Choosing the Ownership Model

Pick the ownership model before you pick the pointer type.

| Relationship | Model | Tool | |---|---|---| | I own, nobody else | exclusive | unique_ptr member | | Shared pool, users outlive creators | shared | shared_ptr | | Back-reference (child → parent) | observing | weak_ptr or T* | | Just using, no lifetime stake | observing | T& / T* parameter | | The container owns it all | value semantics | std::vector<T> |

A worked example — an entity system:

class Scene {
    std::vector<std::unique_ptr<Entity>> entities_;  // scene owns
public:
    Entity& spawn() {
        entities_.push_back(std::make_unique<Entity>());
        return *entities_.back();          // hand back a reference, not ownership
    }
};

Note the signature discipline: spawn() returns Entity& because callers use the entity but the scene keeps owning it. Handing back unique_ptr would invite double-ownership; handing back shared_ptr would make the counters meaningless. Default to unique_ptr and references; escalate to shared_ptr only with a stated reason.