c++ - 基于模板的主题观察者模式——我应该使用 static_cast 还是 dynamic_cast

Question

我指的是文章 Implementing a Subject/Observer pattern with templates

我做了一些修改，变成了如下代码。

template <class T, class A>
class Observer {
public:
    Observer() {}
    virtual ~Observer() {}
    virtual void update(T& subject, A arg) = 0;
};

template <class T, class A>
class Subject
{
public:
    Subject() {}
    virtual ~Subject() {}

    // Take note that, we didn't make the following functions as virtual,
    // as we do not expect them to be overridden.
    void attach(Observer<T, A> &observer) {
        // Ensure no duplication.
        std::vector<Observer<T, A> *>::const_iterator iterator = std::find(observers.begin(), observers.end(), &observer);
        if (iterator == observers.end()) {
            observers.push_back(&observer);
        }
    }

    void dettach(Observer<T, A> &observer) {
        std::vector<Observer<T, A> *>::const_iterator iterator = std::find(observers.begin(), observers.end(), &observer);
        if (iterator != observers.end()) {
            observers.erase(iterator);
        }
    }

    void dettachAll() {
        observers.clear();
    }

    void notify(A arg)
    {
        std::vector<Observer<T, A> *>::const_iterator it;
        for (it = observers.begin(); it != observers.end(); it++) { 
            (*it)->update(*(static_cast<T *>(this)), arg);
        }
    }

private:
    std::vector<Observer<T, A> *> observers;
};

后来，我意识到(*it)->update(*(static_cast<T *>(this)), arg);有限制。例如，

// cause compilation error in static_cast, as it cannot cast cat1 to animal.
class cat1 : public animal, public Subject<animal, int> {
public:
    virtual void speak() {
        notify(888);
    }
};

class zoo1 : public Observer<animal, int> {
public:
    zoo1() {
        c.attach(*this);
        c.speak();
    }

    virtual void update(animal& subject, int arg) {
        cout << "zoo1 received notification " << arg << endl;
    }

    cat1 c;
};

我可以通过更改 static_cast 来解决问题至 dynamic_cast . 但是，我不确定我是否会落入其他陷阱？ 我猜测作者的初衷是static_cast , 是为了确保在编译时进行类型安全检查。

Answer 1

你的问题来自于动物不仅应该是猫的主题，

class animal : public Subject<animal,int>
{
    ...
};

class cat1 : public animal
{
    public:
    virtual void speak() 
    {
        notify(888);
    }
};

class zoo1 : public Observer<animal, int> {
public:
    zoo1() 
    {
        c.attach(*this);
        c.speak();
    }

virtual void update(animal& subject, int arg) 
    {
        cout << "zoo1 received notification " << arg << endl;
    }

cat1 c;
};

通过这样做，每个对象都可以静态地“能够”转换为动物。你的 cat1 不是这种情况