C++ why use public, private or protected inheritance?

Question

Well there is enough information about this subject. For example this thread was very clear to me: Difference between private, public, and protected inheritance

Except one point; Why is it useful?

Answer 1

The answer to this question concerns class interfaces and data encapsulation, rather than language capabilities.

The use cases of protected and private inheritance are rather limited, since there are often other options available which better solve the problem (such as using composition, rather than inheritance). However, there are times when you necessarily must inherit from some type (for example to interface with a third-party library), but you would strongly prefer (for reasons related to user interface of your class) to hide most members inherited from the base class from the users of your new type. A typical scenario would be when you need your type to have the member functions of a certain class for internal use, but it would break the logic of your new type if it was called from outside the class itself.

In these situations, you need to use private or protectedinheritance (depending on whether the interface should be similarly restricted to further derived classes or not.

Bear in mind, however, that this is all just about (strongly) hinting to the users of your class how they should use it. You're adapting its public interface to hide certain features which were public in its base class. This doesn't strictly speaking prevent people from accessing these members, since anyone can still cast a pointer to your derived class to a pointer to the base, and reach the "hidden" resources that way.