std::multimap

std::multimap<Key, Value> is the standard library's sorted associative container of key-value pairs, with the uniqueness rule dropped. Multiple entries can share the same key, which makes it useful for one-to-many relationships: a customer to many orders, a product to many reviews, an author to many books. It uses the same red-black tree as std::map, so every key-touching operation is O(log n), and iteration walks entries in key-sorted order. This chapter covers what changes once duplicate keys are allowed, why operator[] and at are gone, how to access values for a given key, and the workloads where multimap fits well.