What's the Difference Between Virtual and Pure Virtual Functions in C ?

Virtual vs Pure Virtual Functions

In object-oriented programming, virtual functions and pure virtual functions are essential concepts for understanding polymorphism.

What is a Virtual Function?

A virtual function is an inheritable and overridable function in a base class that allows for dynamic dispatch. When a virtual function is called on an object, the runtime system determines the actual function implementation to execute based on the object's type.

Example:

class Base {
public:
    virtual void foo() { std::cout << "Base::foo()" << std::endl; }
};

class Derived : public Base {
public:
    void foo() override { std::cout << "Derived::foo()" << std::endl; }
};

In this example, both Base and Derived classes implement the foo() function. When foo() is called on a Derived object, it executes the overridden version, which prints "Derived::foo()."

What is a Pure Virtual Function?

A pure virtual function, also known as an abstract function, is a virtual function that is not defined in the base class. It serves as a placeholder, requiring derived classes to implement it.

Example:

class Base {
public:
    virtual void foo() = 0; // pure virtual function
};

This example declares foo() as a pure virtual function in the Base class. Derived classes must implement this function to provide a definition. If they do not, the derived class becomes abstract and cannot be instantiated.

Difference between Virtual and Pure Virtual Functions

• Virtual functions can be overridden and customized in derived classes, but they have a default implementation in the base class.
• Pure virtual functions have no implementation in the base class and must be implemented in derived classes.
• Pure virtual functions force a base class to be abstract (i.e., it cannot be instantiated directly).
• Virtual functions can be used to implement polymorphism, allowing objects of different derived classes to behave differently when responding to the same function call.
• Pure virtual functions enforce a contract between base and derived classes, ensuring that implementations exist in all non-abstract derived classes.

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