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## Combining hash values

Say you have a point class, representing a two dimensional location:

    class point
    {
        int x;
        int y;
    public:
        point() : x(0), y(0) {}
        point(int x, int y) : x(x), y(y) {}

        bool operator==(point const& other) const
        {
            return x == other.x && y == other.y;
        }
    };

and you wish to use it as the key for an `unordered_map`. You need to customise the hash for this structure. To do this we need to combine the hash values for `x` and `y`. The function `boost::hash_combine` is supplied for this purpose:

    class point
    {
        ...

        friend std::size_t hash_value(point const& p)
        {
            std::size_t seed = 0;
            boost::hash_combine(seed, p.x);
            boost::hash_combine(seed, p.y);

            return seed;
        }

        ...
    };

Calls to hash_combine incrementally build the hash from the different members of point, it can be repeatedly called for any number of elements. It calls `hash_value` on the supplied element, and combines it with the seed.

Full code for this example is at /libs/functional/hash/examples/point.cpp.

| Note
When using `boost::hash_combine` the order of the calls matters.
        std::size_t seed = 0;
        boost::hash_combine(seed, 1);
        boost::hash_combine(seed, 2);
results in a different seed to:
        std::size_t seed = 0;
        boost::hash_combine(seed, 2);
        boost::hash_combine(seed, 1);
If you are calculating a hash value for data where the order of the data doesn't matter in comparisons (e.g. a set) you will have to ensure that the data is always supplied in the same order.

To calculate the hash of an iterator range you can use `boost::hash_range`:

    std::vector<std::string> some_strings;
    std::size_t hash = boost::hash_range(some_strings.begin(), some_strings.end());

Note that when writing template classes, you might not want to include the main hash header as it's quite an expensive include that brings in a lot of other headers, so instead you can include the `<boost/functional/hash_fwd.hpp>` header which forward declares `boost::hash`, `boost::hash_range` and `boost::hash_combine`. You'll need to include the main header before instantiating `boost::hash`. When using a container that uses `boost::hash` it should do that for you, so your type will work fine with the boost hash containers. There's an example of this in template.hpp and template.cpp.