9.3 강의 보다가 궁금한 점

class Cents {
private:
	int m_cents;

public:
	Cents(int cents = 0) {
		m_cents = cents;
	}

	int getCents() const {
		return m_cents;
	}

	int& getCents() {
		return m_cents;
	}

	Cents operator - () const {
		return Cents(-m_cents);
	}

	bool operator ! () const {
		return (m_cents == 0) ? true : false;
	}

	friend std::ostream& operator << (std::ostream &out, const Cents &cents) {
		out << "(" << cents.m_cents << ")";
		
		return  out;
	}
};

int main() {
	Cents cents1(6);
	Cents cents2(0);

	cout << cents1 << endl;
	cout << cents2 << endl;
	cout << -cents(-10) << endl;

	cout << !cents1 << endl;
	cout << !cents2 << endl;

	return 0;
}

// 결과

(6)
(0)
(10)
0
1

이렇게 1~3번째와 4,5번째가 다른 이유가 궁금합니다.

friend std::ostream& operator << (std::ostream &out, const Cents &cents)
{
  out << "(" << cents.m_cents << ")";
	
  return  out;
}

bool operator ! () const
{
  return (m_cents == 0) ? true : false;
}

//...
out << "(" << cents.m_cents << ")";
//...