[boost] boost::optional<A,B,...>

Subject: [boost] boost::optional<A,B,...>
From: Daniel Larimer (dlarimer_at_[hidden])
Date: 2010-03-04 00:48:00

I have a situation where I have a class that stores two mutually exclusive optional values.  This results in double overhead of two bools and twice the "reserved" space.   I could opt for a boost::optional<boost::variant<A,B> >, but this would incur extra costs associated with the implementation of boost::variant.   Considering the fact that boost::variant<> explicitly guarantees to be always "valid", it seems like it would be reasonable to have boost::optional<A,...> that behaves like boost::variant<>, yet is optimized specifically for allowing "empty" states as well. 

The boolean could then become the type index where 0 is empty, 1 is A, 2 is B etc.   

Thoughts?

Dan