Can Pure Virtual Functions Have Implementations in C ?

Pure Virtual Functions: Unveiling Implementation Possibilities

The concept of pure virtual functions is commonly associated with the absence of an implementation within the base class. However, it may come as a surprise that pure virtual functions can indeed have implementations.

In C , a pure virtual function is declared using "= 0" without a function body within the base class. However, the base class is still allowed to define an implementation for the pure virtual function. This implementation is not accessible by external classes directly but can be explicitly called by a derived class using a fully-scoped name.

Consider the following code snippet:

class A {
public:
    virtual void f() = 0;
};

void A::f() {
    cout << "Test" << endl;
}

In this example, the class A defines a pure virtual function f() but also provides an implementation. The derived class B can then access this base class implementation using the fully-scoped name A::f():

class B : public A {

    virtual void f() {
        // class B doesn't have anything special to do for f()
        //  so we'll call A's

        // note that A's declaration of f() would have to be public or protected to avoid a compile time problem

        A::f();
    }

};

Providing an implementation for a pure virtual function allows for default behavior that can be overridden by derived classes. It also enables a consistent set of functionality to be called explicitly when required, even in cases where derived classes define their own implementation.

This strategy is often employed in situations where reasonable default behavior is desired, but it is essential to explicitly invoke this behavior. Additionally, it can be useful when derived classes are expected to perform their own work but also benefit from a common set of functionality.

It is important to note that providing an implementation for a pure virtual function is not a common practice in C . However, it is a valid option that can provide flexibility and customization in certain scenarios.

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