How to retrieve collisions of unordered map?

Question

I have two elements (6 and 747) that share their key ("eggs"). I want to find all the elements that share a key (let's say "eggs", but I would in real life do that for every key). How to do that?

There must be a way to get a container or something back from the data structure . . .

Answer 1

You're still mistaking key's value with key's hash. But to answer question as asked: you can use unordered_map's bucket() member function with bucket iterators:

std::unordered_map<int,int,dumbest_hash> m;
m[0] = 42;
m[1] = 43;

size_t bucket = m.bucket(1); 

for(auto it = m.begin(bucket), e = m.end(bucket); it != e; ++it) {
    cout << "bucket " << bucket << ": " << it->first << " -> " << it->second << '\n';
}

demo

In simple and mostly correct terms, unordered containers imitate their ordered counterparts in terms of interface. That means that if a map will not allow you to have duplicate keys, then neither will unordered_map.

unordered do employ hashing function to speed up the lookup, but if two keys have the same hash, they will not necessarily have the same value. To keep the behaviour similar to the ordered containers, unordered_set and unordered_map will only consider elements equal when they're actually equal (using operator== or provided comparator), not when their hashed values collide.

To put things in perspective, let's assume that "eggs" and "chicken" have the same hash value and that there's no equality checking. Then the following code would be "correct":

unordered_map<string, int> m;
m["eggs"] = 42;
m.insert(make_pair("chicken", 0)); // not inserted, key already exists
assert(m["chicken"] == 42);

But if you want allow duplicate keys in the same map, simply use unordered_multimap.

Answer 2

Unordered map does not have elements that share a key.

Unordered multi map does.

Use umm.equal_range(key) to get a pair of iterators describing the elements in the map that match a given key.

However, note that "collision" when talking about hashed containers usually refers to elements with the same hashed key, not the same key.

Also, consider using a unordered_map<key, std::vector<value>> instead of a multimap.