void put( const key_t &key, const value_t &value){
auto it = cache_map.find(key);
if (it != cache_map.end()) {
cache_list.erase(it -> second );
cache_map.erase(it);
}
cache_list.push_front(pair(key, value));
cache_map[key] = cache_list.begin();
if (cache_map.size() > max_size)
{
auto last = cache_list.end();
last--;
cache_map.erase( last -> first );
cache_list.pop_back();
}
}