C++ unordered_map Default Value Behavior

In C++, you can write `my_map[key]++` even when `key` has never been inserted into the map. The `[]` operator silently creates a new entry rather than throwing an error. What are the requirements on the key type and value type for this to work? Describe what happens mechanically when you access a non-existent key, and explain what would happen with a custom key type that has no hash function defined.