Remove 'local_iterator_base'.

Merged revisions 42153-42156 via svnmerge from 
https://svn.boost.org/svn/boost/branches/unordered/dev

........
  r42153 | danieljames | 2007-12-18 22:51:31 +0000 (Tue, 18 Dec 2007) | 2 lines
  
  Remove 'local_iterator_base'. It isn't really needed.
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  r42155 | danieljames | 2007-12-18 22:58:12 +0000 (Tue, 18 Dec 2007) | 1 line
  
  Add a helper function for creating null pointers.
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  r42156 | danieljames | 2007-12-18 23:14:49 +0000 (Tue, 18 Dec 2007) | 3 lines
  
  Check that I'm using link_ptr & bucket_ptr correctly (they should be distinct
  types with an explicit conversion from bucket_ptr to link_ptr).
........



[SVN r42189]

include/boost/unordered/detail/allocator.hpp

@@ -42,6 +42,9 @@ namespace boost {
 #if !BOOST_WORKAROUND(BOOST_MSVC, < 1300)
         template <class T>
         inline void reset(T& x) { x  = T(); }
+
+        template <class Ptr>
+        inline Ptr null_ptr() { return Ptr(); }
 #else
         template <class T>
         inline void reset_impl(T& x, ...) { x  = T(); }
@@ -49,6 +52,9 @@ namespace boost {
         inline void reset_impl(T*& x, int) { x  = 0; }
         template <class T>
         inline void reset(T& x) { reset_impl(x); }
+
+        template <class Ptr>
+        inline Ptr null_ptr() { Ptr x; reset(x); return x; }
 #endif
 
         // Work around for Microsoft's ETI bug.