Spaceship operator
- C++ course
- Intermediate
- Lesson 110/116
- 5 min
Learn the C++20 spaceship (<=>) operator.
The spaceship operator (<=>)
The C++20 spaceship operator (<=>) automatically generates all comparison operators with a single function. With = default the compiler compares members in order.
Example: auto operator<=>(const Version&) const = default;. This single line generates ==, !=, <, <=, >, >= all at once. The #include <compare> header is required. You can also write the body by hand instead of = default, but default is enough in most cases. This greatly reduces comparison code.
Default comparison behavior
With = default the operator<=> compares the class members in declaration order. The first differing member gives the result.
Example: in class Version { int major; int minor; }; first major is compared. If equal, minor is compared. So Version(1, 5) < Version(2, 0) is true because 1 < 2. Version(1, 5) < Version(1, 7) is also true because major is equal and 5 < 7. This behavior is ideal for hierarchical data like version numbers.
Practice Add <=> to the Version class
Add the spaceship operator to the Version class in the starting code: auto operator<=>(const Version& other) const = default;. In main compare a < b; print "Less" if less, "More" otherwise.
Starter code
#include <iostream>
#include <compare>
class Version {
public:
int major;
int minor;
Version(int maj, int min) : major(maj), minor(min) {}
// Add the comparison operator
};
int main() {
Version a(1, 0);
Version b(2, 0);
// Compare and print the result
return 0;
}Sample solution
#include <iostream>
#include <compare>
class Version {
public:
int major;
int minor;
Version(int maj, int min) : major(maj), minor(min) {}
auto operator<=>(const Version& other) const = default;
};
int main() {
Version a(1, 0);
Version b(2, 0);
if (a < b) {
std::cout << "Less";
} else {
std::cout << "More";
}
return 0;
}Expected output
Less
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each piece to its role
| Term | Meaning |
|---|---|
<=> |
Spaceship operator |
= default |
Automatic generation |
#include <compare> |
Header file |
auto |
Return type deduction |
2 How many operators does operator<=> generate?
auto operator<=>(const Version&) const = default;- Six Correct answer
- Two
- One
- Ten
Explanation: <=> generates six operators: ==, !=, <, <=, >, >=.
3 Spaceship operator signature
auto ____(const Version& other) const = default;auto operator<=>(const Version& other) const = default;Explanation: operator<=> is the spaceship operator.