When using the boost library, the fuction boost::hash_combine works like this:
seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
http://www.boost.org/doc/libs/1_46_1/doc/html/hash/reference.html#boost.hash_combine
What is the advantage of this approach vs simply XOR-ing?
With XOR-ing, one can even use the hash function to use unordered containers as keys, while this one is order dependent.
There are many ordered containers such as lists. If you would use XOR then you would basically say that [0, 1] is the same as [1, 0]. That's obviously not the case. It's much easier to override the method for unordered containers than to impose one that will create a lot of collisions for ordered ones. XOR has a lot of other nasty properties. For instance, if you have duplicate elements then they will cancel each other out.
In the end the idea for a hash is to be reasonably sure that you don't get the same value for multiple elements. XOR doesn't fit that property by itself.
