equivalent LinkedHashmap in C++? - c++

Question

I have a Java program that I want to convert it to C++. So, there is a Linkedhashmap data structure used in the Java code and I want to convert it to C++. Is there an equivalent datatype for LinkedHashmap in C++?
I tried to use std::unordered_map, however, it does not maintain the order of the insertion.

Answer 1

C++ does not offer a collection template with the behavior that would mimic Java's LinkedHashMap<K,V>, so you would need to maintain the order separately from the mapping.
This can be achieved by keeping the data in a std::list<std::pair<K,V>>, and keeping a separate std::unordered_map<k,std::list::iterator<std::pair<K,V>>> map for quick look-up of the item by key:
On adding an item, add the corresponding key/value pair to the end of the list, and map the key to the iterator std::prev(list.end()).
On removing an item by key, look up its iterator, remove it from the list, and then remove the mapping.
On replacing an item, look up list iterator from the unordered map first, and then replace its content with a new key-value pair.
On iterating the values, simply iterate std::list<std::pair<K,V>>.

Answer 2

The insertion order contract on key iteration can be achieved with a balanced tree for log(n) performance. This is better than maintaining keys in a list as item removal requires n lookup time. My mantra is never put something you look up in a list. If it doesn't have to be sorted, use a hash. If it should be sorted, use a balanced tree. If all you're going to do is iterate, then a list is fine.
In c++ this would be std::map where the key is the item reference and the value is the insertion order, the keys are sorted using red-black trees. See: Is there a sorted container in STL

Answer 3

This is how I do it:
map<TKey, set<MyClass<K1,K2>, greater<MyClass<K1, K2>>>> _objects; // set ordered by timestamp. Does not guarantee uniqueness based on K1 and K2.
map<TKey, map<K2, typename set<MyClass<K1, K2>, greater<MyClass<K1, K2>>>::iterator>> _objectsMap; // Used to locate object in _objects
To add object id:
if (_objectsMap[userId].find(id) == _objectsMap[userId].end())
_objectsMap[userId][id] = _objects[userId].emplace(userId, id).first;
To erase an object id:
if (_objectsMap[userId].find(id) != _objectsMap[userId].end()) {
_objects[userId].erase(_objectsMap[userId][id]);
_objectsMap[userId].erase(id);
}
To retrieve, say the most recent size objects from the list starting from a specific object id:
vector<K2> result;
if (_objectsMap[userId].find(id) != _objectsMap[userId].end() && _objectsMap[userId][id] != _objects[userId].begin()) {
set<MyClass<K2, K2>, greater<MyClass<K1, K2>>>::iterator start = _objects[userId].begin(), end = _objectsMap[userId][id];
size_t counts = distance(_objects[userId].begin(), _objectsMap[userId][id]);
if (counts > size)
advance(start, counts - size);
transform(start,
end,
back_inserter(result),
[](const MyClass<K1, K2>& obj) { return obj.ID(); });
}
return result;