C++ friendship: which statement correctly describes valid use of the friend keyword inside a class?

Introduction / Context:
Friendship in C++ is an access-control feature that allows a specified function or class to access the private and protected members of another class. It is commonly used to implement symmetric operators, tight collaborations between classes, or non-member helper functions that require privileged access without making data public.

Given Data / Assumptions:

• friend grants access to named functions or entire classes, not to specific data members.
• Friendship is neither inherited nor transitive.
• Keywords cannot be reused as identifiers.

Concept / Approach:

Inside a class definition, you can declare either a specific function or another class as a friend. Doing so gives the friend full access to private/protected members of the declaring class. You cannot declare a particular data member as a friend; access always targets functions or whole classes. Friendship is to types or functions, not specific object instances, so “an object as a friend” is meaningless. Finally, the token friend is a reserved keyword and cannot be used as a class name.

Step-by-Step Solution:

Verification / Alternative check:

Create two classes and declare one as friend of the other. Access private fields in the friend class's methods; it compiles. Attempt to mark a single data member as friend or to friend an object variable; the compiler rejects these constructs.

Why Other Options Are Wrong:

Private data member as friend — syntactically and semantically unsupported.

Object as friend — friendship is granted to functions or classes, not instances.

friend as class name — violates the reserved keyword rule.

Common Pitfalls:

• Overusing friendship instead of designing clearer public interfaces.
• Expecting friendship to be inherited down the hierarchy; it is not.

Final Answer:

A class may be declared as a friend.