On 03.09.2009, at 04:43, Andrei Alexandrescu wrote:
I'm defining an "optional" type for D's standard library modeled similarly to Boost.optional. An interesting question came up - should optional<optional<T>> fold itself into optional<T>, or is "double optional" an interesting concept of its own?
I think optional<optional<T>> should not be folded into optional<T>. One use-case occurs with databases. Consider "SELECT myint FROM mytable WHERE mywhatever='foo'". When returning the result of such a query, the C++ function that executed it could return an empty outer optional if the database returned no row (WHERE-clause didn't match anything), an empty inner optional when myint is SQL's NULL and the integer value of myint in the then non-empty inner optional<int> otherwise. Regards, Daniel