virtual keyword in cpp

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In C++, the virtual keyword is used in the context of object-oriented programming to specify that a base class function can be overridden by a function with the same name and signature in a derived class. This enables run-time polymorphism, allowing you to call the appropriate derived class function through a pointer or reference to the base class.

Here’s how the virtual keyword works:

  1. Declaring a Virtual Function: When you declare a function as virtual in the base class, it indicates that this function can be overridden by derived classes. It’s a signal to the compiler that a dynamic dispatch mechanism is involved. 
    class Base {
public:
    virtual void foo() {
        // Base class implementation
    }
};

     

  2. Overriding a Virtual Function: In a derived class, you can use the override keyword to explicitly indicate that you intend to override a virtual function from the base class. This helps catch potential errors during compilation. 
    class Derived : public Base {
public:
    void foo() override {
        // Derived class implementation
    }
};

     

  3. Polymorphic Behavior: When you have a pointer or reference to a base class object, and you call a virtual function through that pointer/reference, the actual function executed is determined at runtime based on the object’s true type (the derived class type). 
    Base* ptr;
Derived derivedObj;
ptr = &derivedObj;
ptr->foo(); // Calls the overridden function in Derived

     

    Without the virtual keyword in the base class, you would get the behavior of the base class’s function, even when calling through a pointer to a derived class object.

Here are some key points to remember about virtual functions:

  • Virtual functions are typically used in the context of inheritance and polymorphism.
  • They allow you to achieve runtime polymorphism, where the appropriate function is called based on the actual object’s type.
  • Not all functions in a class need to be declared as virtual. You should declare only those functions in the base class that are intended to be overridden in derived classes.
  • In C++, virtual functions come with a small performance overhead due to the need for vtables (virtual function tables) to maintain the runtime dispatch.

Using the virtual keyword and overriding functions is essential for building flexible and extensible class hierarchies and is a fundamental concept in object-oriented programming with C++.

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