A good hash function for a vector

Question

I have some vector of integer that I would like to store efficiently in a unordered_map in c++11 my question is this:

How do I best store these and optimize for .find queries?

I came up with the following hasher:

class uint32_vector_hasher {
public:
  std::size_t operator()(std::vector<uint32_t> const& vec) const {
    std::size_t ret = 0;
    for(auto& i : vec) {
      ret ^= std::hash<uint32_t>()(i);
    }
    return ret;
  }
};

and then store the objects in an unordered_map I do however have a couple of questions

1. how often does the hash get calculated, only one, some random number or times?
2. Would it make sense to create a wrapper object with == and hash functions to make memorize the hash and avoid it being calculated more than once?

When profiling I've noticed that a rather large amount of my cpu time is spend doing lookups on the unordered maps, this is not exactly optimal :(

Solution 1

Always go with the experts when possible: http://www.boost.org/doc/libs/release/doc/html/hash/reference.html#boost.hash_combine

Solution 2

So, when not wanting to use boost, Michael Blurr's comment led to the following hash function implementation:

std::size_t operator()(std::vector<uint32_t> const& vec) const {
  std::size_t seed = vec.size();
  for(auto& i : vec) {
    seed ^= i + 0x9e3779b9 + (seed << 6) + (seed >> 2);
  }
  return seed;
}

Seems to work.