When defining an ti(unordered_map) type five template arguments must be
specified :
    itemization(
    it() a i(KeyType) (becoming ti(unordered_map::key_type)),
    it() a i(ValueType) (becoming ti(unordered_map::mapped_type)),
    it() the type of an object computing a hash value from a key value
        (becoming ti(unordered_map::hasher)), and
    it() the type of an object that can compare two keys for equality (becoming
        ti(unordered_map::key_equal)).
    it() the type of its allocator. This is usually left unspecified, using
        the allocator provided by default by the implementor.
    )

The generic definition of an tt(unordered_map) container looks like this:
        verb(    std::unordered_map <KeyType, ValueType, hash type, predicate type,
                        allocator type>)

When tt(KeyType) is tt(std::string) or a built-in type then default types
are available for the hash type and the predicate type. In practice the
allocator type is not specified, as the default allocator suffices.  In these
cases an tt(unordered_map) object can be defined by merely specifying the key-
and value types, like this:
        verb(    std::unordered_map<std::string, ValueType> hash(size_t size = implSize);)

Here, tt(implSize) is the container's default initial size, which is
specified by the implementor. The map's size is automatically enlarged by the
tt(unordered_map) when necessary, in which case the container em(rehashes) all
its elements. In practice the default tt(size) argument provided by the
implementor is completely satisfactory.

The tt(KeyType) frequently consists of text. So, a tt(unordered_map) using a
tt(std::string KeyType) is frequently used. Be careful not to use a plain
tt(char const * key_type) as two tt(char const *) values pointing to equal
bf(C)-strings stored at different locations are considered to be different
keys, as their pointer values rather than their textual content are
compared. Here is an example showing how a tt(char const * KeyType) can be
used. Note that in the example no arguments are specified when constructing
tt(months), since default values and constructors are available:
        verbinclude(-a examples/hash2.cc)

If other tt(KeyTypes) must be used, then the tt(unordered_map)'s constructor
requires (constant references to) a hash function object, computing a hash
value from a key value, and a predicate function object, returning tt(true) if
two tt(unordered_map::key_type) objects are identical.  A em(generic
algorithm) (see chapter ref(GENERIC)) exists performing tests of equality
(i.e., tt(equal_to)). These tests can be used if the key's data type supports
the equality operator. Alternatively, an overloaded ti(operator==) or
specialized i(function object) could be constructed returning tt(true) if two
keys are equal and tt(false) otherwise.

bf(Constructors)

The tt(unordered_map) supports the following constructors:
    itemization(
    it() The copy and move constructors are available;
    itt(explicit unordered_map+CHAR(40)size_type n = implSize,
             hasher const &hf = hasher(),)nl()
             tt(key_equal const &eql = key_equal(),)nl()
             tt(allocator_type const &alloc = allocator_type()CHAR(41)):
        this constructor can also be used as default constructor;
    itt(unordered_map(const_iterator begin,
                    const_iterator end,
                    size_type n = implSize, hasher const &hf = hasher(),
                    key_equal const &eql = key_equal(),
                    allocator_type const &alloc = allocator_type())):
        this constructor expects two iterators specifying
a range of tt(unordered_map::value_type const) objects, and
    itt(unordered_map(initializer_list<value_type> initList,
                    size_type n = implSize, hasher const &hf = hasher(),
                    key_equal const &eql = key_equal(),
                    allocator_type const &alloc = allocator_type())):
            a constructor expecting an tt(initializer_list) of
            tt(unordered_map::value_type) values.
    )

    The following example shows a program using an unordered_map containing
the names of the months of the year and the number of days these months
(usually) have. Then, using the subscript operator the days in several months
are displayed (the predicate used here is the generic algorithm
tt(equal_to<string>), which is provided by the compiler as the default fourth
argument of the tt(unordered_map) constructor):
        verbinclude(-a examples/hash.cc)
