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Virtual Destructors

Virtual Destructors

Tags
c++
Published
October 20, 2020
Author
Chris Chan
 

Virtual Destructors

As a general rule of thumb you should always make the destructor of your base class virtual. Why is that?
Consider the following piece of code which defines a Base and a Derived class.
class Base {
public:
    Base() {
        cout << "Base Constructor" << endl;
    }

    ~Base() {
        cout << "Base Destructor" << endl;
    }
};

class Derived : public Base {
public:
    Derived() {
        cout << "Derived Constructor" << endl;
    }

    ~Derived() {
        cout << "Derived Destructor" << endl;
    }
};

int main()
{
    Base* b = new Derived();
    delete b;
}
This piece of code will print:
Base Constructor
Derived Constructor
Base Destructor
Because the variable b was of type Base*, we only called the destructor of Base. If the Derived class were to dynamically allocate memory in its constructor, our call to delete b would not cleanup all heap allocations as desired.
To correct this behaviour we need to declare the destructor of Base as virtual.
class Base {
public:
    Base() {
        cout << "Base Constructor" << endl;
    }

    virtual ~Base() {
        cout << "Base Destructor" << endl;
    }
};

class Derived : public Base {
public:
    Derived() {
        cout << "Derived Constructor" << endl;
    }

    ~Derived() {
        cout << "Derived Destructor" << endl;
    }
};

int main()
{
    Base* b = new Derived();
    delete b;
}
This piece of code prints:
Base Constructor
Derived Constructor
Derived Destructor
Base Destructor
This is the desired behaviour that we want. So in general when dealing with polymorphic types in C++, make sure your destructors are virtual.
Sources: