<hash_set> equality operator doesn't work in VS2010

Question

Sample code:

std::hash_set<int> hs1; // also i try std::unordered_set<int> - same effect 
std::hash_set<int> hs2;

hs1.insert(15);
hs1.insert(20);

hs2.insert(20);
hs2.insert(15);

assert(hs1 == hs2);

hash_set doesn't stores elements in some order defined by hash function... why? Please note that this code works in VS2008 using stdext::hash_set.

Answer 1

Finally found the reference in the final draft at 23.2.5, note 11:

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent-key group [Ea1,Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1,Eb2) obtained from b.equal_range(Ea1), such that distance(Ea1, Ea2) == distance(Eb1, Eb2) and is_permutation(Ea1, Ea2, Eb1) returns true.

I would bet hash_set is now implemented in term of unordered_set (to begin with), but I still don't understand why in your case it would fail.

The complexity requirement is O(N) in the average case but degenerates to O(N²) in the worst case because of the linear-chaining implementation requirement.

Answer 2

It looks like equality comparisons are broken for both hash_set and unordered_set in Visual C++ 2010.

I implemented a naive equality function for unordered containers using the language from the standard quoted by Matthieu to verify that it's a bug (just to be sure):

template <typename UnorderedContainer>
bool are_equal(const UnorderedContainer& c1, const UnorderedContainer& c2)
{
    typedef typename UnorderedContainer::value_type Element;
    typedef typename UnorderedContainer::const_iterator Iterator;
    typedef std::pair<Iterator, Iterator> IteratorPair;

    if (c1.size() != c2.size())
        return false;

    for (Iterator it(c1.begin()); it != c1.end(); ++it)
    {
        IteratorPair er1(c1.equal_range(*it));
        IteratorPair er2(c2.equal_range(*it));

        if (std::distance(er1.first, er1.second) != 
            std::distance(er2.first, er2.second))
            return false;

        // A totally naive implementation of is_permutation:
        std::vector<Element> v1(er1.first, er1.second);
        std::vector<Element> v2(er2.first, er2.second);

        std::sort(v1.begin(), v1.end());
        std::sort(v2.begin(), v2.end());

        if (!std::equal(v1.begin(), v1.end(), v2.begin()))
            return false;
    }

    return true;
}

It returns that hs1 and hs2 from your example are equal. (Somebody let me know if you spot a bug in that code; I didn't really test it extensively...)

I'll file a defect report on Microsoft Connect.

Answer 3

i ask this question here but give no response =) thanks for your feedback.

i also create some simple console test (just to be sure):

#include <iostream>
#include <hash_set>
int main(int argc, char* argv[])
{   
  stdext::hash_set<int> hs1, hs2;
  hs1.insert(10);
  hs1.insert(15);
  hs2.insert(15);
  hs2.insert(10);
  std::cout << ((hs1 == hs2) ? "It works!" : "It NOT works") << std::endl;
  return EXIT_SUCCESS;
}

and compile it. using vs2008 command prompt:

cl.exe HashSetTest.cpp /oHashSetTest2008.exe 

using vs2010 command prompt:

cl.exe HashSetTest.cpp /oHashSetTest2010.exe

I really see different results =)