Comparison operators
- C++ course
- Intermediate
- Lesson 108/116
- 5 min
Learn to overload == and < operators.
Comparison operators
Comparison operators ==, !=, <, <=, >, >= can be overloaded. They return bool.
Example: bool operator==(const Score& other) const { return value == other.value; }. This lets you compare two Score objects with ==. The expression a == b returns true or false. Comparison operators are usually written as const because they do not modify the object. For sorting, defining operator< is enough; std::sort uses it.
operator< and sorting
operator< tells whether one object is less than another. std::sort and other STL algorithms use this operator.
Example: bool operator<(const Score& other) const { return value < other.value; }. This lets you sort Score objects by value. When you sort a std::vector<Score> with std::sort(v.begin(), v.end()), the compiler uses operator<. If operator< is not defined, sorting gives a compile error. That is why defining operator< is important for sortable classes.
Practice Add operator== to the Score class
Add operator== to the Score class in the starting code: its body is return value == other.value;. In main compare a == b; print "Equal" if equal, "Different" otherwise.
Starter code
#include <iostream>
class Score {
public:
int value;
Score(int v) : value(v) {}
// Add the equality operator
};
int main() {
Score a(10);
Score b(10);
// Compare a == b and print the result
return 0;
}Sample solution
#include <iostream>
class Score {
public:
int value;
Score(int v) : value(v) {}
bool operator==(const Score& other) const {
return value == other.value;
}
};
int main() {
Score a(10);
Score b(10);
if (a == b) {
std::cout << "Equal";
} else {
std::cout << "Different";
}
return 0;
}Expected output
Equal
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What type do comparison operators return?
bool operator==(const Score& other) const;- int
- Score
- bool Correct answer
- void
Explanation: Comparison operators return bool.
2 Equality operator name
bool ____(const Score& other) const { return value == other.value; }bool operator==(const Score& other) const { return value == other.value; }Explanation: operator== performs equality comparison.
3 Match each operator to its purpose
| Term | Meaning |
|---|---|
operator== |
Equality |
operator< |
Less than |
operator!= |
Not equal |
operator>= |
Greater or equal |