diff --git a/doc/quickbook/zip_iterator.qbk b/doc/quickbook/zip_iterator.qbk
index 53daf81..72d57d3 100644
--- a/doc/quickbook/zip_iterator.qbk
+++ b/doc/quickbook/zip_iterator.qbk
@@ -52,9 +52,10 @@ These two iterations can now be replaced with a single one as follows:
 A non-generic implementation of `zip_func` could look as follows:
 
 
-      struct zip_func :
-        public std::unary_function<const boost::tuple<const double&, const int&>&, void>
+      struct zip_func
       {
+        using result_type = void;
+
         void operator()(const boost::tuple<const double&, const int&>& t) const
         {
           m_f0(t.get<0>());