[Note: I don't know why my e-mail address got mangled. It should be "greened@..."] Larry Evans <cppljevans@suddenlink.net> writes:
Could you not use boost::variant
Thanks for your feedback! boost::variant can do some similar things but I think it is really aimed at a different problem. As I understand it, boost::variant's primary use case is to function as a type safe generalized union. Thus it has to deal with things like alignment, etc. that the presented technique (which I call "value dispatch" for lack of a better name) does not. A quick glance at boost::variant source code shows that it is quite complex. IMHO, value dispatch is much simpler. I was aiming for a value-centered analogue to virtual functions as opposed to an enhanced union type. boost::variant feels opaque to me (get<>, etc.) while value dispatch seems more natural and transparent for the use cases I had in mind. Note also that boost::variant appears to require or at least encourage inheritance (e.g. static_visitor) while value dispatch does not. I don't think that's a strong positive or negative as long as value semantics are maintained. That said, I'll now explain how I think inheritance might be useful with value dispatch. :) I think it should be possible to inherit from vehicle and/or visitor<vehicle ...> to allow new behaviors for derived types while maintaining value semantics. The trick is to figure out how to re-use, for example, the vehicle::drive_impl and vehicle::visit_dispatch routines without slicing. Making the object type a type parameter might help. It is an interesting problem but I haven't really looked at it yet. A not-currently-existing C++ feature would also be helpful in avoiding code duplication: enum class my_kind : vehicle::kind { // Inherit vehicle::kind names and values Submarine, Tractor }; Maybe all this isn't compelling over boost::variant but I've found it quite useful in a large project. -Dave