In C++, which of the following is not a valid access specifier ?

Question

Accepted Answer

The correct option is (4):
The keyword mutable is not an access specifier in C++; rather, it is a storage class specifier with a specific, unrelated purpose. Access specifiers—which are limited to private, public, and protected—are primarily used to enforce encapsulation and define the accessibility boundaries of a class’s members (data and methods) to the rest of the program. Mutable, by contrast, is used exclusively to permit a specific non-static data member of a class to be modified even when that member is part of an object that has been declared as constant (`const`). Since its role is to override the const-ness of an object’s member and not to define access policy, it is correctly identified as the option that is not a valid access specifier.

The incorrect option is (1):
The keyword private is one of the three fundamental access specifiers in C++ and is crucial for implementing the core Object-Oriented Programming (OOP) principle of data hiding. Any class members declared after the private specifier can only be accessed by the member functions and friend functions/classes of that very class. This restriction prevents external code and derived classes from directly modifying the internal state of an object, thereby ensuring data integrity and correct usage. Therefore, private is a perfectly valid and foundational access specifier.

The incorrect option is (2):
The keyword public is a standard C++ access specifier that is used to define the external interface of a class. Members declared as public are completely accessible from anywhere in the program: inside the class, by derived classes, and by external code using an object of the class. Public members typically include the class’s constructors, destructors, and accessor/mutator methods that provide a controlled way to interact with the private data. As it defines the unrestricted accessibility of a member, public is a valid access specifier.

The incorrect option is (3):
The keyword protected is the third primary access specifier in C++, designed specifically to manage access in the context of inheritance hierarchies. Protected members are similar to private members in that they cannot be accessed by arbitrary external code outside the class. However, the critical difference is that protected members *can* be accessed by the member functions of any classes derived (inherited) from the original class. This controlled sharing between a parent class and its children makes protected a valid and vital access specifier.