fix synch iterators

Author: tjhei

include/deal.II/base/parallel.h

@@ -216,7 +216,7 @@ namespace parallel
       SyncIterators(x_begin),
       SyncIterators(x_end),
       [function](const auto &it) {
-        *std::get<1>(*it) = function(*std::get<0>(*it));
+        *std::get<1>(it) = function(*std::get<0>(it));
       },
       grainsize);
 
@@ -307,7 +307,7 @@ namespace parallel
       SyncIterators(x_begin),
       SyncIterators(x_end),
       [function](const auto &it) {
-        *std::get<2>(*it) = function(*std::get<0>(*it), *std::get<1>(*it));
+        *std::get<2>(it) = function(*std::get<0>(it), *std::get<1>(it));
       },
       grainsize);
 
@@ -409,8 +409,8 @@ namespace parallel
       SyncIterators(x_begin),
       SyncIterators(x_end),
       [function](const auto &it) {
-        *std::get<3>(*it) =
-          function(*std::get<0>(*it), *std::get<1>(*it), *std::get<2>(*it));
+        *std::get<3>(it) =
+          function(*std::get<0>(it), *std::get<1>(it), *std::get<2>(it));
       },
       grainsize);
 

include/deal.II/base/synchronous_iterator.h

@@ -67,6 +67,12 @@ struct SynchronousIterators
   Iterators &
   operator*();
 
+  using difference_type   = std::size_t;
+  using iterator_category = std::bidirectional_iterator_tag;
+  using value_type        = int;
+  using pointer           = int *;
+  using reference         = int &;
+
 private:
   /**
    * Storage for the iterators represented by the current class.
@@ -181,6 +187,23 @@ advance(std::tuple<I1, I2, I3, I4> &t, const unsigned int n)
 
 
 
+template <typename I1, typename I2>
+inline void
+prev(std::tuple<I1, I2> &t)
+{
+  --std::get<0>(t);
+  --std::get<1>(t);
+}
+
+template <typename I1, typename I2, typename I3>
+inline void
+prev(std::tuple<I1, I2, I3> &t)
+{
+  --std::get<0>(t);
+  --std::get<1>(t);
+  --std::get<2>(t);
+}
+
 /**
  * Advance a tuple of iterators by 1.
  *
@@ -252,6 +275,16 @@ operator++(SynchronousIterators<Iterators> &a)
   return a;
 }
 
+
+template <typename Iterators>
+inline SynchronousIterators<Iterators> &
+operator--(SynchronousIterators<Iterators> &a)
+{
+  dealii::prev(*a);
+  return a;
+}
+
+
 /**
  * Advance the elements of this iterator by 1 (post-increment).
  *