15.7.1. Virtual Destructors

The primary direct impact that inheritance has on copy control for a base class is that a base class generally should define a virtual destructor (§15.2.1, p. 594). The destructor needs to be virtual to allow objects in the inheritance hierarchy to be dynamically allocated.

Recall that the destructor is run when we delete a pointer to a dynamically allocated object (§13.1.3, p. 502). If that pointer points to a type in an inheritance hierarchy, it is possible that the static type of the pointer might differ from the dynamic type of the object being destroyed (§15.2.2, p. 597). For example, if we delete a pointer of type Quote*, that pointer might point at a Bulk_quote object. If the pointer points at a Bulk_quote, the compiler has to know that it should run the Bulk_quote destructor. As with any other function, we arrange to run the proper destructor by defining the destructor as virtual in the base class:

class Quote {
public:
    // virtual destructor needed if a base pointer pointing to a derived object is deleted
    virtual ~Quote() = default; // dynamic binding for the destructor
};

Like any other virtual, the virtual nature of the destructor is inherited. Thus, classes derived from Quote have virtual destructors, whether they use the synthesized destructor or define their own version. So long as the base class destructor is virtual, when we delete a pointer to base, the correct destructor will be run:

Quote *itemP = new Quote;   //  same static and dynamic type
delete itemP;               //  destructor for Quote called
itemP = new Bulk_quote;     //  static and dynamic types differ
delete itemP;               //  destructor for Bulk_quote called

Destructors for base classes are an important exception to the rule of thumb that if a class needs a destructor, it also needs copy and assignment (§13.1.4, p. 504). A base class almost always needs a destructor, so that it can make the destructor virtual. If a base class has an empty destructor in order to make it virtual, then the fact that the class has a destructor does not indicate that the assignment operator or copy constructor is also needed.

Virtual Destructors Turn Off Synthesized Move

The fact that a base class needs a virtual destructor has an important indirect impact on the definition of base and derived classes: If a class defines a destructor—even if it uses = default to use the synthesized version—the compiler will not synthesize a move operation for that class (§13.6.2, p. 537).