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

The virtual keyword in C++ plays a crucial role in enabling polymorphism, particularly in the context of inheritance. When a base class declares a function as virtual, it tells the compiler that this function can be overridden in any derived class. This capability allows a program to determine at runtime which function to execute based on the type of the object being pointed to or referenced, rather than the type of the pointer or reference itself.

This mechanism is essential for achieving dynamic binding, which facilitates the creation of flexible and reusable code structures. For instance, when a base class pointer is used to refer to an object of a derived class, using a virtual function call ensures that the overridden function in the derived class gets executed.

In contrast, the other options relate to different concepts in C++. Declaring a constant variable is done using the const keyword, creating an abstract class involves declaring at least one pure virtual function, and indicating a private variable is typically done using the private access specifier. Each of these options pertains to distinct aspects of C++ and does not capture the primary purpose of the virtual keyword. Thus, the role of virtual is specifically tied to allowing method overriding, making dynamic polymorphism possible in C++.