In C++ object oriented programming, what are virtual functions in the context of inheritance and polymorphism?

Introduction / Context:
Virtual functions are a central mechanism for implementing runtime polymorphism in C++. They allow code that works through a base class pointer or reference to invoke derived class implementations of overridden methods. Understanding virtual functions is essential for designing class hierarchies, using interfaces, and avoiding common pitfalls such as object slicing and missing destructors. This question asks you to identify the correct conceptual description of virtual functions.

Given Data / Assumptions:

• We are working with C++ classes that participate in inheritance relationships.
• Some member functions are declared with the virtual keyword in the base class.
• Derived classes may override these virtual functions to provide specialised behaviour.
• Calls are often made through base class pointers or references.

Concept / Approach:
A virtual function in C++ is a non static member function declared with the virtual keyword in a base class. When a derived class overrides that function with a matching signature, calls to the function through a base class pointer or reference are dispatched dynamically based on the actual object type at runtime. This behaviour is known as dynamic dispatch or late binding. The compiler usually implements virtual dispatch using a table of function pointers often called a vtable, although the exact implementation is not mandated by the standard. Virtual functions enable polymorphic behaviour, where different derived types can respond differently to the same function call interface.

Step-by-Step Solution:
Step 1: Recall that the virtual keyword on a member function in a base class signals that this function is intended to be overridden in derived classes. Step 2: Recognise that when you call a virtual function through a base class pointer or reference, the call is resolved at runtime to the most derived override. Step 3: Understand that this runtime polymorphism allows writing code that depends only on the base interface but works correctly for many derived types. Step 4: Check option a, which explicitly mentions virtual keyword, runtime polymorphism, and dynamic dispatch through a base pointer or reference. Step 5: Confirm that other options either refer to unrelated domains or misstate the properties of virtual functions.

Verification / Alternative check:
Consider a base class Shape with a virtual function draw and derived classes Circle and Square that override draw. If you have Shape* p = new Circle; and then call p->draw;, the Circle implementation will run because draw is virtual. If you later assign Shape* p2 = new Square; and call p2->draw;, the Square implementation runs. This is exactly the behaviour described in option a and demonstrates runtime polymorphism via virtual functions.

Why Other Options Are Wrong:
Option b confuses virtual functions with virtual reality concepts, which have nothing to do with C++ language features. Option c claims that any function defined inside a class body is virtual, but C++ requires the explicit virtual keyword, except for special cases like destructors in polymorphic hierarchies where virtual may be implied by inheritance. Option d states that virtual functions are always inlined and never incur call overhead, which is incorrect; virtual calls normally use an indirect lookup, and while compilers may devirtualise some calls, this is not guaranteed and is not part of the definition of virtual functions.

Common Pitfalls:
Common mistakes include forgetting to declare the base class destructor as virtual in polymorphic hierarchies, which can lead to resource leaks when deleting through a base pointer. Another pitfall is accidentally hiding rather than overriding a virtual function due to mismatched signatures. Developers must also balance the power of virtual functions with performance considerations, using them where polymorphism is genuinely needed. For exam answers, focus on the core concept: virtual functions enable runtime polymorphism and dynamic dispatch via base class interfaces.

Final Answer:
In C++, virtual functions are member functions declared with the virtual keyword that support runtime polymorphism and dynamic dispatch through a base class pointer or reference.