TITLE: Pure virtual destructors


PROBLEM: krames@dbai.tuwien.ac.at (Gerfried Krames)


1) Is the following declaration legal?
	class X {
		...
	public:
		X();
		virtual ~X() = 0;
	};


RESPONSE: rmartin@rcmcon.com (Robert Martin), 17 May 95

Pure virtual destructors are an interesting aspect of C++.  If a
function is pure virtual, then the class that contains it is abstract,
and C++ will not allow it to be instantiated.

Purity is inherited.  Thus:

   struct B {virtual void f() = 0;};  
   struct D : B {};   // abstract class, pure virtual f is inherited.

However, destructors are *not* inherited.  Thus:

   struct B {virtual ~B() = 0;};
   struct D : B {} // concrete class.  Compiler generates destructor.

All classes must have a destructor.  If one is not specified, then the
compiler will write one for you.  If your destructor is pure virtual,
you still have to supply an implementation.

Thus:

  struct B {virtual ~B() = 0;}
  B::~B() {/* do whatever */}

If you don't implement the pure virtual destructor, you will get a 
link error.

-------------------------------------

What all this adds up to is this.  A pure virtual destructor is
exactly like a regular virtual destructor, except that it makes the
class it is declared in, abstract.  It does not affect derived classes
at all.  In particular, it does not make it a compiler error to omit
destructor declarations from derived classes.

Thus, I use pure virtual destructors as a way of making a class
abstract, if that class has no other pure virtual functions.