Access control in C++ gives valuable control to the creator of a class. The users of the class can clearly see exactly what they can use and what to ignore. More important, though, is the ability to ensure that no client programmer becomes dependent on any part of the underlying implementation of a class. If you know this as the creator of the class, you can change the underlying implementation with the knowledge that no client programmer will be affected by the changes because they can’t access that part of the class.

When you have the ability to change the underlying implementation, you can not only improve your design at some later time, but you also have the freedom to make mistakes. No matter how carefully you plan and design, you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn faster, and you’ll finish your project sooner.

The public interface to a class is what the client programmer does see, so that is the most important part of the class to get “right” during analysis and design. But even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more functions, as long as you don’t remove any that client programmers have already used in their code.