Choose true statements about virtual functions in C++:
I: The virtual functions must be member of some class.
II: They cannot be static members.
III: A virtual function can be a friend of another class.

Question

Choose true statements about virtual functions in C++: 
I: The virtual functions must be member of some class. 
II: They cannot be static members. 
III: A virtual function can be a friend of another class.

Accepted Answer

The correct option is (4):
The correct option is (4): I, II and III are all true statements regarding virtual functions in C++. A virtual function is a C++ mechanism that enables runtime polymorphism, ensuring the correct overridden function is called based on the object's actual type, not the pointer or reference type. Statement I is true because the entire virtual mechanism relies on the inheritance structure of classes and the per-object Virtual Table (vtable), meaning the function must be a member of some class. Statement II is also fundamentally true: virtual functions cannot be static. Static member functions belong to the class as a whole (not a specific object instance) and thus cannot use the vtable mechanism for dynamic dispatch, which requires an object instance. Statement III is considered true in C++: while a *friend function itself* cannot be virtual (since it is not a member), a *virtual member function* of one class can certainly be declared as a friend of *another* class, which is a valid C++ concept involving access permissions. All three principles accurately define the rules of C++ virtual functions.

The incorrect option is (1):
Options (1), (2), and (3) are incorrect because they fail to capture the complete set of true statements. Option (1) includes I and II but incorrectly omits statement III, which is a valid concept in C++ regarding friendship and virtual members. The question asks for all *true* statements, and by excluding III, this option is incomplete.

The incorrect option is (2):
Option (2) incorrectly excludes statement I, which is the most basic rule of virtual functions. A function must be a member of a class to be virtual, as the purpose of the virtual keyword is to enable dynamic binding within an object-oriented class hierarchy. Furthermore, a non-member function, which includes a standalone friend function, cannot be declared virtual.

The incorrect option is (3):
Option (3) incorrectly excludes statement II. The C++ standard explicitly forbids declaring a virtual function as static. The core function of the `static` keyword is to bind a function to the class itself (compile-time binding), making it independent of any object instance. This directly clashes with the purpose of the `virtual` keyword, which is to implement dynamic binding at runtime using the object's instance-specific vtable.