Is there any possibility that std::unordered_map collides?

Question

I seen a post in here that you could "meet with the Birthday problem." while using std::unordered_map

When should I use unordered_map and not std::map

Which really surprises me, that is the same that saying std::unordered_map is unsafe to use. Is that true? If i'm not explaining myself, let me show you an example:

unordered_map<size_t, size_t> m;
for (size_t i = 0; i < 100; ++i)
    m[i] = i;
for (size_t i = 0; i < 100; ++i)
    if (m[i] != i)
        cerr << "ERROR!" << endl;

If this code is in main, is there any possibility that it prints ERROR!?

Collisions don't break it, they just make searching for the colliding element slower. — HolyBlackCat, Jan 21 '22 at 20:24
Depends what you mean by unsafe. The chances of collisions are real, but it doesn't mean you'll have that for every scenario. But it's something to keep in mind. — ShadowMitia, Jan 21 '22 at 20:24
If you do want to see a case of some pathological behavior (that's not limited to unordered_map, it affects map as well), try using `double` as the key type and inserting `NaN` keys. — Kyle, Jan 21 '22 at 21:24
Suggested reading: [Blowing up `unordered_map`, and how to stop getting hacked on it](https://codeforces.com/blog/entry/62393). — Evg, Jan 21 '22 at 21:29

eerorika · Accepted Answer · 2022-01-21T20:36:35.937

Is there any possibility that std::unordered_map collides?

It isn't the container that has collisions, but the hash function that you provide for the container.

And yes, all hash functions - when their output range is smaller than the input domain - have collisions.

is there any possibility that it prints ERROR!?

No, it isn't possible. It's completely normal for the hash function to put multiple values into a single bucket. That will happen in case of hash collision, but it also happens with different hash values. The lookup function will get the correct value using linear search. The identity of a key is determined by the equality function, not by the hash function.

Is there any possibility that std::unordered_map collides?

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