c++圆类定义如下，重载输入、输出运算符为友元函数，重载算术减法运算符为成员函数，参考给出代码进行填空

#include <iostream>
using namespace std;

class Circle {
private:
    double radius;
public:
    Circle(double r = 0) : radius(r) {}
    friend istream& operator>>(istream& in, Circle& c) {
        // TODO: 完成输入运算符的重载
        return in;
    }
    friend ostream& operator<<(ostream& out, const Circle& c) {
        // TODO: 完成输出运算符的重载
        return out;
    }
    Circle operator-(const Circle& c) {
        // TODO: 完成算术减法运算符的重载
    }
};

int main() {
    Circle c1, c2;
    cin >> c1 >> c2;
    cout << "c1 = " << c1 << endl;
    cout << "c2 = " << c2 << endl;
    Circle c3 = c1 - c2;
    cout << "c1 - c2 = " << c3 << endl;
    return 0;
}

istream& operator>>(istream& in, Circle& c) {
    double r;
    in >> r;
    c.radius = r;
    return in;
}

ostream& operator<<(ostream& out, const Circle& c) {
    out << c.radius;
    return out;
}

Circle Circle::operator-(const Circle& c) {
    Circle temp(radius - c.radius);
    return temp;
}