Observer Pattern

The Observer Pattern implements a subscription model where a subject notifies observers of state changes. In C++, you should store observer references using std::weak_ptr to avoid circular references.

Preventing Reference Cycles

If a Subject holds a std::shared_ptr to an Observer, and the Observer holds a std::shared_ptr to the Subject, a circular reference occurs.

This prevents reference counts from reaching zero, causing both objects to leak. Storing observers as std::weak_ptr resolves this.

Code Walkthrough

A publish-subscribe pattern that cleans up expired observers during notification.

#include <iostream>
#include <vector>
#include <memory>
// Observer Interface
class Observer {
public:
    virtual void update(int price) = 0;
    virtual ~Observer() = default;
};
// Concrete Observer
class Investor : public Observer {
private:
    std::string m_name;
public:
    Investor(std::string name) : m_name(name) {}
    void update(int price) override {
        std::cout << m_name << " notified of price update: $" << price << "\n";
    }
};
// Subject
class StockTicker {
private:
    std::vector<std::weak_ptr<Observer>> m_observers;
public:
    void attach(std::shared_ptr<Observer> obs) {
        m_observers.push_back(obs);
    }
void setPrice(int price) {
        // Notify observers, cleaning up expired ones dynamically
        for (auto it = m_observers.begin(); it != m_observers.end();) {
            if (auto sharedObs = it->lock()) { // Lock returns shared_ptr if object exists
                sharedObs->update(price);
                ++it;
            } else {
                it = m_observers.erase(it); // Remove expired observer from list
            }
        }
    }
};
int main() {
    StockTicker ticker;
    auto inv1 = std::make_shared<Investor>("Alice");
ticker.attach(inv1);
    ticker.setPrice(150);
{
        auto inv2 = std::make_shared<Investor>("Bob");
        ticker.attach(inv2);
        ticker.setPrice(155);
    } // inv2 goes out of scope and is destroyed
ticker.setPrice(160); // Automatically cleans up inv2 from the observer list
    return 0;
}

Interview-Relevant Information