结构体内重载operator =；结构体重载输出运算符＜＜

• 结构体内重载operator =；结构体内声明，结构体外定义；注意operator 是结构体的operator
• 结构体重载输出运算符<< ；注意 operator 是全局的operator

#include 

class B {
public:
    B(int data) : _data(data) {}
    B& operator = (const B& _b) {
        this->_data = _b._data;
        return *this;
    }
    friend std::ostream& operator << (std::ostream& out, B& _B) { out << _B._data; return out; }
private:
    int _data;
};

struct A {
    A(int _a, double _d, B _b) : a(_a), d(_d), b(_b) {}
    int a;
    double d;
    B b;
    A& operator = (const A& _a);
    friend std::ostream& operator << (std::ostream& out, A& _A);
};

A& A::operator = (const A& _a) {
    this->a = _a.a;
    this->d = _a.d;
    this->b = _a.b;
    return *this;
}

std::ostream& operator << (std::ostream& out, A& _A) {
    out << "a = " << _A.a <<std::endl;
    out << "d = " << _A.d <<std::endl;
    out << "b = " << _A.b <<std::endl;
    return out;
}

int main() {
    std::cout << "test begin! " << std::endl;
    A a(2, 8.99, B(3));
    std::cout << a;
    std::cout << "---- " << std::endl;
    A b(2222, 3338.99, B(43));
    std::cout << b;
    std::cout << "---- " << std::endl;

    b = a;
    std::cout << b;
    std::cout << "---- " << std::endl;
    return 0;
}