我指的是这篇文章使用模板实现主题/观察者模式 http://www.codeproject.com/KB/architecture/observer_with_templates.aspx
我做了一些修改,代码如下。
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);
是有限制的。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;
};
我可以通过更改来解决问题static_cast
to dynamic_cast
. 但是,我不确定我是否会陷入其他陷阱?我对作者初衷的猜测static_cast
,是为了保证编译时的类型安全检查。