Can a Pure Virtual Function Have an Implementation?

Implementation for Pure Virtual Functions

Despite the common understanding that pure virtual functions lack implementations, certain situations allow for an implementation within the base class. Consider the following code:

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

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

While this code appears to violate the principle of pure virtual functions, it is in fact valid. However, this raises two important questions:

1. Is the Code Valid?

Yes, the code is valid syntax. The base class A can define an implementation for its pure virtual function f(), even though derived classes must also provide their own implementations.

2. Purpose of Pure Virtual Function with Implementation:

There are several potential reasons to define an implementation for a pure virtual function:

• Default Behavior: The base class can provide a default implementation that may be useful for most derived classes. Derived classes can override this implementation if necessary.
• Common Functionality: The base class implementation can provide core functionality that all derived classes should perform. This ensures consistency while allowing for flexibility in derived classes.
• Example Usage: Derived classes can explicitly call the base class implementation by fully qualifying the function name (e.g., A::f()). This can be used to invoke the default behavior or common functionality.

It's important to note that while this language feature is allowed, it is not commonly used. However, it can be a useful tool in certain scenarios where a default or shared implementation is desired.

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