usability tweaks: functions that return iterators return them as *const* to allow chaining when the recipient function expects a reference (e.g. foo(Iter&) )

[SVN r35940]

include/boost/fusion/iterator/advance.hpp

@@ -75,14 +75,14 @@ namespace boost { namespace fusion
     }
 
     template <int N, typename Iterator>
-    inline typename result_of::advance_c<Iterator, N>::type
+    inline typename result_of::advance_c<Iterator, N>::type const
     advance_c(Iterator const& i)
     {
         return result_of::advance_c<Iterator, N>::call(i);
     }
 
     template<typename N, typename Iterator>
-    inline typename result_of::advance<Iterator, N>::type
+    inline typename result_of::advance<Iterator, N>::type const
     advance(Iterator const& i)
     {
         return result_of::advance<Iterator, N>::call(i);

include/boost/fusion/iterator/next.hpp

@@ -54,7 +54,7 @@ namespace boost { namespace fusion
     }
 
     template <typename Iterator>
-    typename result_of::next<Iterator>::type
+    typename result_of::next<Iterator>::type const
     next(Iterator const& i)
     {
         return result_of::next<Iterator>::call(i);

include/boost/fusion/iterator/prior.hpp

@@ -54,7 +54,7 @@ namespace boost { namespace fusion
     }
 
     template <typename Iterator>
-    typename result_of::prior<Iterator>::type
+    typename result_of::prior<Iterator>::type const
     prior(Iterator const& i)
     {
         return result_of::prior<Iterator>::call(i);