Steven Watanabe <watanabesj@gmail.com> writes:
AMDG
On 08/15/2012 06:34 PM, greened@obbligato.org wrote:
I was aiming for a value-centered analogue to virtual functions as opposed to an enhanced union type.
Well, what you have is essentially a union type, even if that's not what you intended.
It entirely depends on how the classes arrange member data, no? In my "real" project, the classes all have std::vectors with different kinds of stuff in them, depending on the kind tag. The elements all use the same value dispatch mechanism to implement polymorphism. In that sense I guess it's sort of like a union, but certainly not in the traditional sense.
boost::variant feels opaque to me (get<>, etc.) while value dispatch seems more natural and transparent for the use cases I had in mind.
You should avoid using get<> on a variant. Whenever possible, apply_visitor is better.
That's no less opaque.
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.
Boost.Variant doesn't care about inheritance. static_visitor is like std::iterator or std::binary_function. It's just a helper that provides a result_type typedef. Anyway, static_visitor is for apply_visitor and doesn't have anything to do with the types stored in the variant.
That's why I said it's not a strong positive or negative. :) But I guess I'm sensing value dispatch isn't too interesting outside my own project. That's ok, it's made my life easier anyway and I found the lookup table implementation neat enough to share. :) -Dave