operator overloading() for a user-defined type in unordered_map

Question

I was looking at this post C++ unordered_map using a custom class type as the key

1. I understand that we need to redefine equality and hash code for a custom type key.
2. I know how the operator overloading works in general.

However, what does operator() have to do with the hash code?
Does unordered_map internally evaluate a key with () operator somewhere?

Answer 1

The std::unordered_map uses an std::hash object to calculate the hash-values.

It will use the hash-object like a function, calling the operator() to do the calculation.

As a simple example, for an int key, it will be something like this:

int key = 123;  // Or whatever value...

std::hash<int> hash_object;
auto hash_value = hash_object(key);  // Calls hash_object.operator()(key)