为什么重载“<

Question

这个问题在这里已有答案：

• c++ overloading stream operator, parameters by reference and anonymous instances 1回答

我上课了

class complex
{
 [...]
 complex operator+(const complex &c) const;
 [...]
 friend std::ostream &operator<<(std::ostream &os, const complex &c);
};

complex complex::operator+(const complex &c) const
{
 complex result;
 result.real_m = real_m + c.real_m;
 result.imaginary_m = imaginary_m + c.imaginary_m;
 return result;
}
std::ostream &operator<<(std::ostream &os, const complex &c)
{
 os << "(" << c.real_m << "," << c.imaginary_m << "i)";
 return os;
}

int main()
{
  complex a(3.0, 4.0);
  complex c(5.0, 8.0);
  cout << "a is " << a << '\n';
  cout << "a + c is " << a + c << '\n';
  [...]
 }

并且，一切正常，但如果我在const删除std::ostream &operator<<(std::ostream &os, const complex &c) cout << "a is " << a << '\n';工作正常，但cout << "a + c is " << a + c << '\n';没有，它说：没有运算符“<

Answer 1

a + c不是l值，因此你不能在它上面使用非const引用。