Question

Does such a thing exist? or could anyone please recommend how I could implement such a container?

basically I have a std::map which uses a 64bit integer as its key and a custom datatype as the containing item.

I need to be able to periodically remove items that havent been accessed in a while in the most optimal way. does anyone have any suggestions for this?

cheers

Answer 1

One idea: maintain a std::deque which gets an iterator into your map element pushed to the front whenever accessing the map. You can then easily look at the deque to tell which elements have been used most recently.

Some C++ sketch (void of error checking, the point is to demonstrate that the deque is updated when accessing the map, and you can lateron trim the map).

class MyMap {
  typedef std::map<int64_t, void *> Map;
  Map m_map;
  std::deque<Map::iterator> m_recentlyUsedItems;

public:
  void *getItem( int64_t key ) {
    Map::iterator it = m_map.find( key );
    if ( it == m_map.end() ) {
      return 0;
    }
    m_recentlyUsedItems.push_front( it );
    return it->second;
  }

  void removeAllButMostRecentlyUsedItems( int n ) {
    std::deque<Map::iterator> it = m_recentlyUsedItems.begin();
    advance( it, n );
    std::deque<Map::iterator> it2 = it;
    for ( ; it2 != m_recentlyUsedItems.end(); ++it2 ) {
      m_map.erase( *it2 );
    }
    m_recentlyUsedItems.erase( it, m_recentlyUsedItems.end() );
  }
};