我有一个叫CoffeeShop的类和另一个叫user的类。我想从CoffeeShop类传递一个函数给user类，但是出现了一个错误，我尝试做的是
用户类别：

class user {
public:
    function<void (string)> fun;
    string name;
    void order_coffee() {
        fun(name);
    }
};

然后是咖啡店课程

class CoffeeShop {
public:
    mutex mtx;
    void Make_Coffee(string name) {
        cout << "in queue: " << name << endl;
        unique_lock<mutex> lock(mtx);
        cout << "welcome,order in process, " << name << endl;
        this_thread::sleep_for(chrono::seconds(4));
        cout << "good bye, " << name << endl;
        lock.unlock();
    }

    void assign() {
        user a;
        a.fun = CoffeeShop::Make_Coffee;
        a.name = "amr kamal";
        thread th1(&user::order_coffee, a);

        user b;
        b.fun = a.fun;
        b.name = "hadeer";
        thread th2(&user::order_coffee, b);

        th2.join();
        th1.join();
    }
};

我使用分配的函数开始运行函数，我想做的是让用户使用make_coffee函数并在队列中等待，它一个接一个地处理，我希望能够让用户访问函数。
我主要按如下方式使用该类

int main() {
   CoffeeShop coffeeShop;
   coffeeShop.assign();
}

我得到的错误是关于分配用户make_coffee函数

error C2679: binary '=': no operator found which takes a right-hand operand of type 'void (__cdecl CoffeeShop::* )(std::string)' (or there is no acceptable conversion)