unordered_map is an associated container that stores elements formed by the combination of key-value and a mapped value. The key value is used to uniquely identify the element and the mapped value is the content associated with the key. ... You can read more about this on how-to-use-unordered_map-efficiently-in-c.

What is std :: unordered_map?

Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity. ... std::unordered_map meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

What does unordered_map return?

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.