When apply observer pattern an error occured - c++

Question

I have the following code:
class ISubscriber;
class News {
public:
float getVersion() { return this->version; }
void setVersion(float state) { this->version= state; this->notifyAllSubscribers(); }
void attach(ISubscriber *observer) { this->subscribers.push_back(observer); }
void notifyAllSubscribers() {
for (vector<ISubscriber*>::iterator it = subscribers.begin(); it != subscribers.end(); it++){
(*(*it)).update();
}
}
private:
vector<ISubscriber*> subscribers;
float version;
};
class ISubscriber {
public:
News *news;
virtual void update() = 0;
};
class Subscriber1 : public ISubscriber {
public:
Subscriber1(News *news) { this->news = news; this->news->attach(this); }
void update() override { cout << "Subscriber1: A new version of the newspaper has been launched (v" << this->news->getVersion() << ")" << endl; }
};
class Subscriber2 : public ISubscriber {
public:
Subscriber2(News *news) { this->news = news; this->news->attach(this); }
void update() override { cout << "Subscriber2: A new version of the newspaper has been launched (v" << this->news->getVersion() << ")" << endl; }
};
int main(int argc, char *argv[]) {
News newspaper;
newspaper.setVersion(2.1f);
Subscriber1 sb1(&newspaper);
Subscriber2 sb2(&newspaper);
return 0;
}
But strange errors happened:
The first error points to this code (*(*it)).update(); in news class.
Why that errors happened, what's the reason?

Answer 1

(*(*it)).update(); requires the type ISubscriber to be complete, just the forward declaration is not enough.
You could move the defnition of ISubscriber before the one of News, and give a forward declaration of News before that.
class News;
class ISubscriber {
public:
News *news;
virtual void update() = 0;
};
class News {
public:
float getVersion() { return this->version; }
void setVersion(float state) { this->version= state; this->notifyAllSubscribers(); }
void attach(ISubscriber *observer) { this->subscribers.push_back(observer); }
void notifyAllSubscribers() {
for (vector<ISubscriber*>::iterator it = subscribers.begin(); it != subscribers.end(); it++){
(*(*it)).update();
}
}
private:
vector<ISubscriber*> subscribers;
float version;
};