类内的重载>>运算符

Question

What is the difference between the following ways to oveload the >> operator?

class A {
public:
    istream& operator>>(istream& is) { return is >> x; };
private:
    int x;
};

class B {
public:
    friend istream& operator>>(istream& is, B b) { return is >> b.y; };
private:
    int y;
};

I can't use std::cin >> a but I can do std::cin >> b .

Isn't the first way a legit way to overload?

Answer 1

When you do an in-member overload of an operator, then the left-hand-side of that operator is an instance of the class.

For example, with your A class:

A a;
a >> std::cin;  // Calls a.operator>>(std::cin)

---

When you declare the friend function for class B , it's not a member function but a non-member function. It's basically the same as doing

class B {
public:
    friend istream& operator>>(istream& is, B b);  // Just declare the function
private:
    int y;
};

// Here's the function definition
istream& operator>>(istream& is, B b) { return is >> b.y; };

With this function, the left-hand-side is passed as the first argument, and the right-hand-side is passed as the second argument.

Which means

B b;
std::cin >> b;  // Equal to operator>>(std::cin, b)

Answer 2

不，第一个不是“合法”，因为通常您希望>>以istream作为其左侧参数，但是您的成员函数将A作为其左侧参数（如this ），意味着您不能调用std::cin >> a ，但是可以调用a >> std::cin ，这没有什么意义。