Implementing a Custom Hash Function

To implement a custom hash function for your user-defined type, you need to create a struct that overloads the operator(). Here's an example:

1#include <unordered_set>
2#include <string>
3
4struct Player {
5  std::string Name;
6  int Score;
7};
8
9struct PlayerHash {
10  size_t operator()(const Player& P) const {
11    return std::hash<std::string>{}(P.Name)
12         ^ std::hash<int>{}(P.Score);
13  }
14};

For our type to be compatible with hash-based containers such as std::unordered_set, it additionally needs to implement the == operator:

1#include <string>
2#include <unordered_set>
3
4struct Player {
5  std::string Name;
6  int Score;
7
8  bool operator==(const Player& Other) const {
9    return Name == Other.Name;
10  }
11};
12
13struct PlayerHash {
14  size_t operator()(const Player& P) const {
15    return std::hash<std::string>{}(P.Name)
16       ^ std::hash<int>{}(P.Score); 
17  }
18};
19
20int main() {
21  std::unordered_set<Player, PlayerHash> Players;
22  Players.emplace("Alice", 100);
23  Players.emplace("Bob", 200);
24}

In this example:

• We define a Player struct with Name and Score members, and operator==() to allow Player objects to be compared for equality. In this case, we consider two Player objects to be equal if they have the same Name.
• We create a PlayerHash struct that overloads operator()
• Inside operator(), we combine the hashes of Name and Score using XOR (^). This ensures that players with the same name but different scores (or vice versa) will have different hashes.
• We specify PlayerHash as the second template argument when creating the std::unordered_set

By providing a custom hash function, we enable std::unordered_set to efficiently store and retrieve Player objects.