What is the functionality of the virtual keyword in C++?

In C++, the virtual keyword is used to declare virtual functions. Virtual functions are a special type of function declared in the base class that can be overridden by derived classes. Its main purposes include:

1. Achieving polymorphism: through virtual functions, the actual type of an object can be determined at runtime, allowing the corresponding function to be called based on the actual type. This allows for a base class pointer or reference to point to a derived class object and call the appropriate function based on the actual object’s type.
2. Accessing the members of a derived class through a base class pointer or reference: By assigning an object of the derived class to a pointer or reference of the base class, and calling a virtual function, you can access the overridden functions in the derived class instead of those in the base class.
3. Support dynamic binding: By using virtual functions, dynamic binding can be achieved, meaning the address of the function call is determined at runtime instead of compile time. This can enhance the flexibility and maintainability of the code.

It is important to note that virtual functions can only be declared in the member functions of a class and can only be non-static member functions. Additionally, virtual functions can only be declared in the base class and cannot be re-declared in the derived class.