S: 7. Why does line 15 compile and line 17 not compile?

1 #include

2

3 class Token {

4 private:

5 int a{1};

6 public:

7 std::ostream& operator<<(std::ostream& os) {

8 os << a;

9 return os;

10 }

11 };

12

13 int main() {

14 Token t;

15 t << std::cout;

16

17 std::cout << t;

18

19 return 0;

20 }