Question

I just read recently that its a good practice to implement C++ destructors as virtual functions[1]. Why is this so? Is this a general good practice? If not, in what conditions/cases is a destructor to be made a virtual function?

Reference

1. https://www.blackhat.com/presentations/bh-usa-07/Afek/Whitepaper/bh-usa-07-afek-WP.pdf

Answer 1

If a base class has a destructor and it is NOT virtual, then any child class’s destructor will not be called if delete is invoked on the base class’s pointer.

This can result in memory leaks.

class Shape
{
public:
    Shape()
    {
        cout << "Shape constructor called" << endl;
    }
    // This destructor should be virtual!
    ~Shape()
    {
        cout << "~Shape destructor called" << endl;
    }
};

class Triangle : public Shape
{
public:
    Triangle()
    {
        cout << "Triangle constructor called" << endl;
    }
     ~Triangle()
    {
        cout << "Triangle destructor called" << endl;
    }

}

int main(int argc, char* argv[])
{
     Shape* pShape = new Triangle();
     cout << "About to call delete" << endl;
     delete pShape;
}

This will result in:

Triangle constructor called
Shape constructor called
About to call delete
Shape destructor called

Any resources that should be deallocated in the triangles destructor has now leaked.