通过重载友元的<<使用ostream的cout直接输出对象

class frind_test
{
public :
    string  done()
    {
        return " ok \n";
    }
    friend void operator<< (ostream & out , frind_test & f_m);
};

void  operator << (ostream &out , frind_test &f_m)
{
    out << f_m.done();
   
}
int main(int argc, char** argv) {

    frind_test f_m ;
    cout << f_m ;
    return (EXIT_SUCCESS);
}