What needs to be done to use your class as a key in a map or unordered_map?

For std::map:

You need to define the comparison operator < for your class.

#include <map>
#include <string>

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

    // Define the "<" operator
    bool operator<(const MyKey& other) const {
        if (id != other.id) {
            return id < other.id;
        }
        return name < other.name;
    }
};

int main() {
    std::map<MyKey, int> myMap;
    MyKey key1{1, "apple"};
    MyKey key2{2, "banana"};
    myMap[key1] = 10;
    myMap[key2] = 20;
    return 0;
}

For std::unordered_map:

You need to define a hash function for your class and, if necessary, the equality operator ==. The standard hash std::hash does not support user-defined types.

Ways to define a hash:

1. Overload std::hash in the std namespace (not recommended):

   #include <unordered_map>
   #include <string>
   #include <functional> // For std::hash
   
   struct MyKey {
       int id;
       std::string name;
   
       // Define the "==" operator
       bool operator==(const MyKey& other) const {
           return id == other.id && name == other.name;
       }
   };
   
   // Specialize std::hash for MyKey
   namespace std {
       template <>
       struct hash<MyKey> {
           size_t operator()(const MyKey& key) const {
               // Simple hash combination example
               // More complex and robust hash functions may be preferable
               return hash<int>()(key.id) ^ (hash<std::string>()(key.name) << 1);
           }
       };
   }
   
   int main() {
       std::unordered_map<MyKey, int> myUnorderedMap;
       MyKey key1{1, "apple"};
       MyKey key2{2, "banana"};
       myUnorderedMap[key1] = 10;
       myUnorderedMap[key2] = 20;
       return 0;
   }
   

2. Pass the hasher as a template parameter:

   #include <unordered_map>
   #include <string>
   #include <functional>
   
   struct MyKey {
       int id;
       std::string name;
   
       // Define the "==" operator
       bool operator==(const MyKey& other) const {
           return id == other.id && name == other.name;
       }
   };
   
   // Define the hash structure
   struct MyKeyHash {
       size_t operator()(const MyKey& key) const {
           return std::hash<int>()(key.id) ^ (std::hash<std::string>()(key.name) << 1);
       }
   };
   
   int main() {
       // Pass MyKeyHash as the third template parameter
       std::unordered_map<MyKey, int, MyKeyHash> myUnorderedMap;
       MyKey key1{1, "apple"};
       MyKey key2{2, "banana"};
       myUnorderedMap[key1] = 10;
       myUnorderedMap[key2] = 20;
       return 0;
   }
   

In both cases, == operator is also required for std::unordered_map.