...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, > — A grammar element that associates an externally-specified transform with the grammar. The transform is looked up in the Data parameter using the Grammar as a key.
// In header: <boost/proto/transform/when.hpp> template<typename Grammar> struct when<Grammar, proto::external_transform> : proto::transform< when<Grammar, proto::external_transform> > { // types typedef typename Grammar::proto_grammar proto_grammar; // member classes/structs/unions template<typename Expr, typename State, typename Data> struct impl : boost::remove_reference< Data >::type ::template when< Grammar > ::template impl< Expr, State, Data > { }; };
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, proto::external_transform>
indicates that the associated transform is not yet known. It should be looked up when the transform
is about to be applied. It is found by looking it up in the passed-in Data parameter, which
behaves like a compile-time map from grammar types to transform types. The map is indexed using
Grammar
as a key. The associated value type is used as the transform
to apply. In this way, the same grammar can be used to define multiple evaluating strategies that
can be added post-hoc.
See proto::external_transforms
for an example.