3-way comparison operator (Space Ship Operator) in C++ 20

The three-way comparison operator "<=>" is called a spaceship operator. The spaceship operator determines for two objects A and B whether A < B, A = B, or A > B. The spaceship operator or the compiler can auto-generate it for us. Also, a three-way comparison is a function that will give the entire relationship in one query. Traditionally, strcmp() is such a function. Given two strings it will return an integer where,

• < 0 means the first string is less
• == 0 if both are equal
• > 0 if the first string is greater.

It can give one of the three results, hence it’s a three-way comparison.

From the above table, it can be seen that the spaceship operator is a primary operator i.e., it can be reversed and corresponding secondary operators can be written in terms of it.

(A <=> B) < 0 is true if A < B
(A <=> B) > 0 is true if A > B
(A <=> B) == 0 is true if A and B are equal/equivalent.

Program 1:

Below is the implementation of the three-way comparison operator for two float variables:

// C++ program to illustrate the 3 way comparison
// (spaceship) operator
#include <compare>
#include <iostream>
using namespace std;

int main()
{
    int x = 10;
    int y = 20;

    // saving the result of 3 way comparison operator
    auto res = x <=> y;

    // executing statements based on the above comparison
    if (res < 0)
        cout << "Less";
    else if (res > 0)
        cout << "Greater";
    else if (res == 0)
        cout << "Same";
    else
        cout << "Unordered";
  
    return 0;
}

Output

Program 2:

Below is the implementation of the three-way comparison operator for two vectors:

// C++ 20 program for the illustration of the
// 3-way comparison operator for 2 vectors
#include <bits/stdc++.h>
using namespace std;

// Driver Code
int main()
{
    // Given vectors
    vector<int> v1{ 3, 6, 9 };
    vector<int> v2{ 3, 6, 9 };

    auto ans2 = v1 <=> v2;

    // If ans is less than zero
    if (ans2 < 0) {

        cout << "v1 < v2" << endl;
    }

    // If ans is equal to zero
    else if (ans2 == 0) {

        cout << "v1 == v2" << endl;
    }

    // If ans is greater than zero
    else if (ans2 > 0) {

        cout << "v1 > v2" << endl;
    }

    return 0;
}

Output

Note: You should download the adequate latest compiler to run C++ 20.

Needs of Spaceship Operators:

• It's the common generalization of all other comparison operators (for totally-ordered domains): >, >=, ==, <=, <. Using <=>, every operation can be implemented in a completely generic way in the case of user-defined data type like a structure where one has to define the other 6 comparison operators one by one instead.
• For strings, it's equivalent to the old strcmp() function of the C standard library. So it is useful for lexicographic order checks, such as data in vectors, or lists, or other ordered containers.