[def __tr1__ [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1745.pdf
    C++ Standard Library Technical Report]]
[def __hash-function__ [@http://en.wikipedia.org/wiki/Hash_function
    hash function]]

[section:intro Introduction]

For accessing data based on keys, the C++ standard library offers `std::set`,
`std::map`, `std::multiset` and `std::multimap`. These are generally
implemented using balanced binary trees so accessing data by key is
consistently of logarithmic complexity. Which is generally okay, but not great.

Also, they require their elements to be ordered, and to supply a 'less than'
comparison object. For some data types this is impractacle, It might be slow
to calculate, or even impossible.

So the __tr1__ provides unordered associative containers. These will store data
with no ordering, and typically allow for fast constant time access. Their
worst case complexity is linear, but this is generally rare, and with
care can be avoided. There are four containers to match the existing
associate containers:
[classref boost::unordered_set unordered_set],
[classref boost::unordered_map unordered_map],
[classref boost::unordered_multiset unordered_multiset] and
[classref boost::unordered_multimap unordered_multimap].

The fast lookup speeds are acheived using a __hash-function__. The basic idea
is that a function is called for the key value, which is used to index the
data. If this hash function is carefully chosen the different keys will usually
get different indicies, so there will only be a very small number of keys for
each index, so a key lookup won't have to look at many keys to find the correct
one.

[endsect]