How can I make the base class unable to instantiate?

For instance, classes A, B, and C derive from base class X. How can I make sure it's only A, B, and C that can instantiate, and not X? Thanks in advanced.

Provide X with a pure virtual function, e.g. virtual ~X() = 0;

pure virtual methods:

Code:

class X
{
  X();
  ~X();

  virtual void PureVirtualMethod() = 0;
};

Of course it must be implemented in a subclass.

Making all constructors of X or its destructor protected also effectively prevents instantiation (except within X's member functions or its friends). That ability to instantiate only in defined places can be useful at times.

That should do the trick. Thanks.