BUG: hashtable key_iterator ++ operator returning incorrect type

ENH: ensure std::distance works with hashtable iterators

applications/test/HashTable/Test-hashTable.C

@@ -25,6 +25,8 @@ License
 
 #include "HashTable.H"
 #include "List.H"
+#include "SortableList.H"
+#include "DynamicList.H"
 #include "FlatOutput.H"
 #include "IOstreams.H"
 #include "IStringStream.H"
@@ -194,7 +196,24 @@ int main()
     // These do not yet work. Issues resolving the distance.
     //
     //  List<scalar> table1vals(table1.begin(), table1.end());
-    //  wordList table1keys(table1.begin(), table1.end());
+
+    {
+        Info<<"distance/size: "
+            << std::distance(table1.begin(), table1.end())
+            << "/" << table1.size()
+            << " and "
+            << std::distance(table1.keys().begin(), table1.keys().end())
+            << "/" << table1.keys().size()
+            << nl;
+
+        SortableList<word> sortKeys
+        // DynamicList<word> sortKeys
+        (
+            table1.keys().begin(),
+            table1.keys().end()
+        );
+        Info<<"sortKeys: " << flatOutput(sortKeys) << nl;
+    }
 
     Info<< "\nFrom table1: " << flatOutput(table1.sortedToc()) << nl
         << "retain keys: " << flatOutput(table3.sortedToc()) << nl;

src/OpenFOAM/containers/HashTables/HashTable/HashTable.H

@@ -61,6 +61,7 @@ SourceFiles
 #include "nullObject.H"
 
 #include <initializer_list>
+#include <iterator>
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -179,10 +180,17 @@ public:
         //- Type of values that the HashTable contains.
         typedef T value_type;
 
+        //- The type used for storing into value_type objects.
+        //  This type is usually value_type&.
+        typedef T* pointer;
+
         //- The type used for storing into value_type objects.
         //  This type is usually value_type&.
         typedef T& reference;
 
+        //- The type used for reading from constant value_type objects.
+        typedef const T* const_pointer;
+
         //- The type used for reading from constant value_type objects.
         typedef const T& const_reference;
 
@@ -441,6 +449,7 @@ protected:
             // Public typedefs
             using table_type = this_type;
             using key_type   = this_type::key_type;
+            using iterator_category = std::forward_iterator_tag;
             using difference_type  = this_type::difference_type;
 
         private:
@@ -518,16 +527,20 @@ public:
             public WrappedIterator
         {
         public:
+            using value_type = this_type::key_type;
+            using pointer    = const Key*;
             using reference  = const Key&;
-            using difference_type = typename WrappedIterator::difference_type;
 
             //- Implicit conversion
             inline key_iterator_base(const WrappedIterator& iter);
 
             //- Return the key
             inline reference operator*() const;
-        };
+            inline reference operator()() const;
 
+            inline key_iterator_base& operator++();
+            inline key_iterator_base operator++(int);
+        };
 
 
     // STL iterator
@@ -544,9 +557,9 @@ public:
 
           // Public typedefs
             using table_type = this_type;
-            using key_type   = this_type::key_type;
+            using value_type = this_type::value_type;
+            using pointer    = this_type::pointer;
             using reference  = this_type::reference;
-            using difference_type = typename iterator_base::difference_type;
 
           // Constructors
 
@@ -592,9 +605,9 @@ public:
 
           // Public typedefs
             using table_type = const this_type;
-            using key_type   = this_type::key_type;
+            using value_type = const this_type::value_type;
+            using pointer    = this_type::const_pointer;
             using reference  = this_type::const_reference;
-            using difference_type = typename iterator_base::difference_type;
 
           // Constructors
 

src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H

@@ -363,6 +363,39 @@ Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
 }
 
 
+template<class T, class Key, class Hash>
+template<class WrappedIterator>
+inline const Key&
+Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
+::operator()() const
+{
+    return this->key();
+}
+
+
+template<class T, class Key, class Hash>
+template<class WrappedIterator>
+inline Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>&
+Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
+::operator++()
+{
+    this->increment();
+    return *this;
+}
+
+
+template<class T, class Key, class Hash>
+template<class WrappedIterator>
+inline Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
+Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
+::operator++(int)
+{
+    key_iterator_base old = *this;
+    this->increment();
+    return old;
+}
+
+
 // * * * * * * * * * * * * * * * * STL iterator  * * * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>