Niels Dekker - mail address until 2008-12-31 wrote:
Howard Hinnant wrote: swap1 time is 0.172796 seconds swap2 time is 1.3e-05 seconds
David Abrahams wrote:
Your program gives about the results I'd have expected for the case you tested, but it doesn't address what I was concerned about.
Howard's example shows that a swap overload for std::tr1::array effectively reduces the complexity of swapping a tr1::array<vector<int>,N> from O(M*N) to O(N), when the array has N vectors, each having size M. And it provides no-throw.
I know what it shows.
Aren't those the issues you were concerned about?
No. swap for a given array<> is not guaranteed to be nothrow unless all the elements have a nothrow swap. But I guess in a world where all swaps are nothrow, this is a non-issue. But the efficiency is a different issue. It's like the same reason we don't provide random access for list iterators: operator++ on an iterator is supposed to be fundamentally O(1) (notwithstanding filter_iterator -- how do you measure /that/?!) I am inclined to agree that a fast swap for array is useful and thus should be provided, but I still have a nagging doubt about whether it's "the right thing to do." Maybe I've been brainwashed by a certain disciple of Alex Stepanov's, because nobody else seems to share that doubt... so feel free to ignore me if you're unconvinced.
Any type that is both CopyConstructible and Assignable is Swappable, by definition.
Unless you define Swappable to mean "has an O(1) swap." or unless you meant "by implementation" instead of "by definition" ;-)
Within generic code, people typically use the well-known idiom to swap a pair of Swappable objects:
// Allow argument-dependent lookup to find a custom swap. using std::swap; swap(lhs, rhs);
As long as boost::swap hasn't come out of the sandbox, of course (hint)!
??
Don't you think it /always/ makes sense to provide a custom swap overload for a Swappable type, as long as it outperforms the default std::swap, in one way or the other?
That's the whole question. I'm not 100% sure. -- Dave Abrahams BoostPro Computing http://www.boostpro.com