Why should the destructor of base classes be virtual?

Question

in C++: Why should the destructor of base classes be virtual?

Answer 1

You want them to be virtual so that all subclass destructors are automatically called when the object is destroyed, even if it is destroyed through a pointer to the base class. In the following code:

class base {
public:
  virtual ~base() { }
};

class derived : public base {
public:
  ~derived() { }  // Inherits the virtual designation
};

int main(void)
{
  base *b = new derived;

  delete b;
}

The derived destructor will only be called if the base destructor is virtual.

As Magnus indicates, you don't have to do this if you aren't taking advantage of polymorphism. However, I try to develop the habit of declaring all my destructors virtual. It protects me against the case where I should have declared them virtual but forget to do so. As Johannes indicates, this habit can impose a small space and performance penalty when the virtual designation is not needed.