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### Tutorial

When using a hash index with Boost.MultiIndex, you don't need to do anything to use `boost::hash` as it uses it by default. To find out how to use a user-defined type, read the section on extending boost::hash for a custom data type.

If your standard library supplies its own implementation of the unordered associative containers and you wish to use `boost::hash`, just use an extra template parameter:

    std::unordered_multiset<std::vector<int>, boost::hash<int> >
            set_of_ints;

    std::unordered_set<std::pair<int, int>, boost::hash<std::pair<int, int> >
            set_of_pairs;

    std::unordered_map<int, std::string, boost::hash<int> > map_int_to_string;

To use `boost::hash` directly, create an instance and call it as a function:

    #include <boost/functional/hash/hash.hpp>

    int main()
    {
        boost::hash<std::string> string_hash;

        std::size_t h = string_hash("Hash me");
    }

If you wish to make use of the extensions, you will need to include the appropriate header (see the reference documentation for the full list).

    #include <boost/functional/hash/pair.hpp>

    int main()
    {
        boost::hash<std::pair<int, int> > pair_hash;

        std::size_t h = pair_hash(std::make_pair(1, 2));
    }

Or alternatively, include `<boost/functional/hash.hpp>` for the full library.

For an example of generic use, here is a function to generate a vector containing the hashes of the elements of a container:

    template <class Container>
    std::vector<std::size_t> get_hashes(Container const& x)
    {
        std::vector<std::size_t> hashes;
        std::transform(x.begin(), x.end(), std::insert_iterator(hashes),
            boost::hash<typename Container::value_type>());

        return hashes;
    }

| Copyright © 2005 Daniel James