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Struct template impl

boost::proto::when<Grammar, >::impl


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

template<typename Expr, typename State, typename Data> 
struct impl :  
      typename mpl::eval_if_c<
        proto::result_of::has_env_var<Data, proto::transforms_type>::value,
        proto::result_of::env_var<Data, proto::transforms_type>,
        proto::result_of::env_var<Data, proto::data_type>
      ::template when< Grammar >
        ::template impl< Expr, State, Data >


The implementation of the impl struct depends on whether the Data parameter is a transform environment that contains a value corresponding to the proto::transforms_type key. If so, that value is treated as a map from rules to transforms. Otherwise, the Data type itself is treated as such a map.