• 观察者保存一个目标指针，用于获取目标对象的状态

// 目标
class Subject;

// 观察者接口
class Observer {
 public:
  explicit Observer(const std::shared_ptr<Subject>& p) : subject_(p) {}
  virtual void Update() = 0;
  virtual ~Observer() = default;

 protected:
  std::weak_ptr<Subject> subject_;
};

• 目标保存一系列观察者，发出通知时，更新所有观察者，以使观察者获取目标对象的新状态

class Subject {
 public:
  void Append(const std::shared_ptr<Observer>& o) {
    observers_.emplace_back(o);
  }

  void Remove(const std::shared_ptr<Observer>& o) { observers_.remove(o); }

  void Notify() {
    for (auto&& x : observers_) {
      x->Update();
    }
  }

  void SetValue(std::int32_t n) { value_ = n; }

  std::int32_t Value() const { return value_; }

 private:
  std::list<std::shared_ptr<Observer>> observers_;
  std::int32_t value_ = 0;
};

• 具体的观察者

class ObserverA : public Observer {
 public:
  using Observer::Observer;

  void Update() override {
    if (const auto p = subject_.lock()) {
      std::cout << "Subject value in A: " << p->Value() << '\n';
    }
  }
};

class ObserverB : public Observer {
 public:
  using Observer::Observer;

  void Update() override {
    if (const auto p = subject_.lock()) {
      std::cout << "Subject value in B: " << p->Value() << '\n';
    }
  }
};

int main() {
  auto subject = std::make_shared<Subject>();
  auto a = std::make_shared<ObserverA>(subject);
  auto b = std::make_shared<ObserverB>(subject);

  subject->Append(a);
  subject->Append(b);

  subject->SetValue(3);
  subject->Notify();  // subject_ value in A: 3\nsubject value in B: 3\n
}