Stream operators
- C++ course
- Intermediate
- Lesson 109/116
- 5 min
Learn to overload << and >> operators.
Stream operators
operator<< and operator>> let you write an object to a stream and read from a stream. These operators are defined as global functions, not as members of the class.
Example: std::ostream& operator<<(std::ostream& os, const Point& p) { os << p.x << " " << p.y; return os; }. This lets you write std::cout << p;. The function returns std::ostream&; this enables chaining (std::cout << a << b). Why global? Because the left side is not your class but std::ostream. A member operator would need to be added inside std::ostream, which is not possible.
Chaining and return os
The operator<< function returns the stream with return os;. This lets you write multiple objects in sequence.
Example: std::cout << a << " " << b; works like this: first operator<<(std::cout, a) is called and returns std::cout. Then " " is written to the returned stream and the stream is returned again. Finally b is written. If the function returned void, chaining would not work; std::cout << a << b would be a compile error. That is why return os; is mandatory.
Practice Add operator<< for Point
Add a global operator<< function for the Point class in the starting code: std::ostream& operator<<(std::ostream& os, const Point& p) { os << p.x << " " << p.y; return os; }. In main print the point with std::cout << p;.
Starter code
#include <iostream>
class Point {
public:
int x;
int y;
Point(int x_, int y_) : x(x_), y(y_) {}
// The output operator is defined outside the class
};
// Add the output operator here
int main() {
Point p(3, 7);
// Print p with std::cout
return 0;
}Sample solution
#include <iostream>
class Point {
public:
int x;
int y;
Point(int x_, int y_) : x(x_), y(y_) {}
};
std::ostream& operator<<(std::ostream& os, const Point& p) {
os << p.x << " " << p.y;
return os;
}
int main() {
Point p(3, 7);
std::cout << p;
return 0;
}Expected output
3 7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Stream operator signature
std::ostream& ____(std::ostream& os, const Point& p) { os << p.x; return os; }std::ostream& operator<<(std::ostream& os, const Point& p) { os << p.x; return os; }Explanation: operator<< is the output stream operator.
2 Match each piece to its role
| Term | Meaning |
|---|---|
operator<< |
Writing to output stream |
operator>> |
Reading from input stream |
std::ostream& |
Return type |
return os |
Returning the stream |
3 Why is operator<< defined as a global function?
std::ostream& operator<<(std::ostream& os, const Point& p);- Because it is faster
- Because the compiler requires it
- Because it cannot be written inside the class
- Because the left side is std::ostream Correct answer
Explanation: The left side is not your class but std::ostream.