C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 20:
20.1 What is a "virtual member function"?
20.2 How can C++ achieve dynamic binding yet also static typing?
20.3 Difference between calling virtual and non-virtual member functions?
20.4 Implementation details and efficiency of calling a virtual function?
20.5 How can a member function in my derived class call the same function from its base class?
20.6 Using dynamic binding with objects in heterogeneous containers?
20.7 When should my destructor be virtual?
20.8 What is a "virtual constructor"?
[20.7] When should my destructor be virtual?

When someone will delete a derived-class object via a base-class pointer.

In particular, here's when you need to make your destructor virtual:

  • if someone will derive from your class,
  • and if someone will say new Derived, where Derived is derived from your class,
  • and if someone will say delete p, where the actual object's type is Derived but the pointer p's type is your class.

Confused? Here's a simplified rule of thumb that usually protects you and usually doesn't cost you anything: make your destructor virtual if your class has any virtual functions. Rationale:

  • that usually protects you because most base classes have at least one virtual function.
  • that usually doesn't cost you anything because there is no added per-object space-cost for the second or subsequent virtual in your class. In other words, you've already paid all the per-object space-cost that you'll ever pay once you add the first virtual function, so the virtual destructor doesn't add any additional per-object space cost. (Everything in this bullet is theoretically compiler-specific, but in practice it will be valid on almost all compilers.)

Note: in a derived class, if your base class has a virtual destructor, your own destructor is automatically virtual. You might need an explicitly defined destructor for other reasons, but there's no need to redeclare a destructor simply to make sure it is virtual. No matter whether you declare it with the virtual keyword, declare it without the virtual keyword, or don't declare it at all, it's still virtual.

BTW, if you're interested, here are the mechanical details of why you need a virtual destructor when someone says delete using a Base pointer that's pointing at a Derived object. When you say delete p, and the class of p has a virtual destructor, the destructor that gets invoked is the one associated with the type of the object *p, not necessarily the one associated with the type of the pointer. This is A Good Thing. In fact, violating that rule makes your program undefined. The technical term for that is, "Yuck."