std::unordered_map 如何表现？ [C++]

Question

I was reading about C++ unordered_map and have few questions which I can't answer clearly.

I have noticed that unordered_map puts one element per index and not multiple (hash tables can be implemented using on of these methods to solve 2 elements being mapped to same index)

1. If an element with key A in the map, and I insert another one it will override the previous element right? (no matter if the values match or not).

2. Is there an option to change the behavior of unordered_map so it can contains multiple elements per index like I explained above?

3. I have noticed from my code that unordered_map gets bigger by one every-time I insert a new element but that's really bad, I learnt in hash table we need to multiply size by 2 every-time it's needed so complexity for inserting is O(1) Amortized and not O(n).

4. How can I give my unordered_map an initial size but with no elements? ie I know it will reach size 1000 so instead of making it bigger multiple times and doing heavy copying we can do it straightforward.

Answer 1

Let me shortly answer your questions

1. No, you cannot insert a new key. The function will fail. Keys must be unique.
2. No, there is no option. If you want to store multiple same keys, the you need to use a std::unordered_multimap instead
3. You mix up bucket with number elements. Of course the mapwill grow, if you enter a new key. If there would be the same hash value for 2 different keys, then those would be stored in a bucket for the hash value.
4. This does not make that much sense. See 3. And, also logically: A map contains keys. And if a key is in, then you cannot overwrite. However, you may use its reserve function, to allocate internal space. . .

All this is explained in the CPP Reference. Please see here