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## Struct template pass_through

boost::proto::pass_through — 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.

## Synopsis

    // In header: <boost/proto/transform/pass_through.hpp>

    template<typename Grammar, typename Domain = proto::deduce_domain>
    struct pass_through :  proto::transform< pass_through<Grammar, Domain> > {
      // member classes/structs/unions
      template<typename Expr, typename State, typename Data>
      struct impl :  proto::transform_impl<Expr, State, Data> {
        // types
        typedef typename proto::result_of::child_c<Grammar, N>::type       GN;           // For each N in [0,Expr arity), for exposition only
        typedef typename proto::result_of::child_c<Expr, N>::type          EN;           // For each N in [0,Expr arity), for exposition only
        typedef typename boost::result_of<GN(EN,State,Data)>::type         RN;           // For each N in [0,Expr arity), for exposition only
        typedef typename Expr::proto_tag                                   T;            // For exposition only
        typedef boost::is_same<Domain, deduce_domain>                      Deduce;       // For exposition only
        typedef typename Expr::proto_domain                                DD;           // For exposition only
        typedef typename mpl::if_<Deduce, DD, Domain>::type                D;            // For exposition only
        typedef typename D::proto_generator                                G;            // For exposition only
        typedef proto::listN<R0,...RN>                                     A;            // For exposition only
        typedef proto::expr<T, A>                                          E;            // For exposition only
        typedef proto::basic_expr<T, A>                                    BE;           // For exposition only
        typedef typename mpl::if_<proto::wants_basic_expr<G>, BE, E>::type expr_type;    // For exposition only
        typedef typename boost::result_of<D(expr_type)>::type              result_type;

        // public member functions
        result_type operator()(typename impl::expr_param,
                               typename impl::state_param,
                               typename impl::data_param) const;
      };
    };

## Description

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> >
      >
    {};