C++类内重载operator<<

#include <iostream>
using namespace std;
class A {
 public:
  int a;
  friend ostream& operator<<(ostream& os, A);
};

//友元函数定义在外部 不在某个类的作用域内
//ostream& A::operator<<(ostream& os, A A)
ostream& operator<<(ostream& os, A A) {
  cout << A.a << endl;
  return cout;
}

int main()
{
  A a{};
  cout << a;
  return 0;
}