smart pointer v old pointer DI

di.cpp

#include <iostream>
#include <memory>

// Interface
class Engine {
public:
    virtual ~Engine() = default;
    virtual void start() = 0;
    virtual int getHorsepower() const = 0;
};

// Concrete implementations
class V6Engine : public Engine {
public:
    void start() override {
        std::cout << "V6 engine starting with a smooth hum..." << std::endl;
    }
    int getHorsepower() const override { return 280; }
};

class V8Engine : public Engine {
public:
    void start() override {
        std::cout << "V8 engine roaring to life!" << std::endl;
    }
    int getHorsepower() const override { return 450; }
};

// ===== RAW POINTER VERSION =====
class Car {
private:
    Engine* engine;
    std::string model;

public:
    // Constructor - can accept nullptr
    Car(const std::string& model)
        : model(model) {
    }

    void setEngine(Engine* newEngine) {
        engine = newEngine;
    }

    void startCar() {
        std::cout << model << " attempting to start..." << std::endl;

        if (engine == nullptr) {
            std::cout << "ERROR: No engine installed!" << std::endl;
            return;
        }

        engine->start();
        std::cout << "Power: " << engine->getHorsepower() << " HP"
            << std::endl;
    }
};

// ===== SMART POINTER VERSION =====
class SmartCar {
private:
    std::unique_ptr<Engine> engine = nullptr;
    std::string model;

public:
    // Constructor - can start with no engine
    SmartCar(const std::string& model)
        : model(model) {
    }

    void setEngine(std::unique_ptr<Engine> newEngine) {
        engine = std::move(newEngine);
    }

    void startCar() {
        std::cout << model << " attempting to start..." << std::endl;

        if (!engine) {  // or: if (engine == nullptr)
            std::cout << "ERROR: No engine installed!" << std::endl;
            return;
        }

        engine->start();
        std::cout << "Power: " << engine->getHorsepower() << " HP"
            << std::endl;
    }
};

int main() {
    std::cout << "===== RAW POINTER EXAMPLE =====" << std::endl;

    // Start with no engine
    Car car1("Honda Civic");
    car1.startCar();

    std::cout << "\n--- Installing engine ---\n" << std::endl;
    Engine* v6 = new V6Engine();
    car1.setEngine(v6);
    car1.startCar();

    delete v6;

    std::cout << "\n\n===== SMART POINTER EXAMPLE =====" << std::endl;

    // Start with no engine (nullptr or omit the parameter)
    SmartCar car2("Tesla Model S");  // No engine parameter
    car2.startCar();

    std::cout << "\n--- Installing engine ---\n" << std::endl;
    car2.setEngine(std::make_unique<V8Engine>());
    car2.startCar();

    std::cout << "\n--- Removing engine ---\n" << std::endl;
    car2.setEngine(nullptr);
    car2.startCar();

    return 0;
}