on Sat Dec 27 2008, Howard Hinnant <hinnant-AT-twcny.rr.com> wrote:
On Dec 27, 2008, at 6:57 PM, David Abrahams wrote:
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.
I take it you mean that nobody has ever complained to you that sizeof(vector) should have been one word.
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.
By the same token, it seems logical to me that if reducing the size of a vector to 3 words matters, reducing it to one word matters even more ;-)
Concerning your suggestion about container<unique_ptr<unordered_set<A>>> as a substitute for container<unordered_set<A>> (or equivalent),
That's a twisty way to say it. Just to be sure we understand one another, I'll clarify that I never suggested about asking clients to use the unique_ptr approach. Rather, my suggestion was that the library use unique_ptr (or equivalent) under the covers to minimize the size of empty data structures.
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.
I'm guessing that most of those clients either haven't had the opportunity to make the measurement, or else they use unique_ptr (or equivalent) explicitly to get the effect they want. Also, the lack, up to now, of a conforming auto_ptr-sized thingy that can be stored in containers has surely impacted the measurements people are able to conveniently make.
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).
Me too...
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... ).
Probably you should be preaching to me. That protection was added in 2004, and although I worry about unintended ADL a lot, the technique I used there wouldn't help anyone trying to leverage EBO. In fact overuse of boost::noncopyable can undermine EBO for reasons pointed out in...
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 ).
...Nathan's article. Nice article, Nathan! I never saw that before. The conundrum here is that if we use that technique for tuple, then a tuple of empty types can never be empty itself. I'm starting to get ill just thinking about a generic framework for composing "logically empty" types that undoes this "EBO erasure" effect when necessary. -- Dave Abrahams BoostPro Computing http://www.boostpro.com