pre C++11 nullptr anonymous class

null.h

#include <core/Preprocessor.h>

namespace core {
namespace types {

  /**
   * Anonymous class, to be used instead of <code>0</code> or <code>NULL</code>.
   * Enables the selection of the correct form when methods are overloaded for
   * both pointers & integrals types.
   */
  const class
  {
    public:
#if defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ == 5) && (__GNUC_PATCHLEVEL < 2)
    ALWAYS_INLINE operator void*() const // works around bug #45383
    {
      return 0;
    }
#endif // #if defined(__GNUC__) && (__GNUC__ == 4) && __(__GNUC_MINOR__ == 5) && (__GNUC_PATCHLEVEL < 3)

    template<typename T>
    ALWAYS_INLINE operator T*() const
    {
      return 0;
    }

    template<class C, typename T>
    ALWAYS_INLINE operator T C::*() const
    {
      return 0;
    }

    private:
    void operator &() const; // not implemented on purpose

  } null = {};  // the empty brace initializer fills the ISO C++
                // (in 8.5 [dcl.ini] paragraph 9) requirements:
                //
                //   If no initializer is specified for an object, and the
                //   object is of (possibly cv-qualified) non-POD class
                //   type (or array thereof), the object shall be
                //   default-initialized; if the object is of
                //   const-qualified type, the underlying class type shall
                //   have a user-declared default constructor

} // namespace types
} // namespace core

source:

Scott Meyers, "Item 25: Avoid overloading on a pointer and a numerical type", Effective C++: 50 Specific Ways to Improve Your Programs And Designs, 2nd Edition, Addison-Wesley Professional, September 1997, ISBN: 0201924889