What needs to be done to use a class as a key in an unordered_map?

To use a custom class as a key in std::unordered_map, you need to:

1. Overload the equality comparison operator (operator==) for your class. unordered_map uses it to determine key equality.
2. Provide a hash function for your class. This can be done in one of the following ways:
   • Specialize the template struct std::hash for your class.
   • Pass a hash functor object as the third argument to the std::unordered_map constructor.

Example of specializing std::hash:

#include <functional>

struct MyClass {
    int id;
    std::string name;

    // operator== required
    bool operator==(const MyClass& other) const {
        return id == other.id && name == other.name;
    }
};

// Specialization of std::hash for MyClass
namespace std {
    template <>
    struct hash<MyClass> {
        size_t operator()(const MyClass& obj) const {
            // Combine hashes of members
            return std::hash<int>()(obj.id) ^ (std::hash<std::string>()(obj.name) << 1);
        }
    };
}

Example of passing a hash functor to the constructor:

#include <functional>
#include <unordered_map>

struct MyClass {
    int id;
    std::string name;

    bool operator==(const MyClass& other) const {
        return id == other.id && name == other.name;
    }
};

struct MyClassHasher {
    size_t operator()(const MyClass& obj) const {
        return std::hash<int>()(obj.id) ^ (std::hash<std::string>()(obj.name) << 1);
    }
};

// Usage
std::unordered_map<MyClass, int, MyClassHasher> myMap;

It is important that the hash function is deterministic (always returns the same hash for the same object) and provides good distribution of hashes to minimize collisions.