IDL4CPP — underlying_type traits

The underlying_type traits is maybe something to remove, std::underlying_type does the same at the moment bitmask is implemented using an enum. Also I am doubting whether an enum or enum class is the best mapping, have you tried to implement some user code, there is no operator bool() for example, some for checking if a bit is set we always have to cast. Maybe mapping to a class with operator is easier for the user, or use an enum class with operators.