为自定义的类写<<（输出）操作符

#include <iostream>

using namespace std;
class item;

ostream & operator <<(ostream & os, item& _item);

class item {
public:
        friend ostream& operator<< (ostream& os, item& _item);
        int i;
        item(int m) : i(m) {}
        void operator <<(ostream& os) {
                cout << this->i;
//              return os;
        }
};

ostream & operator <<(ostream & os, item & _item) {
        os << "在操作符中：" << _item.i;
        return os;
}

int main() {
        item a(1), b(2);
        a = b;
        cout << a.i << endl;
        cout << &a << endl;
        cout << a << endl;
        return 0;
}