While the associative containers use an ordering relation to specify how the elements are stored, the unordered associative containers use an equality predicate and a hash function. For example, boost::unordered_map is declared as:

template <
    class Key, class Mapped,
    class Hash = boost::hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<Key const, Mapped> > >
class unordered_map;

The hash function comes first as you might want to change the hash function but not the equality predicate. For example, if you wanted to use the FNV-1 hash you could write:

boost::unordered_map<std::string, int, hash::fnv_1>
    dictionary;

There is an implementation of FNV-1 in the examples directory.

If you wish to use a different equality function, you will also need to use a matching hash function. For example, to implement a case insensitive dictionary you need to define a case insensitive equality predicate and hash function:

struct iequal_to
{
    bool operator()(std::string const& x,
        std::string const& y) const
    {
        return boost::algorithm::iequals(x, y, std::locale());
    }
};

struct ihash
{
    std::size_t operator()(std::string const& x) const
    {
        std::size_t seed = 0;
        std::locale locale;

        for(std::string::const_iterator it = x.begin();
            it != x.end(); ++it)
        {
            boost::hash_combine(seed, std::toupper(*it, locale));
        }

        return seed;
    }
};

Which you can then use in a case insensitive dictionary:

boost::unordered_map<std::string, int, ihash, iequal_to>
    idictionary;

This is a simplified version of the example at /libs/unordered/examples/case_insensitive.hpp which supports other locales and string types.

Caution

Be careful when using the equality (==) operator with custom equality predicates, especially if you're using a function pointer. If you compare two containers with different equality predicates then the result is undefined. For most stateless function objects this is impossible - since you can only compare objects with the same equality predicate you know the equality predicates must be equal. But if you're using function pointers or a stateful equality predicate (e.g. boost::function) then you can get into trouble.

Custom Types

Similarly, a custom hash function can be used for custom types:

struct point {
    int x;
    int y;
};

bool operator==(point const& p1, point const& p2)
{
    return p1.x == p2.x && p1.y == p2.y;
}

struct point_hash
{
    std::size_t operator()(point const& p) const
    {
        std::size_t seed = 0;
        boost::hash_combine(seed, p.x);
        boost::hash_combine(seed, p.y);
        return seed;
    }
};

boost::unordered_multiset<point, point_hash> points;

Since the default hash function is Boost.Hash, we can extend it to support the type so that the hash function doesn't need to be explicitly given:

struct point {
    int x;
    int y;
};

bool operator==(point const& p1, point const& p2)
{
    return p1.x == p2.x && p1.y == p2.y;
}

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;
}

// Now the default function objects work.
boost::unordered_multiset<point> points;

See the Boost.Hash documentation for more detail on how to do this. Remember that it relies on extensions to the draft standard - so it won't work for other implementations of the unordered associative containers, you'll need to explicitly use Boost.Hash.