug.hpp> struct display_expr { // types typedef void result_type; // construct/copy/destruct display_expr(std::ostream & = std::cout, int = 0); // public member functions template<typename Expr> void operator()(Expr const &) const; };