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Struct template when<Grammar, R(A...)>

boost::proto::when<Grammar, R(A...)> — A grammar element and a PrimitiveTransform that associates a transform with the grammar.


// 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_base_expr proto_base_expr;
  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;


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.