Virtual Destructors in C++

Introduction

In C++, destructors are special member functions invoked when an object goes out of scope or is explicitly deleted. A virtual destructor ensures that the destructor of a derived class is called when an object is deleted through a base class pointer. This is crucial for proper resource management and to avoid memory leaks.

The Need for Virtual Destructors

When you delete an object through a base class pointer, and the base class has a non-virtual destructor, the derived class’s destructor won’t be called. This can lead to resource leaks because the derived class may allocate resources that need to be released in its destructor.

#include <iostream>

class Base {
public:
    ~Base() {
        std::cout << "Base Destructor" << std::endl;
    }
};

class Derived : public Base {
public:
    ~Derived() {
        std::cout << "Derived Destructor" << std::endl;
    }
};

int main() {
    Base* b = new Derived();
    delete b;
    return 0;
}
                    

Using Virtual Destructors

To ensure the derived class’s destructor is called, you should declare the base class’s destructor as virtual.

#include <iostream>

class Base {
public:
    virtual ~Base() {
        std::cout << "Base Destructor" << std::endl;
    }
};

class Derived : public Base {
public:
    ~Derived() {
        std::cout << "Derived Destructor" << std::endl;
    }
};

int main() {
    Base* b = new Derived();
    delete b;
    return 0;
}
                    

Key Points

• Always declare destructors as virtual in base classes intended to be inherited from.
• Virtual destructors ensure that the destructors of derived classes are invoked, preventing resource leaks.
• Even if a base class destructor is empty, it should be declared virtual if the class is intended to be a base class.

Conclusion

Virtual destructors are a crucial aspect of C++ polymorphism. They ensure that objects are cleaned up correctly when deleted through a base class pointer. This tutorial has provided an overview of virtual destructors, why they are necessary, and how to use them properly in C++.