[boost] Generic non-derivable base class

From: Matthew Vogt (mvogt_at_[hidden])
Date: 2004-04-06 17:25:09

Is there any interest in creating a generic base class to encapsulate the
non-derivable class idiom, as recently discussed wrt Reece Dunn's error check
component?

Since the the idiom is on a similar complexity level to that of noncopyable, I
feel it would be appropriate to encapsulate it, if a general solution is
possible.

The following simple base class implements the mechanism as dicussed on 
Stroustrup's FAQ page, and encapsulates the virtual inheritance so that the
class can be non-virtually inherited, removing the most obvious chance of
implementation error.
 
Will this code work on multiple platforms?  I have only tested on gcc 3.2...

//-------------------------------

namespace boost {

template <typename T>
struct nonderivable;

namespace detail {

template <typename T>
class nonderivable_base
{
private:
  template <typename X>
  struct identity { typedef X type; };

  friend typename identity<T>::type;
  friend class nonderivable<T>;

  nonderivable_base() {}
  nonderivable_base(const nonderivable_base&) {}
};

} //namespace detail

template <typename T>
struct nonderivable : public virtual detail::nonderivable_base<T>
{
};

} // namespace boost

// ------------------------

struct A : public boost::nonderivable<A> {}; 
struct B : public A {};

A a;  // Ok
B b;  // Error - cannot be instantiated

// ------------------------

Matt