Question

Explain how in a private inheritance, the members of the base class are inherited by a derived class.

Short answer

In private inheritance, all public and protected members in the base class become private in the derived class.

Step-by-step solution

Understanding Private Inheritance

In private inheritance, the derived class inherits members from the base class, but their access levels are modified. All public and protected members of the base class become private members in the derived class.

Implications of Private Members

Once inherited, the base class's public and protected members become private in the derived class. This means they cannot be accessed by functions outside the derived class, except friend functions.

Accessing Private Members

To access private members of the derived class that are inherited, the derived class itself can use its inherited private methods and properties internally but cannot expose them to any other function or class directly unless through another public or protected method it provides.

Visibility to Further Derived Classes

If another class derives from this privately derived class, it will not have access to the base class's members directly as they have turned private at the first derivation and aren't publicly accessible.

Key concepts

Inheritance Access Control

Inheritance access control in C++ defines how the members of a base class are visible to a derived class. When using private inheritance, a derived class inherits all members of the base class, but their access levels are changed. Specifically, all public and protected members become private in the context of the derived class.
This change implies that these members, now private, cannot be accessed from outside the derived class. However, the derived class itself can still access them internally.

• Public members become private, limiting external access
• Protected members also become private in the derived class

This mechanism allows the derived class to utilize, yet conceal, the base class’s functionality from external entities.

Base and Derived Classes

The relationship between base and derived classes is fundamental in C++. A base class serves as the parent or superclass, providing properties and behaviors that can be utilized by derived, or subclass, classes. In private inheritance, these relationships focus on internal reuse rather than external interface exposure.
A derived class, when inheriting a base class privately, indicates a clear inheritance hierarchy but restricts what is exposed outside.

• Base class provides core functionality
• Derived class extends or modifies base functionality
• Private inheritance implies members are not externally visible

This setup is beneficial when you want to use base class features without making them part of the derived class’s public API.

Friend Functions

Friend functions play a special role in accessing private members that are inherited in C++. They are not part of a class but are given special access to its private members.
In the case of private inheritance, friend functions can access the inherited private members of the derived class. This means that even though these members cannot be accessed by regular functions outside the derived class, friend functions have the privilege to interact with them.

• Friend functions are defined outside of the class
• They acquire access to private members due to friendship
• Can access otherwise inaccessible inherited members

This special access allows designed interactions and utility functions while keeping access control tight.

Visibility in Derived Classes

Visibility of members in a derived class is greatly influenced by the type of inheritance. In private inheritance, the visibility of base class members transforms, thereby affecting how future derivations can interact with these members.
When a further class derives from a privately inherited class, it cannot access the private base members directly because they have been made private in the immediate derived class. This limits the propagation of those members through multiple inheritance levels.

• Members become private and hence not accessible by further derived classes
• This restricts inheritance hierarchies and forces abstraction
• Ensures encapsulation of the base class's internal structure

This encapsulation enforces strict access and promotes secure coding practices by limiting exposure of sensitive data or methods.