/** Reference operator.
*
* Returns a reference to the object that is associated with a
* particular key. If the cache_map does not already contain such an
* object, operator[] inserts the default object data_type().
*
* @return a reference to an item, found using the key or newly created.
*/
data_type& operator[]( const key_type& key )
{ return m_ht.find_or_insert( key ).second; }
