...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::proto::when<Grammar, R(A...)> — A grammar element and a Transform 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_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; }; };
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.