2008/11/24 Thorsten Ottosen <thorsten.ottosen@dezide.com>:
1. storing two functors had to do with implementing exception safety if copying of the functors may throw. Was it swap() that was the problem?
Here's the original post where I described the problem: http://lists.boost.org/Archives/boost/2005/02/80211.php I think it also applies to assignment.
2. the space optimization only gave little less space.
The cost from not using EBO amounts to a few bytes - which can sometimes be offset due to alignment. Since 4 bytes is typically required per bucket it doesn't seem like a large amount. Early versions used boost::compressed_pair but that had poor support for Visual C++ 6 (which I supported at the time). I've been thinking about reintroducing EBO, but there have also been plenty of other more pressing tasks. I'd probably implement it manually though, as I don't think using compressed_pair would buy much. I can't imagine a situation where the hash function will be empty and the equality predicate won't so there isn't much point in the extra complexity and dependency. Similarly, I'm not aware of an allocator that is empty for a bucket, but not for a node. Daniel