Virtual Destructors

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A destructor can be declared as virtual. This allows a pointer to a base class object to call the correct destructor in the event that the pointer actually refers to a derived class object. The destructor of a class derived from a class with a virtual destructor is itself virtual.

/* How virtual affects the order of destructor calls.
   Without a virtual destructor in the base class, the derived
   class destructor won't be called. */
#include <iostream>
class color {
public:
   virtual ~color() {  // Virtual destructor
      std::cout << "color dtor\n";
      }
};
class red : public color {
public:
   ~red() {  // This destructor is also virtual
      std::cout << "red dtor\n";
      }
};
class brightred : public red {
public:
   ~brightred() {  // This destructor is also virtual
       std::cout << "brightred dtor\n";
       }
};
int main()
{
   color *palette[3];
   palette[0] = new red;
   palette[1] = new brightred;
   palette[2] = new color;
   // The destructors for red and color are called.
   delete palette[0];
   std::cout << std::endl;
   // The destructors for bright red, red, and color are called.
   delete palette[1];
   std::cout << std::endl;
   // The destructor for color is called.
   delete palette[2];
   return 0;
}

Program Output:

red dtor
color dtor
brightred dtor
red dtor
color dtor
color dtor

However, if no destructors are declared as virtual, delete palette[0], delete palette[1], and delete palette[2] would all call only the destructor for class color. This would incorrectly destruct the first two elements, which were actually of type red and brightred.

See Also