Entity e1(10, 20);
Entity e2 = e1;          // ← copy constructor called here
Entity e3(e1);           // ← also copy ctor



class Entity {
public:
    int* data;          // raw pointer – classic trap

    Entity(int value) {
        data = new int(value);
    }

    ~Entity() {
        delete data;    // boom when double delete
    }
};

Entity e1(42);
Entity e2 = e1;         // default copy ctor: e2.data = e1.data (same pointer!)


Result:

e1 dies → delete data
e2 still points to deleted memory → dangling pointer
e2 dies → delete same address again → double free crash

Cherno:
"This is why default copy constructor is evil when you have raw resources.
Shallow copy = shared pointers = death."



Default copy = shallow → deadly with raw pointers/resources
Deep copy = manual allocation + copy in copy ctor
Rule of Zero = use RAII (unique_ptr, vector, string) → no manual copy functions
Copy elision / RVO = compiler often skips copy ctor when returning by value — free performance
Self-assignment check in operator=: if (this == &other) return *this;