On Dec 27, 2008, at 6:57 PM, David Abrahams wrote:
on Tue Nov 25 2008, Howard Hinnant <howard.hinnant-AT-gmail.com> wrote:
<nod> Believe it or not vector<unordered_set<A>> is a viable container, even when many of those inner containers are empty. If the overhead(unordered_set<A>) can be 5 words instead of 9 words (or more), then the ability to fit a data structure into nearly half the space is a significant size *and* speed optimization (for some clients).
It is never the number of words that matter in shaving off the space overhead of a container. It is the percent difference in space overhead between an optimized and unoptimized implementation that matters. Because for the clients who care, they have a very large number of containers which statically have a small size(). If you can cut the overhead(container) by 25% or more, it is a big win.
One thing I've always wondered about this: in that case -- at least when you're not also trying to use the small object optimization -- why not always use a Pimpl idiom where an empty container (with no capacity) can be represented by a single null pointer?
I've simply had clients complain to me: why is sizeof(vector) 4 words instead of 3? Please make it 3. Given that complaint, I can attempt to generate reasonable use cases which motivate such a complaint. But my basic premise is that those complaining know their needs far better than I do. I've never actually had anyone complain to me about the sizeof an unordered_set. However it seems logical to me that if clients are concerned about the overhead of one container, they could be concerned about the overhead of another (and thus any) container. Concerning your suggestion about container<unique_ptr<unordered_set<A>>> as a substitute for container<unordered_set<A>> (or equivalent), my best guess is that there may be clients with use cases where the median size of the inner container is small but non-zero, and thus the extra heap allocation and indirection ends up costing more. Suffice it to say that from a general purpose library point of view, more overhead is never better if you can get the equivalent functionality/performance out of less overhead. And historically, the design of the std::containers has given precedence to speed and space performance as opposed to turning basic exception safety into strong exception safety (and I agree with that design philosophy for a general purpose library).
It seems like you think compressed pair can only compress away one member, but IIUC you can chain them and then derive your hash table from that to compress as much as you like. Of course it would be nice to have a compressed_tuple that handles it all for us, but I thikn you _can_ do it.
I have high hopes that std::tuple will become the compressed_pair of the future. Technically, and especially with variadic template language support, it is not that hard to do (tuple is a pain no matter what without variadic template language support). Standards-wise, the current C++0X draft (N2798) does not outlaw, nor mandate, the empty member optimization for tuple,
Except that there is no "empty member optimization," right? Unless this is one of the langauge changes I've failed to notice (and there are a few), I think what you mean is that tuple is free to use derivation to take full advantage of the EBO, right?
and I hope to keep it that way. Recent straw polls on the LWG have been favorable to allowing the empty member optimization for std::tuple.
Until we get CWG to agree that the EMO exists, I doubt straw polls in LWG can make much difference ;-)
I used the term "empty member optimization" to refer to the library technique for optimization as opposed to a language optimization. Your assumption regarding my meaning referring to derivation under the covers is correct. As long as we're on the subject, I should warn off naive attempts at this optimization which look like: namespace std { template <class T, class A> class vector : private A { ... }; } // std Such an implementation of "EMO" is error prone as it associates the namespace of the client-supplied A with that of the vector. I'm not preaching to Dave with this statement as he has a proven track record in this department with his implementation of boost::noncopyable (http://www.boost.org/doc/libs/1_37_0/libs/utility/utility.htm#Class_noncopya... ). I mention this warning aimed at the widest possible audience. Instead one should use the techniques employed by boost::compressed_pair, or go back to (as far as I know) the originator of this technique, Nathan Myers (http://www.cantrip.org/emptyopt.html ). -Howard