如何在调用 cout 时 output 结构中的常量文本？

Question

I have this struct:

struct sample {
  int x;
};

and then I have this overload of operator<<:

std::ostream &operator<<(const std::ostream &os, const sample &s) {
  if (s.x == 0)
    return os << "zero";
  else
    return os << "not zero";
}

main:

int main() {
  sample sam;
  sam.x = 0;
  std::cout << sam << std::endl;
  sam.x = 1;
  std::cout << sam << std::endl;
  return 0;
}

But the compiler gives me this error: Complile Error

What can I do?

Answer 1

You are right except for a small error in your operator signature:

std::ostream &operator<<(const std::ostream &os, const sample &s)
//                       ^^^^^ Problem

You mark the output stream as const but then modify it inside the function:

os << "zero";

and

os << "not zero";

Because

std::basic_ostream<CharT,Traits>::operator<< is not const .

---

So, remove that const and the code will work.