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DiscussionThis simple guideline illustrates a subtle issue and reflects modern uses of inheritance and object-oriented design principles. For a base class Base, calling code might try to delete derived objects through pointers to Base. If Base's destructor is public and nonvirtual (the default), it can be accidentally called on a pointer that actually points to a derived object, in which case the behavior of the attempted deletion is undefined. This state of affairs has led older coding standards to impose a blanket requirement that all base class destructors must be virtual. This is overkill (even if it is the common case); instead, the rule should be to make base class destructors virtual if and only if they are public. To write a base class is to define an abstraction (see Items 35 through 37). Recall that for each member function participating in that abstraction, you need to decide:
As described in Item 39, for a normal member function, the choice is between allowing it to be called via a Base* nonvirtually (but possibly with virtual behavior if it invokes virtual functions, such as in the NVI or Template Method patterns), virtually, or not at all. The NVI pattern is a technique to avoid public virtual functions. Destruction can be viewed as just another operation, albeit with special semantics that make nonvirtual calls dangerous or wrong. For a base class destructor, therefore, the choice is between allowing it to be called via a Base* virtually or not at all; "nonvirtually" is not an option. Hence, a base class destructor is virtual if it can be called (i.e., is public), and nonvirtual otherwise. Note that the NVI pattern cannot be applied to the destructor because constructors and destructors cannot make deep virtual calls. (See Items 39 and 55.) Corollary: Always write a destructor for a base class, because the implicitly generated one is public and nonvirtual. |
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