[section:hash_equality Equality Predicates and Hash Functions]

[/TODO: A better introduction to hash functions?]

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 [classref boost::unordered_set]
is declared as:

    template<typename Value,
           typename Hash = ``[classref boost::hash]``<Value>, 
           typename Pred = std::equal_to<Value>, 
           typename Alloc = std::allocator<Value> > 
    class ``[classref boost::unordered_set unordered_set]``;

The hash function comes first as you might want to change the hash function
but not the equality predicate, while if you were to change the behaviour
of the equality predicate you would have to change the hash function to match
it.

For example, if you wanted to use

[endsect]