What is the Rule of Three, Five and Zero?

These rules describe which special member functions must be defined together.

The Rule of Three (C++98): if you need to write any one of the destructor, copy constructor, or copy assignment operator, you almost certainly need all three. The reason is that needing a custom destructor implies the class manages a resource, and the compiler-generated copy operations will copy the pointer rather than the resource — producing a double free.

The Rule of Five (C++11) adds the move constructor and move assignment operator. Declaring a destructor or copy operation suppresses the implicit move operations, so a class following only the Rule of Three silently loses move optimisation.

The Rule of Zero is the modern guidance and the one to lead with: design classes so you need none of them. If every member is a type that manages itself — std::string, std::vector, std::unique_ptr — the compiler-generated versions are all correct, and there is no code to get wrong.

Note: The practical consequence is a separation of duties. Write resource-managing wrapper classes rarely and carefully, and let everything else follow the Rule of Zero. A class that both manages a raw resource and holds business logic is where these bugs live.