Definition of the proto::pass_through<> transform, which is the default transform of all of the expression generator metafunctions such as proto::unary_plus<>, proto::plus<> and proto::nary_expr<>. proto::transform< pass_through<Grammar, Domain> > A PrimitiveTransform that transforms the child expressions of an expression node according to the corresponding children of a Grammar. The resulting expression is in the specified domain. Given a Grammar such as proto::plus<T0, T1>, an expression type that matches the grammar such as proto::plus<E0, E1>::type, a state S and a data D, the result of applying the proto::pass_through<proto::plus<T0, T1> > transform is: proto::plus< boost::result_of<T0(E0, S, D)>::type, boost::result_of<T1(E1, S, D)>::type >::type The above demonstrates how child transforms and child expressions are applied pairwise, and how the results are reassembled into a new expression node with the same tag type as the original. The Domain template parameter determines which domain the resulting expression should be in. If it is proto::deduce_domain, which is the default, the resulting expression is in the same domain as the expression passed in. Otherwise, the resulting expression is in the specified domain. Practically, that means the specified domain's generator is used to post-process the resulting expression. The explicit use of proto::pass_through<> is not usually needed, since the expression generator metafunctions such as proto::plus<> have proto::pass_through<> as their default transform. So, for instance, these are equivalent: proto::when< proto::plus<X, Y>, proto::pass_through< proto::plus<X, Y> > > proto::when< proto::plus<X, Y>, proto::plus<X, Y> > proto::when< proto::plus<X, Y> > // because of proto::when<class X, class Y=X> proto::plus<X, Y> // because plus<> is both a grammar and a transform For example, consider the following transform that promotes all float terminals in an expression to double. // This transform finds all float terminals in an expression and promotes // them to doubles. struct Promote : proto::or_< proto::when<proto::terminal<float>, proto::terminal<double>::type(proto::_value) >, // terminal<>'s default transform is a no-op: proto::terminal<proto::_>, // nary_expr<> has a pass_through<> transform: proto::nary_expr<proto::_, proto::vararg<Promote> > > {}; proto::transform_impl<Expr, State, Data> For each N in [0,Expr arity), for exposition only typename proto::result_of::child_c<Grammar, N>::type For each N in [0,Expr arity), for exposition only typename proto::result_of::child_c<Expr, N>::type For each N in [0,Expr arity), for exposition only typename boost::result_of<GN(EN,State,Data)>::type For exposition only typename Expr::proto_tag For exposition only boost::is_same<Domain, deduce_domain> For exposition only typename Expr::proto_domain For exposition only typename mpl::if_<Deduce, DD, Domain>::type For exposition only typename D::proto_generator For exposition only proto::listN<R0,...RN> For exposition only proto::expr<T, A> For exposition only proto::basic_expr<T, A> For exposition only typename mpl::if_<proto::wants_basic_expr<G>, BE, E>::type typename boost::result_of<D(expr_type)>::type result_type typename impl::expr_param typename impl::state_param typename impl::data_param proto::matches<Expr, Grammar>::value is true. D()(expr_type::make( G0()(proto::child_c<0>(expr), state, data), ... GN()(proto::child_c<N>(expr), state, data) ))