Remove element from std::map based on the time of insertion

Question

I need to erase elements from an std::map based on the time of insertion (or something else more efficient than that).

The map will probably hold thousands of elements and if I store the time and iterate the map to check each elements time, it will probably end up being quite time consuming.

Does anyone have any good idea how to erase elements from a std::map when they are getting old?

Answer 1

The std::map<> type has no notion of when an element was inserted. It only serves to hold a key / value pair mapping. It also has no notion of insert order so it can't even provide a relative type of insert.

To do what you want you'll need to add an association between the elements and the time they were inserted. If all you want is relative order then you could use a std::queue paired with the map. Every time you insert into the map you insert into the std::queue as well. Elements at front of the queue are older than the back and you can use that for relative age

Answer 2

Pretty close to LRU Cache.

The Boost.MultiIndex library shows an example of MRU Cache (Most Recently Used), so adapting it to LRU should be trivial.

Basically the idea is to maintain two data structures in parallel:

• a map with the items in
• a deque with references into the map

Basic code:

static double const EXPIRY = 3600; // seconds

std::map<Key, Value> map;
std::deque<std::pair<std::map<Key, Value>::iterator, time_t>> deque;

bool insert(Key const& k, Value const& v) {
  std::pair<std::map<Key, Value>::iterator, bool> result =
    map.insert(std::make_pair(k, v));

  if (result.second) {
    deque.push_back(std::make_pair(result.first, time()));
  }

  return result.second;
}

// to be launched periodically
void clean() {
  while (not deque.empty() and difftime(time(), deque.front().second) > EXPIRY) {
    map.erase(deque.front().first);
    deque.pop_front();
  }
}

Of course, those structures need be synchronized if the intent is to get multi-threaded code.

Answer 3

You can use a queue, and insert pointers to objects as they are inserted into the map. The next item in the queue will be the oldest one. Or you can store a pair in the queue if you also need the time of insertion.