Do We Need to Declare Both Constructor and Destructor as Pure Virtual in a Derived Class?

The question often arises in object-oriented programming: do we need to declare both the constructor and destructor as pure virtual functions in a derived class if they have already been declared as pure virtual functions in the base class? This article explores this topic and provides insights based on C concepts and practices.

Understanding Virtual Functions

A virtual function in C allows a function to be overridden in a derived class. A pure virtual function (often referred to as a 0-function) in C is a function declaration with an equal sign (``) and an inline zero, indicating that the class cannot be instantiated and serves as a contract or a template for derived classes to implement.

Constructors and Virtuality

The first important point to note is that constructors cannot be virtual, let alone pure virtual. This inherent property stems from the fact that constructors are used to initialize objects, and in order to do so, they must be called directly and not through the base class.

Constructors are crucial for creating and initializing objects, and virtuality would disrupt this process. Therefore, marking a constructor as pure virtual is not possible, as it would prevents the creation of any derived class instances.

Destructor and Virtuality

The behavior of destructors is quite different from constructors. While constructors decide object creation, destructors manage object destruction. As discussed earlier, declaring a destructor as pure virtual forces a derived class to provide an implementation for it, otherwise, the derived class fails to instantiate.

Inheritance and Virtuality

When a destructor in the base class is pure virtual, the derived class must either provide its own implementation or inherit the default destructor provided by the base class. This is because, if a base class provides a default destructor (non-virtual, non-pure), the derived class will automatically inherit it unless explicitly overridden.

The derived class destructor can also be declared virtual, but this is not strictly necessary, especially in a case where the base class destructor is already pure virtual. The derived class destructor is not declared as pure virtual unless it needs to be explicitly overridden to provide specific cleanup behavior.

Example Scenarios

Base Class with Pure Virtual Destructor

Consider a Shape class with a pure virtual destructor:

class Shape {public:    Shape() {}   // Base class constructor    virtual ~Shape()  0;  // Pure virtual destructor};

In this scenario, the derived class, Circle, must either provide its own implementation or inherit the base class destructor:

class Circle : public Shape {public:    Circle() {}  // Derived class constructor    virtual ~Circle();  // Required for the derived class to be used};

A virtual method does not have to be declared as pure virtual, but in the context of destructors, if a pure virtual destructor is declared in the base class, it forces the derived class to implement it.

Base Class without Virtual Destructor

If the base class does not declare a virtual destructor, it is not mandatory for the derived class to implement one:

class Shape {public:    Shape() {}   // Base class constructor    void someMethod() {}  // Non-virtual method};

Derived class may or may not have a virtual destructor depending on its specific needs:

class Circle : public Shape {public:    Circle() {}  // Derived class constructor    ~Circle() {} // No pure virtual required};

Overriding and Polymorphism

When a base class destructor is not pure virtual, there is no compulsion for the derived class to implement a virtual destructor. If a derived class needs to override the destructor in a specific way, it must be marked as virtual:

class Shape {public:    virtual ~Shape();  // Non-pure virtual destructor};

Derived class can then override the virtual destructor if needed:

class Circle : public Shape {public:    ~Circle() {}  // Overriding the virtual destructor};

Conclusion

In summary, constructors cannot be pure virtual, and whether a destructor needs to be declared as pure virtual in the derived class depends on the design and intent of the base class. If a base class has a pure virtual destructor, the derived class is obligated to implement it, whereas if the base class destructor is non-virtual or not pure virtual, the derived class has no such requirement.

Key Takeaways:

Constructors cannot be virtual. A pure virtual destructor in the base class compels the derived class to implement it. Destructors in the derived class default to inheriting the base class's destructor, unless overridden to provide specific cleanup behavior.

Understanding these principles can help in designing more robust and maintainable C applications. For further reading, please see the resources provided for a deeper dive into C concepts.