Explanation on the set--unordered feature

Unordered set is a container that stores unique elements in no specific order, which allows fast retrieval of each element based on its value.
In an unordered_set, an element's value is at the same time its key, identifying it uniquely. Keys are immutable, so elements in an unordered_set cannot be modified once in the container - they can be inserted or removed, though.
Internally, the elements of the unordered_set are sorted in any particular order, but organized according to their hash values ​​to allow fast access to individual elements directly by their values ​​(with average constant average time complexity ) value area.
Unordered_set containers can access individual elements by key faster than group containers, although they are generally inefficient for range iteration through a subset of their elements.

Iterators in containers are at least forward iterators.

Regarding the issue of set sorting being out of order, the reason is that set uses a hash table as a memory index.

Detailed introduction can be found at:

and wiki:

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