Struct template pass_through

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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.

## Synopsis

    template<typename Grammar>
    struct pass_through :  proto::transform< pass_through<Grammar> > {
      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 proto::expr<T, proto::listN<R0,...RN> >              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 `V`, the result of applying the `proto::pass_through<plus<T0, T1> >` transform is:

    plus<
      boost::result_of<T0(E0, S, V)>::type,
      boost::result_of<T1(E1, S, V)>::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 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> >
      >
    {};