C++ Observer

// Example program
#include <iostream>
#include <string>
#include <memory>
#include <algorithm>

struct Event {
    std::string message;
};

class Observer {
public:
    Observer(int pNum) : num(pNum) {}

    void notify(Event& e) {
        std::cout << "Observer #" << num << " received event: " << e.message << std::endl;
    }

    int getNum() {return num;}

private:
    int num;
};

class Subject {
public:
    Subject(int pNum) : num(pNum) {}

    int getNum() {return num;}

    void addObserver(std::shared_ptr<Observer> pObserver) {
        std::cout << "Added observer #" << pObserver->getNum() << " to observers of subject #" << num << std::endl;
        observers.emplace_back(pObserver);
    }

    void process(std::string pMessage) {
        Event event{pMessage};
        for(auto it=observers.begin(); it!=observers.end();) {
            auto sharedObserver = (*it).lock();
            if(sharedObserver) {
                sharedObserver->notify(event);
                ++it;
            } else {
                std::cout << "Removed invalid observer" << std::endl;
                it = observers.erase(it);
            }
        }
    }

private:
    int num;
    std::vector<std::weak_ptr<Observer>> observers;
};

int main()
{
    Subject subject(1);
    auto obs1 = std::make_shared<Observer>(1);
    auto obs2 = std::make_shared<Observer>(2);
    subject.addObserver(obs1);
    subject.addObserver(obs2);
    subject.process("Message 1");
    obs2 = nullptr;
    {
        auto obs3 = std::make_shared<Observer>(3);
        subject.addObserver(obs3);
        subject.process("Message 2");
    }
    subject.process("Message 3");
    subject.process("Message 4");
}