C++ private inheritance: under private inheritance, how are the base class’s public members treated in the derived class interface?

Introduction / Context:
Access conversion during inheritance is a subtle but important part of C++. With private inheritance, the intent is often to implement in terms of, rather than to expose an “is-a” relationship. Understanding how base-class access levels map into the derived class’s interface avoids surprises and preserves encapsulation.

Given Data / Assumptions:

• We consider a declaration like class D : private B { … };
• We focus on the visibility of B’s members through D’s interface.
• Private members of B are never directly accessible to D regardless of inheritance mode.

Concept / Approach:

Under private inheritance, B’s public and protected members become private within D. That means clients of D cannot access B’s public API through a D object unless D explicitly re-exposes selected members (e.g., via using-declarations or forwarding functions). Private members of B remain inaccessible in D in all inheritance modes; they are part of B’s internal implementation.

Step-by-Step Solution:

Verification / Alternative check:

Create B with a public method and derive D privately. Attempt to call the method via D from non-friend code; it fails. Add using B::method; inside D to re-expose it; the call then succeeds, illustrating the mapping.

Why Other Options Are Wrong:

A: protected is not the result of private inheritance.

C: base private never becomes accessible.

D: public → public occurs only with public inheritance.

Common Pitfalls:

• Using private inheritance where composition would be clearer (“has-a” vs “is-a”).
• Forgetting to re-expose necessary members when adopting private inheritance.

Final Answer:

Public members of the base class become private members of derived class.