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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