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

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  • Difficulty Level : Expert
  • Last Updated : 24 Nov, 2020
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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.

Equality Ordering
Primary == <=>
Secondary != <, >, <=, >=

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++




// C++ 20 program to illustrate the
// 3 way comparison operator
#include <bits/stdc++.h>
using namespace std;
  
// Driver Code
int main()
{
    float A = -0.0;
    float B = 0.0;
  
    // Find the value of 3 way comparison
    auto ans = A <= > B;
  
    // If ans is less than zero
    if (ans < 0)
        cout << "-0 is less than 0";
  
    // If ans is equal to zero
    else if (ans == 0)
        cout << "-0 and 0 are equal";
  
    // If ans is greater than zero
    else if (ans > 0)
        cout << "-0 is greater than 0";
  
    return 0;
}


Output:

Program 2:

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

C++




// 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.
     

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