Repair Unsafe C++ Ownership with Smart Pointers

Review this ownership sketch:

class Engine {
public:
 void start();
};

class Controller {
 Engine* engine_;
public:
 Controller() : engine_(new Engine()) {}
 Engine* engine() const { return engine_; }
};

Controller* make_controller() {
 return new Controller();
}

Identify the lifetime and ownership defects. Redesign the interfaces using RAII and the most restrictive appropriate smart-pointer types. Discuss copying, moving, exception safety, non-owning access, and what would change if an Engine had to be shared by several controllers.

Clarifying Questions to Ask

• Is a controller the sole owner of its engine in the normal design?
• Must controllers be copyable, movable, or polymorphic?
• Can an engine outlive every controller that uses it?
• Does the factory transfer ownership to its caller?