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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());

| Copyright © 2005, 2006 Daniel James