STRUCT TE(3)

NAME
boost::proto::when<Grammar,_R(A...)> - A grammar element and a Transform that associates a transform with the grammar.

SYNOPSIS

// In header: <boost/proto/transform/when.hpp>
template<typename Grammar, typename R, typename... A>
struct when<Grammar, R(A...)> :  proto::transform< when<Grammar, R(A...)> > {
  // types
  typedef typename Grammar::proto_grammar proto_grammar;
  // member classes/structs/unions
  template<typename Expr, typename State, typename Data>
  struct impl :  proto::transform_impl< Expr, State, Data > {
    // types
    typedef proto::call<R(A...)>                                       call_;        // For exposition only
    typedef proto::make<R(A...)>                                       make_;        // For exposition only
    typedef typename mpl::if_<proto::is_callable<R>,call_,make_>::type which;        // For exposition only
    typedef typename boost::result_of<which(Expr, State, Data)>::type  result_type;
    // public member functions
    result_type operator()(typename impl::expr_param,
                           typename impl::state_param,
                           typename impl::data_param) const;
  };
};

DESCRIPTION

Use proto::when<> to override a grammar's default transform with a custom transform. It is for use when composing larger transforms by associating smaller transforms with individual rules in your grammar.

The when<G, R(A...)> form accepts either a CallableTransform or an ObjectTransform as its second parameter. proto::when<> uses proto::is_callable<R>::value to distinguish between the two, and uses proto::call<> to evaluate CallableTransforms and proto::make<> to evaluate ObjectTransforms.