Question

Assume the declaration of Exercise \(22 .\) Suppose that class third is derived from class first using the statement: class third: protected first Determine which members of class first are private, protected, and public in class third.

Short answer

In class `third`, only the protected and public members of `class first` become protected. Private members remain inaccessible.

Step-by-step solution

Understanding Class Inheritance

In the exercise, we have a class `third` which is derived from a class `first` with the inheritance type `protected`. The statement `class third: protected first` indicates that the inheritance model used here is protected, which affects how members of `first` are accessed in `third`.

Reviewing Access Specifiers

In C++, there are three access specifiers for class members: private, protected, and public. Private members are accessible only within the class; protected members are accessible within the class and by derived classes; public members are accessible from anywhere the class is visible.

Effect of Protected Inheritance

With protected inheritance, public and protected members of the base class (`first`) become protected members of the derived class (`third`). Private members of the base class remain inaccessible to the derived class.

Identifying Member Access in Class third

Given the protected inheritance (`class third: protected first`), in class `third`: - Private members of `first` are not accessible. - Protected members of `first` become protected members. - Public members of `first` become protected members.

Key concepts

Access Specifiers

In C++, a fundamental concept for classes is the use of access specifiers to control the visibility and accessibility of class members. There are three main types of access specifiers:

• Private: Members declared as private are only accessible within the class itself. This means that any private member variables or functions cannot be accessed or modified outside of the class definition. Private access ensures encapsulation by restricting access to the internal workings of the class.
• Protected: The protected access specifier allows access to class members for the class itself and any derived classes. However, they are not accessible from outside the class hierarchy. This specifier is useful in inheritance scenarios where derived classes need to operate on the base class's members.
• Public: Members declared public are accessible from anywhere in the program where the class is visible. This specifier is used when you want certain functions or variables of a class to be accessed freely, facilitating outside interaction with the class.

Understanding these access levels is crucial for designing secure and efficient class structures, as they dictate how objects of the class can interact with their components and each other.

Protected Inheritance

In protected inheritance, the base class is accessed by the derived class through the `protected` keyword. This inheritance model has particular effects on the access levels of the inherited members:

• Protected and Public Members become Protected: When derivation is through protected inheritance, both protected and public members of the base class transition to protected members in the derived class. This means they become available only to the derived class and its subclasses, not to external code.
• Private Members Stay Private: As with any form of inheritance in C++, the private members of the base class remain inaccessible to the derived class. To allow derived classes access to these, you'd need to define protected or public member functions in the base class.

This mode of inheritance is suitable when you want to expose the functionality to derived classes without allowing clients of your class to access base class functionalities directly. It's a middle ground between private and public inheritance.

Class Members Accessibility

The accessibility of class members in a derived class is governed by the type of inheritance used and the original access specifiers of the base class members.

• In protected inheritance, like in our exercise scenario, both public and protected members of the base class become protected in the derived class. This retains an element of internal control while allowing derived class extensions.
• Private members of the base class remain private, ensuring that they are completely encapsulated from any sort of access by the derived class. To provide a mechanism for derived classes to access these members, base class methods, either protected or public, should be utilized.
• Any attempt to access base class members from the context of a derived class must respect the accessibility rules derived from both the base class's definition and the type of inheritance.

This framework lets you design a flexible class architecture where you can selectively expose class internals, providing power without undermining encapsulation, a key principle of Object Oriented Programming (OOP).