Virtual destructor with example

A virtual destructor is a destructor declared with the keyword virtual in the base class.

Need

When a derived class object is deleted through a base class pointer, and the base destructor is not virtual, only the base destructor is called. The derived destructor is skipped, so resources acquired by the derived class (memory, files) are never released, causing a memory leak and undefined behaviour. Making the base destructor virtual ensures the derived destructor runs first, then the base destructor, in the correct order.

Without virtual destructor

#include <iostream>
using namespace std;

class Base {
public:
    ~Base() { cout << "Base destructor" << endl; }
};
class Derived : public Base {
    int *p;
public:
    Derived() { p = new int[100]; }
    ~Derived() { delete[] p; cout << "Derived destructor" << endl; }
};

int main() {
    Base *b = new Derived;
    delete b;        // only Base destructor runs: memory leak
    return 0;
}

Output: Base destructor

With virtual destructor

#include <iostream>
using namespace std;

class Base {
public:
    virtual ~Base() { cout << "Base destructor" << endl; }
};
class Derived : public Base {
    int *p;
public:
    Derived() { p = new int[100]; }
    ~Derived() { delete[] p; cout << "Derived destructor" << endl; }
};

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

Output:

Derived destructor
Base destructor

Rule: any class meant to be used as a base class with virtual functions should have a virtual destructor. Constructors cannot be virtual.