Use unordered_map with Custom Keys

With an ordered map, the type of the key must be sortable, which means it must at least support the less-than < comparison operator. Suppose we want to use an associative container with a custom type that is not sortable. For example, a vector where $(0, 1)$ is not smaller or larger than $(1, 0)$, it simply points in a different direction. In such cases, we may still use the ...