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Thinking in C++
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Destructors and virtual destructors

You cannot use the virtual keyword with constructors, but destructors can and often must be virtual.

The constructor has the special job of putting an object together piece-by-piece, first by calling the base constructor, then the more derived constructors in order of inheritance (it must also call member-object constructors along the way). Similarly, the destructor has a special job: it must disassemble an object that may belong to a hierarchy of classes. To do this, the compiler generates code that calls all the destructors, but in the reverse order that they are called by the constructor. That is, the destructor starts at the most-derived class and works its way down to the base class. This is the safe and desirable thing to do because the current destructor can always know that the base-class members are alive and active. If you need to call a base-class member function inside your destructor, it is safe to do so. Thus, the destructor can perform its own cleanup, then call the next-down destructor, which will perform its own cleanup, etc. Each destructor knows what its class is derived from, but not what is derived from it.

You should keep in mind that constructors and destructors are the only places where this hierarchy of calls must happen (and thus the proper hierarchy is automatically generated by the compiler). In all other functions, only that function will be called (and not base-class versions), whether it’s virtual or not. The only way for base-class versions of the same function to be called in ordinary functions (virtual or not) is if you explicitly call that function.

Normally, the action of the destructor is quite adequate. But what happens if you want to manipulate an object through a pointer to its base class (that is, manipulate the object through its generic interface)? This activity is a major objective in object-oriented programming. The problem occurs when you want to delete a pointer of this type for an object that has been created on the heap with new. If the pointer is to the base class, the compiler can only know to call the base-class version of the destructor during delete. Sound familiar? This is the same problem that virtual functions were created to solve for the general case. Fortunately, virtual functions work for destructors as they do for all other functions except constructors.

//: C15:VirtualDestructors.cpp
// Behavior of virtual vs. non-virtual destructor
#include <iostream>
using namespace std;

class Base1 {
  ~Base1() { cout << "~Base1()\n"; }

class Derived1 : public Base1 {
  ~Derived1() { cout << "~Derived1()\n"; }

class Base2 {
  virtual ~Base2() { cout << "~Base2()\n"; }

class Derived2 : public Base2 {
  ~Derived2() { cout << "~Derived2()\n"; }

int main() {
  Base1* bp = new Derived1; // Upcast
  delete bp;
  Base2* b2p = new Derived2; // Upcast
  delete b2p;
} ///:~

When you run the program, you’ll see that delete bp only calls the base-class destructor, while delete b2p calls the derived-class destructor followed by the base-class destructor, which is the behavior we desire. Forgetting to make a destructor virtual is an insidious bug because it often doesn’t directly affect the behavior of your program, but it can quietly introduce a memory leak. Also, the fact that some destruction is occurring can further mask the problem.

Even though the destructor, like the constructor, is an “exceptional” function, it is possible for the destructor to be virtual because the object already knows what type it is (whereas it doesn’t during construction). Once an object has been constructed, its VPTR is initialized, so virtual function calls can take place.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire