In C++ std::unordered_map is a data structure that implements a hash table and allows fast access to each element based on its key. However, when we want to use user-defined types as keys, we need to provide a custom hash function. In this article, we will learn how to implement a custom hash function for user-defined types in std::unordered_map.

Implement Custom Hash Functions for User-Defined Types in std::unordered_map

To implement a custom hash function, we need to define a functor (a struct or class that overloads the operator()) that should take an object of our user-defined type as input and return the hash value. We need to pass that custom hash function as a template argument while declaring the std::unordered_map using the below syntax.

Syntax to Use a Custom Hash Function in std::unordered_map

unordered_map<KeyType, ValueType, HashType>

Here, 

• KeyType is a user-defined type, 
• ValueType is the type of the values in the map.
• HashType is the custom hash function.

Approach:

• First, define a user-defined type(class, struct or any other user-defined type) for which you want to implement a custom hash function.
• Next, define a custom hash function that should be a struct or class that overloads the operator() and it should take an object of your user-defined type as input and return a std::size_t representing the hash value.
• Finally, use the custom hash function as a template argument while declaring the std::unordered_map.

C++ Program to Implement Custom Hash Functions for User-Defined Types in std::unordered_map

The below program demonstrates how we can implement a custom hash function for user-defined types in std::unordered_map in C++.

// C++ program to Implement Custom Hash Functions for
// User-Defined Types in std::unordered_map

#include <iostream>
#include <unordered_map>
using namespace std;

struct Point {
    int x, y;
    // Constructor
    Point(int x, int y)
        : x(x)
        , y(y)
    {
    }
    bool operator==(const Point& other) const
    {
        return x == other.x && y == other.y;
    }
};
// Define a custom hash function for the Point
struct PointHasher {
    size_t operator()(const Point& p) const
    {
        // Combine hashes of x and y using the bitwise XOR
        return hash<int>()(p.x) ^ (hash<int>()(p.y) << 1);
    }
};
int main()
{
    // Create an unordered_map with the Point as key and int
    // as value and pass hash function as template arguement
    unordered_map<Point, int, PointHasher> point_map;
    point_map[Point(1, 2)] = 10;
    point_map[Point(3, 4)] = 20;
    point_map[Point(5, 6)] = 30;
    // printing the key-value pairs of point_map
    for (const auto& pair : point_map) {
        cout << "Point(" << pair.first.x << ", "
             << pair.first.y << "): " << pair.second
             << endl;
    }
    return 0;
}

// C++ program to Implement Custom Hash Functions for

// User-Defined Types in std::unordered_map

​

#include <iostream>

#include <unordered_map>

using namespace std;

​

struct Point {

    int x, y;

    // Constructor

    Point(int x, int y)

        : x(x)

        , y(y)

    {

    }

    bool operator==(const Point& other) const

    {

        return x == other.x && y == other.y;

    }

};

// Define a custom hash function for the Point

struct PointHasher {

    size_t operator()(const Point& p) const

    {

        // Combine hashes of x and y using the bitwise XOR

        return hash<int>()(p.x) ^ (hash<int>()(p.y) << 1);

    }

};

int main()

{

    // Create an unordered_map with the Point as key and int

    // as value and pass hash function as template arguement

    unordered_map<Point, int, PointHasher> point_map;

    point_map[Point(1, 2)] = 10;

    point_map[Point(3, 4)] = 20;

    point_map[Point(5, 6)] = 30;

    // printing the key-value pairs of point_map

    for (const auto& pair : point_map) {

        cout << "Point(" << pair.first.x << ", "

             << pair.first.y << "): " << pair.second

             << endl;

    }

    return 0;

}

Point(5, 6): 30
Point(3, 4): 20
Point(1, 2): 10

Time Complexity: O(1), for insertion, deletion, and access in std::unordered_map
Auxilliary Space: O(n), where n is the number of elements in the map.