How is the comparison of objects used as keys in std::unordered_map implemented?

In std::unordered_map, objects used as keys are compared using the equality function, which by default is std::equal_to<Key>.

When inserting or searching for a key:

1. The hash code of the key is computed using a hash function (by default std::hash<Key>).
2. The corresponding bucket is searched using key comparison via the == operator or a specified equality predicate.

If the keys are user-defined types, it is necessary to:

• Define a specialization of std::hash for that type or provide your own hash function.
• Define the == operator or provide your own equality comparison function.

Example:

struct Key {
    int id;
    bool operator==(const Key& other) const {
        return id == other.id;
    }
};

namespace std {
    template<>
    struct hash<Key> {
        size_t operator()(const Key& k) const {
            return std::hash<int>()(k.id);
        }
    };
}

std::unordered_map<Key, std::string> map;