What is the purpose of the virtual keyword in C++?To define a function that can be overridden in a derived class.To define a function that cannot be overridden.To declare a function outside of a class.To create a template function.

The purpose of the virtual keyword in C++ is to define a function that can be overridden in a derived class. When a function is declared as virtual in a base class, the function can be redefined in any derived class. This allows for polymorphism, or the ability for objects of different classes related by inheritance to respond differently to the same function call.

Here's a step-by-step explanation:

1. Declare a function in the base class as virtual. This makes the function "overridable" in any class that inherits from this base class.

class Base {
public:
    virtual void print() {
        cout << "This is the base class." << endl;
    }
};

2. In the derived class, you can override the virtual function from the base class.

class Derived : public Base {
public:
    void print() override { // 'override' keyword is optional but good practice
        cout << "This is the derived class." << endl;
    }
};

3. Now, if you have a pointer or reference to the base class, you can call the print function and it will execute the version of the function that corresponds to the actual object type, not the type of the pointer or reference.

Base* ptr = new Derived();
ptr->print(); // Outputs: "This is the derived class."

In this example, even though ptr is a pointer to Base, the print function of the Derived class is called. This is because print was declared as virtual in the Base class, and then overridden in the Derived class.