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

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

template<typename Grammar> 
struct pass_through :  proto::transform< pass_through<Grammar> > {
  // 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 typename Expr::proto_domain                                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 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> >
  >
{};

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