Access control in C++ allows you to separate interface from implementation, but the implementation hiding is only partial. The compiler must still see the declarations for all parts of an object in order to create and manipulate it properly. You could imagine a programming language that requires only the public interface of an object and allows the private implementation to be hidden, but C++ performs type checking statically (at compile time) as much as possible. This means that you’ll learn as early as possible if there’s an error. It also means that your program is more efficient. However, including the private implementation has two effects: the implementation is visible even if you can’t easily access it, and it can cause needless recompilation.