diff --git a/applications/test/HashTable/Make/files b/applications/test/HashTable/Make/files
deleted file mode 100644
index 53d1e07699..0000000000
--- a/applications/test/HashTable/Make/files
+++ /dev/null
@@ -1,3 +0,0 @@
-Test-hashTable.C
-
-EXE = $(FOAM_USER_APPBIN)/Test-hashTable
diff --git a/applications/test/HashTable1/Make/files b/applications/test/HashTable1/Make/files
new file mode 100644
index 0000000000..05c776d65c
--- /dev/null
+++ b/applications/test/HashTable1/Make/files
@@ -0,0 +1,3 @@
+Test-HashTable1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-HashTable1
diff --git a/applications/test/HashTable/Make/options b/applications/test/HashTable1/Make/options
similarity index 100%
rename from applications/test/HashTable/Make/options
rename to applications/test/HashTable1/Make/options
diff --git a/applications/test/HashTable/Test-hashTable.C b/applications/test/HashTable1/Test-HashTable1.C
similarity index 100%
rename from applications/test/HashTable/Test-hashTable.C
rename to applications/test/HashTable1/Test-HashTable1.C
diff --git a/applications/test/HashTable4/Make/files b/applications/test/HashTable4/Make/files
new file mode 100644
index 0000000000..fcc01c49bd
--- /dev/null
+++ b/applications/test/HashTable4/Make/files
@@ -0,0 +1,3 @@
+Test-HashTable4.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-HashTable4
diff --git a/applications/test/HashTable4/Make/options b/applications/test/HashTable4/Make/options
new file mode 100644
index 0000000000..6a9e9810b3
--- /dev/null
+++ b/applications/test/HashTable4/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = -I$(LIB_SRC)/cfdTools/include */
+/* EXE_LIBS = -lfiniteVolume */
diff --git a/applications/test/HashTable4/Test-HashTable4.C b/applications/test/HashTable4/Test-HashTable4.C
new file mode 100644
index 0000000000..b9333f320d
--- /dev/null
+++ b/applications/test/HashTable4/Test-HashTable4.C
@@ -0,0 +1,312 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Description
+    Test HashTable resizing
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "HashSet.H"
+#include "HashTable.H"
+#include "Map.H"
+#include "cpuTime.H"
+#include "memInfo.H"
+
+#include <map>
+#include <set>
+#include <unordered_map>
+#include <unordered_set>
+
+// #undef ORDERED
+// #define ORDERED
+
+using namespace Foam;
+
+template<class T>
+Ostream& printInfo(Ostream& os, const HashTable<T, T, Hash<T>>& ht)
+{
+    os << " (size " << ht.size() << " capacity " << ht.capacity() << ") ";
+    return os;
+}
+
+
+template<class K, class V>
+inline void insertElem
+(
+    #ifdef ORDERED
+    std::set<K>& container,
+    #else
+    std::unordered_set<K, Hash<K>>& container,
+    #endif
+    K k,
+    V v
+)
+{
+    container.insert(k);
+}
+
+
+template<class K, class V>
+inline void insertElem
+(
+    #ifdef ORDERED
+    std::map<K, V>& container,
+    #else
+    std::unordered_map<K, V, Hash<K>>& container,
+    #endif
+    K k,
+    V v
+)
+{
+    container.insert(std::make_pair(k, v));
+}
+
+
+template<class K, class V>
+inline void insertElem
+(
+    HashSet<K, Hash<K>>& container,
+    K k,
+    V v
+)
+{
+    container.insert(k);
+}
+
+
+template<class K, class V>
+inline void insertElem
+(
+    HashTable<K, V, Hash<K>>& container,
+    K k,
+    V v
+)
+{
+    container.insert(k, v);
+}
+
+
+template<class Container>
+inline void loopInsert(Container& container, const label n)
+{
+    for (label i = 0; i < n; i++)
+    {
+        insertElem(container, i, i);
+    }
+}
+
+
+template<class Container>
+inline unsigned long loopFind(const Container& container, const label n)
+{
+    const auto endIter = container.end();
+    unsigned long sum = 0;
+
+    for (label i = 0; i < n; i++)
+    {
+        if (container.find(i) != endIter)
+        {
+            ++sum;
+        }
+    }
+
+    return sum;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    const label nLoops = 200;
+    const label nFind  = 10;
+    const label nElem  = 1000000;
+
+    argList::noBanner();
+    argList::addBoolOption("std", "use std::unordered_map or std::set");
+    argList::addBoolOption("set", "test HashSet");
+    argList::addBoolOption("find", "test find");
+
+    argList args(argc, argv);
+
+    const bool optStd = args.optionFound("std");
+    const bool optSet = args.optionFound("set");
+    const bool optFnd = args.optionFound("find");
+
+
+    cpuTime timer;
+    memInfo mem;
+
+    Info<< "insert " << nElem << " (int) elements";
+    if (optFnd)
+    {
+        Info<< ", then find " << (nFind*nLoops) << " times\n";
+    }
+    else
+    {
+        Info<< " repeated " << nLoops << " times " << endl;
+    }
+
+    if (false)
+    {
+        // verify that resizing around (0) doesn't fail
+        HashTable<label, label, Hash<label>> map(32);
+        printInfo(Info, map) << endl;
+
+        map.insert(10, 1000);
+
+        map.resize(0);
+        printInfo(Info, map) << endl;
+
+        map.resize(10);
+        printInfo(Info, map) << endl;
+
+        map.clear();
+        printInfo(Info, map) << endl;
+
+        map.resize(0);
+        printInfo(Info, map) << endl;
+        return 0;
+    }
+
+
+    if (optStd)
+    {
+        if (optFnd)
+        {
+            #ifdef ORDERED
+            Info<< "using stl::set" << endl;
+            std::set<label> map;
+            #else
+            Info<< "using stl::unordered_set" << endl;
+            std::unordered_set<label, Hash<label>> map(32);
+            #endif
+
+            loopInsert(map, nElem);
+            (void)timer.cpuTimeIncrement();
+
+            unsigned long sum = 0;
+            for (label loopi = 0; loopi < nFind*nLoops; ++loopi)
+            {
+                sum += loopFind(map, nElem);
+            }
+
+            // check result (suppress compiler optimizations?)
+            if (sum == 0)
+            {
+                Info<<"sum=0\n";
+            }
+        }
+        else if (optSet)
+        {
+            #ifdef ORDERED
+            Info<< "using stl::set" << endl;
+            #else
+            Info<< "using stl::unordered_set" << endl;
+            #endif
+
+            for (label loopi = 0; loopi < nLoops; ++loopi)
+            {
+                #ifdef ORDERED
+                std::set<label> map;
+                #else
+                std::unordered_set<label, Hash<label>> map(32);
+                #endif
+
+                loopInsert(map, nElem);
+            }
+        }
+        else
+        {
+            #ifdef ORDERED
+            Info<< "using stl::map" << endl;
+            #else
+            Info<< "using stl::unordered_set" << endl;
+            #endif
+
+            for (label loopi = 0; loopi < nLoops; ++loopi)
+            {
+                #ifdef ORDERED
+                std::map<label, label> map;
+                #else
+                std::unordered_map<label, label, Hash<label>> map(32);
+                #endif
+
+                loopInsert(map, nElem);
+            }
+        }
+    }
+    else
+    {
+        if (optFnd)
+        {
+            Info<< "using HashSet" << endl;
+
+            HashSet<label, Hash<label>> map(32);
+
+            loopInsert(map, nElem);
+            (void)timer.cpuTimeIncrement();
+
+            unsigned long sum = 0;
+            for (label loopi = 0; loopi < nFind*nLoops; ++loopi)
+            {
+                sum += loopFind(map, nElem);
+            }
+
+            // check result (suppress compiler optimizations?)
+            if (sum == 0)
+            {
+                Info<<"sum=0\n";
+            }
+        }
+        else if (optSet)
+        {
+            Info<< "using HashSet" << endl;
+            for (label loopi = 0; loopi < nLoops; ++loopi)
+            {
+                HashSet<label, Hash<label>> map(32);
+
+                loopInsert(map, nElem);
+            }
+        }
+        else
+        {
+            Info<< "using HashTable" << endl;
+            for (label loopi = 0; loopi < nLoops; ++loopi)
+            {
+                HashTable<label, label, Hash<label>> map(32);
+
+                loopInsert(map, nElem);
+            }
+        }
+    }
+
+    Info<< timer.cpuTimeIncrement() << " s\n";
+    Info<< "mem info: " << mem.update() << endl;
+
+    return 0;
+}
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
index 9370dd26ed..e7ad07ec2f 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C
@@ -28,6 +28,13 @@ License
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
+template<class T, class Key, class Hash>
+Foam::HashPtrTable<T, Key, Hash>::HashPtrTable()
+:
+    parent_type()
+{}
+
+
 template<class T, class Key, class Hash>
 Foam::HashPtrTable<T, Key, Hash>::HashPtrTable(const label size)
 :
@@ -157,6 +164,7 @@ void Foam::HashPtrTable<T, Key, Hash>::operator=
     }
 }
 
+
 // * * * * * * * * * * * * * * * Friend Operators  * * * * * * * * * * * * * //
 
 #include "HashPtrTableIO.C"
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
index f83c04e313..daf19328d0 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H
@@ -91,8 +91,11 @@ public:
 
     // Constructors
 
+        //- Construct null with default table size
+        HashPtrTable();
+
         //- Construct given initial table size
-        HashPtrTable(const label size = 128);
+        explicit HashPtrTable(const label size);
 
         //- Construct from Istream using given Istream constructor class
         template<class INew>
@@ -102,7 +105,7 @@ public:
         HashPtrTable(Istream& is);
 
         //- Construct from dictionary with default dictionary constructor class
-        HashPtrTable(const dictionary& dict);
+        explicit HashPtrTable(const dictionary& dict);
 
         //- Construct as copy
         HashPtrTable(const this_type& ht);
@@ -127,7 +130,7 @@ public:
         //- Erase an entry specified by the given key
         bool erase(const Key& key);
 
-        //- Clear all entries from table
+        //- Clear all entries from table and deleting any allocated pointers
         void clear();
 
         //- Write
diff --git a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableIO.C b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableIO.C
index 55004189fe..bbc7351857 100644
--- a/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableIO.C
+++ b/src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableIO.C
@@ -41,7 +41,7 @@ void Foam::HashPtrTable<T, Key, Hash>::read(Istream& is, const INew& inewt)
 
     is.fatalCheck
     (
-        "HashPtrTable<T, Key, Hash>::read(Istream&, const INew&) : "
+        "HashPtrTable::read(Istream&, const INew&) : "
         "reading first token"
     );
 
@@ -50,7 +50,7 @@ void Foam::HashPtrTable<T, Key, Hash>::read(Istream& is, const INew& inewt)
         const label s = firstToken.labelToken();
 
         // Read beginning of contents
-        const char delimiter = is.readBeginList("HashPtrTable<T, Key, Hash>");
+        const char delimiter = is.readBeginList("HashPtrTable");
 
         if (s)
         {
@@ -69,8 +69,8 @@ void Foam::HashPtrTable<T, Key, Hash>::read(Istream& is, const INew& inewt)
 
                     is.fatalCheck
                     (
-                        "HashPtrTable<T, Key, Hash>::"
-                        "read(Istream&, const INew&) : reading entry"
+                        "HashPtrTable::read(Istream&, const INew&) : "
+                        "reading entry"
                     );
                 }
             }
@@ -114,7 +114,7 @@ void Foam::HashPtrTable<T, Key, Hash>::read(Istream& is, const INew& inewt)
 
             is.fatalCheck
             (
-                "HashPtrTable<T, Key, Hash>::read(Istream&, const INew&) : "
+                "HashPtrTable::read(Istream&, const INew&) : "
                 "reading entry"
             );
 
@@ -155,7 +155,7 @@ void Foam::HashPtrTable<T, Key, Hash>::read
 template<class T, class Key, class Hash>
 void Foam::HashPtrTable<T, Key, Hash>::write(Ostream& os) const
 {
-    for (const_iterator iter = this->begin(); iter != this->end(); ++iter)
+    for (const_iterator iter = this->cbegin(); iter != this->cend(); ++iter)
     {
         const T* ptr = iter.object();
         if (ptr)
@@ -209,29 +209,34 @@ Foam::Ostream& Foam::operator<<
     const HashPtrTable<T, Key, Hash>& tbl
 )
 {
-    using const_iterator = typename HashPtrTable<T, Key, Hash>::const_iterator;
+    const label sz = tbl.size();
 
-    // Write size and start delimiter
-    os << nl << tbl.size() << nl << token::BEGIN_LIST << nl;
-
-    // Write contents
-    for (const_iterator iter = tbl.cbegin(); iter != tbl.cend(); ++iter)
+    if (sz)
     {
-        const T* ptr = iter.object();
+        // Size and start list delimiter
+        os << nl << sz << nl << token::BEGIN_LIST << nl;
 
-        os << iter.key();
-        if (ptr)
+        // Contents
+        for (auto iter = tbl.cbegin(); iter != tbl.cend(); ++iter)
         {
-            os << token::SPACE << *ptr;
+            const T* ptr = iter.object();
+
+            os << iter.key();
+            if (ptr)
+            {
+                os << token::SPACE << *ptr;
+            }
+            os << nl;
         }
-        os << nl;
+        os << token::END_LIST; // End list delimiter
+    }
+    else
+    {
+        // Empty hash table
+        os << sz << token::BEGIN_LIST << token::END_LIST;
     }
 
-    // Write end delimiter
-    os << token::END_LIST;
-
     os.check(FUNCTION_NAME);
-
     return os;
 }
 
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
index 020d3e36d9..3070831f26 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.C
@@ -79,7 +79,7 @@ inline Foam::label Foam::HashSet<Key, Hash>::assignMultiple
 template<class Key, class Hash>
 Foam::HashSet<Key, Hash>::HashSet(const UList<Key>& lst)
 :
-    HashTable<nil, Key, Hash>(2*lst.size())
+    parent_type(2*lst.size())
 {
     for (const auto& k : lst)
     {
@@ -92,7 +92,7 @@ template<class Key, class Hash>
 template<unsigned Size>
 Foam::HashSet<Key, Hash>::HashSet(const FixedList<Key, Size>& lst)
 :
-    HashTable<nil, Key, Hash>(2*lst.size())
+    parent_type(2*lst.size())
 {
     for (const auto& k : lst)
     {
@@ -104,7 +104,7 @@ Foam::HashSet<Key, Hash>::HashSet(const FixedList<Key, Size>& lst)
 template<class Key, class Hash>
 Foam::HashSet<Key, Hash>::HashSet(std::initializer_list<Key> lst)
 :
-    HashTable<nil, Key, Hash>(2*lst.size())
+    parent_type(2*lst.size())
 {
     for (const auto& k : lst)
     {
@@ -120,7 +120,7 @@ Foam::HashSet<Key, Hash>::HashSet
     const HashTable<AnyType, Key, AnyHash>& tbl
 )
 :
-    HashTable<nil, Key, Hash>(tbl.capacity())
+    parent_type(tbl.capacity())
 {
     using other_iter =
         typename HashTable<AnyType, Key, AnyHash>::const_iterator;
@@ -349,7 +349,7 @@ template<class Key, class Hash>
 inline typename Foam::HashSet<Key, Hash>::const_iterator
 Foam::HashSet<Key, Hash>::begin() const
 {
-    return HashTableCore::iterator_begin<const_iterator>
+    return HashTableCore::iterator_cbegin<const_iterator>
     (
         static_cast<const parent_type&>(*this)
     );
@@ -360,7 +360,7 @@ template<class Key, class Hash>
 inline typename Foam::HashSet<Key, Hash>::const_iterator
 Foam::HashSet<Key, Hash>::cbegin() const
 {
-    return HashTableCore::iterator_begin<const_iterator>
+    return HashTableCore::iterator_cbegin<const_iterator>
     (
         static_cast<const parent_type&>(*this)
     );
@@ -379,7 +379,7 @@ template<class Key, class Hash>
 inline const typename Foam::HashSet<Key, Hash>::const_iterator&
 Foam::HashSet<Key, Hash>::end() const
 {
-    return HashTableCore::iterator_end<const_iterator>();
+    return HashTableCore::iterator_cend<const_iterator>();
 }
 
 
diff --git a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
index ace2ca6d7a..f20e905920 100644
--- a/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
+++ b/src/OpenFOAM/containers/HashTables/HashSet/HashSet.H
@@ -25,7 +25,24 @@ Class
     Foam::HashSet
 
 Description
-    A HashTable with keys but without contents.
+    A HashTable with keys but without contents that is similar to
+    \c std::unordered_set.
+    The entries are considered \a unordered since their placement
+    depends on the method used to generate the hash key index, the
+    table capacity, insertion order etc. When the key order is
+    important, use the sortedToc() method to obtain a list of sorted
+    keys and use that for further access.
+
+Note
+    The HashSet iterator dereferences to the key, so the following
+    range-for works as expected:
+    \code
+        HashSet<label> someLabels{10, 20, 30, 40, ...};
+        for (const label i : someLabels)
+        {
+            Info<< "val:" << i << nl;
+        }
+    \endcode
 
 Typedef
     Foam::wordHashSet
@@ -45,7 +62,6 @@ Description
 #define HashSet_H
 
 #include "HashTable.H"
-#include "nil.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -66,7 +82,7 @@ Ostream& operator<<(Ostream& os, const HashSet<Key, Hash>& tbl);
 template<class Key=word, class Hash=string::hash>
 class HashSet
 :
-    public HashTable<nil, Key, Hash>
+    public HashTable<zero::null, Key, Hash>
 {
     // Private Member Functions
 
@@ -94,7 +110,7 @@ public:
     typedef HashSet<Key, Hash> this_type;
 
     //- The template instance used for the parent HashTable
-    typedef HashTable<nil, Key, Hash> parent_type;
+    typedef HashTable<zero::null, Key, Hash> parent_type;
 
     //- An iterator, returning reference to the key
     using iterator = typename parent_type::key_iterator;
@@ -105,13 +121,19 @@ public:
 
     // Constructors
 
+        //- Construct null with default (128) table size
+        HashSet()
+        :
+            parent_type()
+        {}
+
         //- Construct given initial size
-        HashSet(const label size = 128)
+        explicit HashSet(const label size)
         :
             parent_type(size)
         {}
 
-        //- Construct from Istream
+        //- Construct from Istream with default table size
         HashSet(Istream& is)
         :
             parent_type(is)
@@ -140,13 +162,13 @@ public:
         {}
 
         //- Construct by transferring the parameter contents
-        HashSet(const Xfer<HashSet<Key, Hash>>& hs)
+        HashSet(const Xfer<this_type>& hs)
         :
             parent_type(hs)
         {}
 
         //- Construct by transferring the parameter contents
-        HashSet(const Xfer<HashTable<nil, Key, Hash>>& hs)
+        HashSet(const Xfer<parent_type>& hs)
         :
             parent_type(hs)
         {}
@@ -166,7 +188,7 @@ public:
         //  not previously exist in the set.
         bool insert(const Key& key)
         {
-            return this->parent_type::insert(key, nil());
+            return this->parent_type::insert(key, zero::null());
         }
 
         //- Insert keys from the list of Key
@@ -182,26 +204,26 @@ public:
         //  \return The number of new elements inserted
         label insert(std::initializer_list<Key> lst);
 
-        //- Same as insert (cannot overwrite nil content)
+        //- Same as insert (no value to overwrite)
         bool set(const Key& key)
         {
             return insert(key);
         }
 
-        //- Same as insert (cannot overwrite nil content)
+        //- Same as insert (no value to overwrite)
         label set(const UList<Key>& lst)
         {
             return insert(lst);
         }
 
-        //- Same as insert (cannot overwrite nil content)
+        //- Same as insert (no value to overwrite)
         template<unsigned Size>
         label set(const FixedList<Key, Size>& lst)
         {
             return insert(lst);
         }
 
-        //- Same as insert (cannot overwrite nil content)
+        //- Same as insert (no value to overwrite)
         label set(std::initializer_list<Key> lst)
         {
             return insert(lst);
@@ -331,22 +353,22 @@ public:
       // Logical operations
 
         //- Combine entries from HashSets
-        void operator|=(const HashSet<Key, Hash>& rhs);
+        void operator|=(const this_type& rhs);
 
         //- Only retain entries found in both HashSets
-        inline void operator&=(const HashSet<Key, Hash>& rhs);
+        inline void operator&=(const this_type& rhs);
 
         //- Only retain unique entries (xor)
-        void operator^=(const HashSet<Key, Hash>& rhs);
+        void operator^=(const this_type& rhs);
 
         //- Add entries listed in the given HashSet to this HashSet
-        inline void operator+=(const HashSet<Key, Hash>& rhs)
+        inline void operator+=(const this_type& rhs)
         {
             this->operator|=(rhs);
         }
 
         //- Remove entries listed in the given HashSet from this HashSet
-        inline void operator-=(const HashSet<Key, Hash>& rhs);
+        inline void operator-=(const this_type& rhs);
 
 
     // IOstream Operator
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
index 1c1d7b4360..68faaafb49 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.C
@@ -61,21 +61,27 @@ Foam::label Foam::HashTable<T, Key, Hash>::eraseMultiple
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
+template<class T, class Key, class Hash>
+Foam::HashTable<T, Key, Hash>::HashTable()
+:
+    HashTable<T, Key, Hash>(128)
+{}
+
+
 template<class T, class Key, class Hash>
 Foam::HashTable<T, Key, Hash>::HashTable(const label size)
 :
     HashTableCore(),
-    nElmts_(0),
-    tableSize_(HashTableCore::canonicalSize(size)),
+    size_(0),
+    capacity_(HashTableCore::canonicalSize(size)),
     table_(nullptr)
 {
-    if (tableSize_)
+    if (capacity_)
     {
-        table_ = new hashedEntry*[tableSize_];
-
-        for (label hashIdx = 0; hashIdx < tableSize_; ++hashIdx)
+        table_ = new node_type*[capacity_];
+        for (label i=0; i < capacity_; ++i)
         {
-            table_[hashIdx] = nullptr;
+            table_[i] = nullptr;
         }
     }
 }
@@ -84,7 +90,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(const label size)
 template<class T, class Key, class Hash>
 Foam::HashTable<T, Key, Hash>::HashTable(const HashTable<T, Key, Hash>& ht)
 :
-    HashTable<T, Key, Hash>(ht.tableSize_)
+    HashTable<T, Key, Hash>(ht.capacity_)
 {
     for (const_iterator iter = ht.cbegin(); iter != ht.cend(); ++iter)
     {
@@ -96,10 +102,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(const HashTable<T, Key, Hash>& ht)
 template<class T, class Key, class Hash>
 Foam::HashTable<T, Key, Hash>::HashTable(HashTable<T, Key, Hash>&& ht)
 :
-    HashTableCore(),
-    nElmts_(0),
-    tableSize_(0),
-    table_(nullptr)
+    HashTable<T, Key, Hash>(0)
 {
     transfer(ht);
 }
@@ -111,10 +114,7 @@ Foam::HashTable<T, Key, Hash>::HashTable
     const Xfer<HashTable<T, Key, Hash>>& ht
 )
 :
-    HashTableCore(),
-    nElmts_(0),
-    tableSize_(0),
-    table_(nullptr)
+    HashTable<T, Key, Hash>(0)
 {
     transfer(ht());
 }
@@ -150,110 +150,10 @@ Foam::HashTable<T, Key, Hash>::~HashTable()
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
-template<class T, class Key, class Hash>
-bool Foam::HashTable<T, Key, Hash>::found(const Key& key) const
-{
-    if (nElmts_)
-    {
-        const label hashIdx = hashKeyIndex(key);
-
-        for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
-        {
-            if (key == ep->key_)
-            {
-                return true;
-            }
-        }
-    }
-
-    #ifdef FULLDEBUG
-    if (debug)
-    {
-        InfoInFunction << "Entry " << key << " not found in hash table\n";
-    }
-    #endif
-
-    return false;
-}
-
-
-template<class T, class Key, class Hash>
-typename Foam::HashTable<T, Key, Hash>::iterator
-Foam::HashTable<T, Key, Hash>::find
-(
-    const Key& key
-)
-{
-    if (nElmts_)
-    {
-        const label hashIdx = hashKeyIndex(key);
-
-        for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
-        {
-            if (key == ep->key_)
-            {
-                return iterator(this, ep, hashIdx);
-            }
-        }
-    }
-
-    #ifdef FULLDEBUG
-    if (debug)
-    {
-        InfoInFunction << "Entry " << key << " not found in hash table\n";
-    }
-    #endif
-
-    return iterator();
-}
-
-
-template<class T, class Key, class Hash>
-typename Foam::HashTable<T, Key, Hash>::const_iterator
-Foam::HashTable<T, Key, Hash>::find
-(
-    const Key& key
-) const
-{
-    return this->cfind(key);
-}
-
-
-template<class T, class Key, class Hash>
-typename Foam::HashTable<T, Key, Hash>::const_iterator
-Foam::HashTable<T, Key, Hash>::cfind
-(
-    const Key& key
-) const
-{
-    if (nElmts_)
-    {
-        const label hashIdx = hashKeyIndex(key);
-
-        for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
-        {
-            if (key == ep->key_)
-            {
-                return const_iterator(this, ep, hashIdx);
-            }
-        }
-    }
-
-    #ifdef FULLDEBUG
-    if (debug)
-    {
-        InfoInFunction << "Entry " << key << " not found in hash table\n";
-    }
-    #endif
-
-    return const_iterator();
-}
-
-
 template<class T, class Key, class Hash>
 Foam::List<Key> Foam::HashTable<T, Key, Hash>::toc() const
 {
-    List<Key> keyLst(nElmts_);
+    List<Key> keyLst(size_);
     label count = 0;
 
     for (const_iterator iter = cbegin(); iter != cend(); ++iter)
@@ -297,7 +197,7 @@ Foam::List<Key> Foam::HashTable<T, Key, Hash>::tocKeys
     const bool invert
 ) const
 {
-    List<Key> keyLst(nElmts_);
+    List<Key> keyLst(size_);
     label count = 0;
 
     for (const_iterator iter = cbegin(); iter != cend(); ++iter)
@@ -323,7 +223,7 @@ Foam::List<Key> Foam::HashTable<T, Key, Hash>::tocValues
     const bool invert
 ) const
 {
-    List<Key> keyLst(nElmts_);
+    List<Key> keyLst(size_);
     label count = 0;
 
     for (const_iterator iter = cbegin(); iter != cend(); ++iter)
@@ -349,7 +249,7 @@ Foam::List<Key> Foam::HashTable<T, Key, Hash>::tocEntries
     const bool invert
 ) const
 {
-    List<Key> keyLst(nElmts_);
+    List<Key> keyLst(size_);
     label count = 0;
 
     for (const_iterator iter = cbegin(); iter != cend(); ++iter)
@@ -438,36 +338,36 @@ bool Foam::HashTable<T, Key, Hash>::set
 (
     const Key& key,
     const T& obj,
-    const bool protect
+    const bool overwrite
 )
 {
-    if (!tableSize_)
+    if (!capacity_)
     {
         resize(2);
     }
 
-    const label hashIdx = hashKeyIndex(key);
+    const label index = hashKeyIndex(key);
 
-    hashedEntry* existing = nullptr;
-    hashedEntry* prev = nullptr;
+    node_type* curr = nullptr;
+    node_type* prev = nullptr;
 
-    for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
+    for (node_type* ep = table_[index]; ep; ep = ep->next_)
     {
-        if (key == ep->key_)
+        if (key == ep->key())
         {
-            existing = ep;
+            curr = ep;
             break;
         }
         prev = ep;
     }
 
-    if (!existing)
+    if (!curr)
     {
         // Not found, insert it at the head
-        table_[hashIdx] = new hashedEntry(key, obj, table_[hashIdx]);
-        nElmts_++;
+        table_[index] = new node_type(key, obj, table_[index]);
+        ++size_;
 
-        if (double(nElmts_)/tableSize_ > 0.8 && tableSize_ < maxTableSize)
+        if (double(size_)/capacity_ > 0.8 && capacity_ < maxTableSize)
         {
             #ifdef FULLDEBUG
             if (debug)
@@ -476,13 +376,35 @@ bool Foam::HashTable<T, Key, Hash>::set
             }
             #endif
 
-            resize(2*tableSize_);
+            resize(2*capacity_);
         }
     }
-    else if (protect)
+    else if (overwrite)
     {
-        // Found - but protected from overwriting
-        // this corresponds to the STL 'insert' convention
+        // Overwrite current entry (Perl convention).
+
+        node_type* ep = curr->next_;  // next in the linked list
+
+        // In some cases the delete/new could be avoided in favour of move
+        // assignment, but cannot be certain that all objects support this
+        // or that it behaves the same as a copy construct.
+
+        delete curr;
+        ep = new node_type(key, obj, ep);
+
+        // Replace current element - within list or insert at the head
+        if (prev)
+        {
+            prev->next_ = ep;
+        }
+        else
+        {
+            table_[index] = ep;
+        }
+    }
+    else
+    {
+        // Do not overwrite existing entry (STL 'insert' convention)
         #ifdef FULLDEBUG
         if (debug)
         {
@@ -492,99 +414,22 @@ bool Foam::HashTable<T, Key, Hash>::set
         #endif
         return false;
     }
-    else
-    {
-        // Found - overwrite existing entry
-        // this corresponds to the Perl convention
-        hashedEntry* ep = new hashedEntry(key, obj, existing->next_);
-
-        // Replace existing element - within list or insert at the head
-        if (prev)
-        {
-            prev->next_ = ep;
-        }
-        else
-        {
-            table_[hashIdx] = ep;
-        }
-
-        delete existing;
-    }
 
     return true;
 }
 
 
-template<class T, class Key, class Hash>
-bool Foam::HashTable<T, Key, Hash>::iterator_base::erase()
-{
-    // Note: entryPtr_ is nullptr for end(), so this catches that too
-    if (entryPtr_)
-    {
-        // Search element before entryPtr_
-        entry_type* prev = nullptr;
-
-        for
-        (
-            entry_type* ep = hashTable_->table_[hashIndex_];
-            ep;
-            ep = ep->next_
-        )
-        {
-            if (ep == entryPtr_)
-            {
-                break;
-            }
-            prev = ep;
-        }
-
-        if (prev)
-        {
-            // Has an element before entryPtr - reposition to there
-            prev->next_ = entryPtr_->next_;
-            delete entryPtr_;
-            entryPtr_ = prev;
-        }
-        else
-        {
-            // entryPtr was first element on SLList
-            hashTable_->table_[hashIndex_] = entryPtr_->next_;
-            delete entryPtr_;
-
-            // Assign any non-nullptr value so it doesn't look like end()
-            entryPtr_ = reinterpret_cast<hashedEntry*>(this);
-
-            // Mark with special hashIndex value to signal it has been rewound.
-            // The next increment will bring it back to the present location.
-            //
-            // From the current position 'curPos', we wish to continue at
-            // prevPos='curPos-1', which we mark as markPos='-curPos-1'.
-            // The negative lets us notice it is special, the extra '-1'
-            // is needed to avoid ambiguity for position '0'.
-            // To retrieve prevPos, we would later use '-(markPos+1) - 1'
-            hashIndex_ = -hashIndex_ - 1;
-        }
-
-        hashTable_->nElmts_--;
-
-        return true;
-    }
-    else
-    {
-        return false;
-    }
-}
-
-
 template<class T, class Key, class Hash>
 bool Foam::HashTable<T, Key, Hash>::erase(const iterator& iter)
 {
-    // NOTE: We use (const iterator&) here, but manipulate its contents anyhow.
+    // NOTE: we use (const iterator&) here, but treat its contents as mutable.
+    //
     // The parameter should be (iterator&), but then the compiler doesn't find
     // it correctly and tries to call as (iterator) instead.
-    //
-    // Adjust iterator after erase
-    return const_cast<iterator&>(iter).erase();
+
+    iterator& it = const_cast<iterator&>(iter);
+
+    return iterator_erase(it.entry_, it.index_);
 }
 
 
@@ -599,7 +444,7 @@ bool Foam::HashTable<T, Key, Hash>::erase(const Key& key)
 template<class T, class Key, class Hash>
 Foam::label Foam::HashTable<T, Key, Hash>::erase(const UList<Key>& keys)
 {
-    return eraseMultiple(keys.begin(), keys.end());
+    return eraseMultiple(keys.cbegin(), keys.cend());
 }
 
 
@@ -610,7 +455,7 @@ Foam::label Foam::HashTable<T, Key, Hash>::erase
     const FixedList<Key, Size>& keys
 )
 {
-    return eraseMultiple(keys.begin(), keys.end());
+    return eraseMultiple(keys.cbegin(), keys.cend());
 }
 
 
@@ -634,17 +479,14 @@ Foam::label Foam::HashTable<T, Key, Hash>::erase
     const label nTotal = this->size();
     label changed = 0;
 
-    using other_iter =
-        typename HashTable<AnyType, Key, AnyHash>::const_iterator;
-
     if (other.size() <= nTotal)
     {
         // The other is smaller/same-size, use its keys for removal
 
         for
         (
-            other_iter iter = other.begin();
-            changed < nTotal && iter != other.end(); // terminate early
+            auto iter = other.cbegin();
+            changed < nTotal && iter != other.cend(); // Terminate early
             ++iter
         )
         {
@@ -660,7 +502,7 @@ Foam::label Foam::HashTable<T, Key, Hash>::erase
         for
         (
             iterator iter = begin();
-            changed < nTotal && iter != end(); // terminate early
+            changed < nTotal && iter != end(); // Terminate early
             ++iter
         )
         {
@@ -711,9 +553,10 @@ Foam::label Foam::HashTable<T, Key, Hash>::retain
 template<class T, class Key, class Hash>
 void Foam::HashTable<T, Key, Hash>::resize(const label sz)
 {
-    const label newSize = HashTableCore::canonicalSize(sz);
+    const label newCapacity = HashTableCore::canonicalSize(sz);
+    const label oldCapacity = capacity_;
 
-    if (newSize == tableSize_)
+    if (newCapacity == oldCapacity)
     {
         #ifdef FULLDEBUG
         if (debug)
@@ -724,46 +567,85 @@ void Foam::HashTable<T, Key, Hash>::resize(const label sz)
 
         return;
     }
-
-    HashTable<T, Key, Hash>* tmpTable = new HashTable<T, Key, Hash>(newSize);
-
-    for (const_iterator iter = cbegin(); iter != cend(); ++iter)
+    else if (!newCapacity)
     {
-        tmpTable->insert(iter.key(), iter.object());
+        // Special treatment for resize(0)
+        if (size_)
+        {
+            WarningInFunction
+                << "HashTable contains " << size_ << " cannot resize(0)"
+                << endl;
+        }
+        else
+        {
+            if (table_)
+            {
+                delete[] table_;
+                capacity_ = 0;
+            }
+
+            table_ = nullptr;
+        }
+
+        return;
     }
 
-    const label oldSize = tableSize_;
-    tableSize_ = tmpTable->tableSize_;
-    tmpTable->tableSize_ = oldSize;
+    // Swap primary table entries: size_ is left untouched
 
-    hashedEntry** oldTable = table_;
-    table_ = tmpTable->table_;
-    tmpTable->table_ = oldTable;
+    auto oldTable = table_;
+    capacity_ = newCapacity;
 
-    delete tmpTable;
+    table_ = new node_type*[capacity_];
+    for (label i=0; i < capacity_; ++i)
+    {
+        table_[i] = nullptr;
+    }
+
+    // Move to new table[] but with new chaining.
+
+    label nMove = size_;  // Allow early completion
+    for (label i=0; nMove && i < oldCapacity; ++i)
+    {
+        for (node_type* ep = oldTable[i]; ep; /*nil*/)
+        {
+            node_type* next = ep->next_;
+
+            // Move to new location
+            {
+                const label newIdx = hashKeyIndex(ep->key());
+
+                ep->next_ = table_[newIdx];  // add to head
+                table_[newIdx] = ep;
+            }
+
+            ep = next;  // continue in the linked-list
+            --nMove;    // note any early completion
+        }
+        oldTable[i] = nullptr;
+    }
+
+    if (oldTable)
+    {
+        delete[] oldTable;
+    }
 }
 
 
 template<class T, class Key, class Hash>
 void Foam::HashTable<T, Key, Hash>::clear()
 {
-    if (nElmts_)
+    for (label i=0; size_ && i<capacity_; ++i)
     {
-        for (label hashIdx = 0; hashIdx < tableSize_; hashIdx++)
+        for (node_type* ep = table_[i]; ep; /*nil*/)
         {
-            if (table_[hashIdx])
-            {
-                hashedEntry* ep = table_[hashIdx];
-                while (hashedEntry* next = ep->next_)
-                {
-                    delete ep;
-                    ep = next;
-                }
-                delete ep;
-                table_[hashIdx] = nullptr;
-            }
+            node_type* next = ep->next_;
+
+            delete ep;
+
+            ep = next;  // continue in the linked-list
+            --size_;    // note any early completion
         }
-        nElmts_ = 0;
+        table_[i] = nullptr;
     }
 }
 
@@ -779,30 +661,31 @@ void Foam::HashTable<T, Key, Hash>::clearStorage()
 template<class T, class Key, class Hash>
 void Foam::HashTable<T, Key, Hash>::swap(HashTable<T, Key, Hash>& ht)
 {
-    Foam::Swap(table_,     ht.table_);
-    Foam::Swap(tableSize_, ht.tableSize_);
-    Foam::Swap(nElmts_,    ht.nElmts_);
+    Foam::Swap(size_, ht.size_);
+    Foam::Swap(capacity_, ht.capacity_);
+    Foam::Swap(table_, ht.table_);
 }
 
 
 template<class T, class Key, class Hash>
 void Foam::HashTable<T, Key, Hash>::transfer(HashTable<T, Key, Hash>& ht)
 {
-    // As per the Destructor
+    // As per destructor
     if (table_)
     {
         clear();
         delete[] table_;
     }
 
-    tableSize_ = ht.tableSize_;
-    ht.tableSize_ = 0;
+    size_ = ht.size_;
+    ht.size_ = 0;
+
+    capacity_ = ht.capacity_;
+    ht.capacity_ = 0;
 
     table_ = ht.table_;
     ht.table_ = nullptr;
 
-    nElmts_ = ht.nElmts_;
-    ht.nElmts_ = 0;
 }
 
 
@@ -904,9 +787,9 @@ void Foam::HashTable<T, Key, Hash>::operator=
     }
 
     // Could be zero-sized from a previous transfer()
-    if (!tableSize_)
+    if (!capacity_)
     {
-        resize(rhs.tableSize_);
+        resize(rhs.capacity_);
     }
     else
     {
@@ -927,7 +810,7 @@ void Foam::HashTable<T, Key, Hash>::operator=
 )
 {
     // Could be zero-sized from a previous transfer()
-    if (!tableSize_)
+    if (!capacity_)
     {
         resize(2*lst.size());
     }
@@ -975,7 +858,7 @@ bool Foam::HashTable<T, Key, Hash>::operator==
 
     for (const_iterator iter = rhs.cbegin(); iter != rhs.cend(); ++iter)
     {
-        const_iterator other = this->cfind(iter.key());
+        const const_iterator other(this->cfind(iter.key()));
 
         if (!other.found() || other.object() != iter.object())
         {
@@ -997,8 +880,11 @@ bool Foam::HashTable<T, Key, Hash>::operator!=
 }
 
 
-// * * * * * * * * * * * * * * * Friend Operators  * * * * * * * * * * * * * //
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
+// Iterators, Friend Operators
+
+#include "HashTableIter.C"
 #include "HashTableIO.C"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
index 1c42c9426a..a496b6b630 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTable.H
@@ -25,14 +25,15 @@ Class
     Foam::HashTable
 
 Description
-    An STL-conforming hash table.
+    A HashTable similar to \c std::unordered_map.
+    The entries are considered \a unordered since their placement
+    depends on the method used to generate the hash key index, the
+    table capacity, insertion order etc. When the key order is
+    important, use the sortedToc() method to obtain a list of sorted
+    keys and use that for further access.
 
-Note
-    Hashing index collisions are handled via chaining using a singly-linked
-    list with the colliding entry being added to the head of the linked
-    list. Thus copying the hash table (or indeed even resizing it) will
-    often result in a different hash order. Use a sorted table-of-contents
-    when the hash order is important.
+    Internally the table uses closed addressing into a flat storage space
+    with collisions handled by linked-list chaining.
 
     The end iterator of all hash-tables has a nullptr to the hash entry.
     Thus avoid separate allocation for each table and use a single one with
@@ -41,10 +42,38 @@ Note
     a nullptr as its first data member.
     The nullObject is such an item (with a nullptr data member).
 
+Note
+    For historical reasons, dereferencing the table iterator
+    (eg, \a *iter) returns a reference to the stored object value
+    value rather than the stored key/object value like std::unordered_map
+    does.
+
+    The HashTable iterator:
+    \code
+        forAllConstIters(table, iter)
+        {
+            Info<< "val:" << *iter << nl
+                << "key:" << iter.key() << nl;
+                << "val:" << iter.object() << nl;
+        }
+    \endcode
+    whereas for the \c std::unordered_map iterator:
+    \code
+        forAllConstIters(stdmap, iter)
+        {
+            Info<< "key/val:" << *iter << nl
+                << "key:" << iter->first << nl
+                << "val:" << iter->second << nl;
+        }
+    \endcode
+    This difference is most evident when using range-for syntax.
+
 SourceFiles
     HashTableI.H
+    HashTableIterI.H
     HashTable.C
     HashTableIO.C
+    HashTableIter.C
 
 \*---------------------------------------------------------------------------*/
 
@@ -52,6 +81,7 @@ SourceFiles
 #define HashTable_H
 
 #include "word.H"
+#include "zero.H"
 #include "Xfer.H"
 #include "Hash.H"
 #include "HashTableCore.H"
@@ -80,7 +110,7 @@ Ostream& operator<<(Ostream& os, const HashTable<T, Key, Hash>& tbl);
 
 
 /*---------------------------------------------------------------------------*\
-                           Class HashTable Declaration
+                          Class HashTable Declaration
 \*---------------------------------------------------------------------------*/
 
 template<class T, class Key=word, class Hash=string::hash>
@@ -88,6 +118,140 @@ class HashTable
 :
     public HashTableCore
 {
+    // Private types for table entries
+
+        //- Structure with a (K,V) tuple and a linked-list for collisions
+        //  Could store key/object as std::pair, but no particular advantage
+        //  unless the iterator dereference type changes.
+        struct pair_entry
+        {
+            //- Type of key
+            typedef Key key_type;
+
+            //- Object content type
+            typedef T mapped_type;
+
+            //- The lookup key
+            key_type key_;
+
+            //- The data object
+            mapped_type obj_;
+
+            //- Addressing (next in collision list)
+            pair_entry* next_;
+
+            //- Construct from key, object, next pointer
+            pair_entry(const Key& key, const T& obj, pair_entry* next)
+            :
+                key_(key),
+                obj_(obj),
+                next_(next)
+            {}
+
+            //- The key
+            const key_type& key() const
+            {
+                return key_;
+            }
+
+            //- The mapped object
+            const mapped_type& mapped() const
+            {
+                return obj_;
+            }
+            mapped_type& mapped()
+            {
+                return obj_;
+            }
+
+        private:
+            //- Disallow default bitwise copy construct / assignment
+            pair_entry(const pair_entry&) = delete;
+            void operator=(const pair_entry&) = delete;
+        };
+
+
+        //- Structure with a single (K) value and a linked-list for collisions
+        struct unary_entry
+        {
+            //- Type of key
+            typedef Key key_type;
+
+            //- Object content type
+            typedef zero::null mapped_type;
+
+            //- Content storage type to the entry
+            typedef key_type value_type;
+
+            //- The lookup key == content
+            key_type key_;
+
+            //- Addressing (next in collision list)
+            unary_entry* next_;
+
+            //- Construct from key, (ununsed) object, next pointer
+            unary_entry(const Key& key, const T&, unary_entry* next)
+            :
+                key_(key),
+                next_(next)
+            {}
+
+            //- The key
+            const key_type& key() const
+            {
+                return key_;
+            }
+
+            //- Dummy mapped object
+            const mapped_type& mapped() const
+            {
+                return zeroNullElement;
+            }
+            mapped_type& mapped()
+            {
+                return zeroNullElement;
+            }
+
+
+        private:
+            //- Disallow default bitwise copy construct / assignment
+            unary_entry(const unary_entry&) = delete;
+            void operator=(const unary_entry&) = delete;
+        };
+
+
+        //- Hashed node with a linked-list for collisions
+        typedef typename std::conditional
+        <
+            std::is_same<zero::null, typename std::remove_cv<T>::type>::value,
+            unary_entry,
+            pair_entry
+        >::type node_type;
+
+
+    // Private Data
+
+        //- The number of nodes currently stored in table
+        label size_;
+
+        //- Number of nodes allocated in table
+        label capacity_;
+
+        //- The table of primary nodes
+        node_type** table_;
+
+
+    // Private Member Functions
+
+        //- Return the hash index of the Key within the current table size.
+        //  No checks for zero-sized tables.
+        inline label hashKeyIndex(const Key& key) const;
+
+        //- Assign a new hash-entry to a possibly already existing key.
+        //  \return True if the new entry was set.
+        bool set(const Key& key, const T& obj, const bool overwrite);
+
+
 public:
 
     //- The template instance used for this HashTable
@@ -96,18 +260,25 @@ public:
 
     // STL type definitions
 
-        //- Type of keys that the HashTable uses.
+        //- The second template parameter, type of keys used.
         typedef Key key_type;
 
-        //- Type of values that the HashTable contains.
+        //- The first template parameter, type of objects contained.
+        typedef T mapped_type;
+
+        //- Same as mapped_type for OpenFOAM HashTables
+        //  Note that this is different than the std::map definition.
         typedef T value_type;
 
+        //- The third template parameter, the hash index method.
+        typedef Hash hasher;
+
         //- The type used for storing into value_type objects.
-        //  This type is usually value_type&.
+        //  This type is usually 'value_type*'.
         typedef T* pointer;
 
         //- The type used for storing into value_type objects.
-        //  This type is usually value_type&.
+        //  This type is usually 'value_type&'.
         typedef T& reference;
 
         //- The type used for reading from constant value_type objects.
@@ -129,65 +300,14 @@ public:
         //- Forward iterator with const access
         class const_iterator;
 
-
-private:
-
-    // Private data type for table entries
-
-        //- Structure to hold a hashed entry, with a linked-list for collisions
-        struct hashedEntry
-        {
-            //- The lookup key
-            Key key_;
-
-            //- The data object
-            T obj_;
-
-            //- Pointer to next hashedEntry in sub-list
-            hashedEntry* next_;
-
-            //- Construct from key, object, next pointer
-            inline hashedEntry(const Key& key, const T& obj, hashedEntry* next);
-
-        private:
-            //- Disallow default bitwise copy construct
-            hashedEntry(const hashedEntry&) = delete;
-
-            //- Disallow default bitwise assignment
-            void operator=(const hashedEntry&) = delete;
-        };
-
-
-    // Private data: size of table, the table and current number of elements
-
-        //- The current number of elements in table
-        label nElmts_;
-
-        //- Number of primary entries allocated in table
-        label tableSize_;
-
-        //- The table of primary entries
-        hashedEntry** table_;
-
-
-    // Private Member Functions
-
-        //- Return the hash index of the Key within the current table size.
-        //  No checks for zero-sized tables.
-        inline label hashKeyIndex(const Key& key) const;
-
-        //- Assign a new hash-entry to a possibly already existing key.
-        //  \return True if the new entry was set.
-        bool set(const Key& key, const T& obj, const bool protect);
-
-
 protected:
 
         //- Internally used base for iterator and const_iterator
-        class iterator_base;
+        template<bool Const> class Iterator;
 
-        //- Friendship with the iterator_base is required.
-        friend class iterator_base;
+        //- Friendship with the base iterator is required.
+        friend class Iterator<true>;
+        friend class Iterator<false>;
 
 
     // Protected Member Functions
@@ -205,20 +325,23 @@ public:
 
     // Constructors
 
-        //- Construct given initial table size
-        HashTable(const label size = 128);
+        //- Construct null with default (128) table size
+        HashTable();
 
-        //- Construct from Istream
+        //- Construct given initial table size
+        explicit HashTable(const label size);
+
+        //- Construct from Istream with default table size
         HashTable(Istream& is, const label size = 128);
 
         //- Construct as copy
-        HashTable(const HashTable<T, Key, Hash>& ht);
+        HashTable(const this_type& ht);
 
         //- Move construct
-        HashTable(HashTable<T, Key, Hash>&& ht);
+        HashTable(this_type&& ht);
 
         //- Construct by transferring the parameter contents
-        HashTable(const Xfer<HashTable<T, Key, Hash>>& ht);
+        HashTable(const Xfer<this_type>& ht);
 
         //- Construct from an initializer list
         HashTable(std::initializer_list<std::pair<Key, T>> lst);
@@ -242,19 +365,19 @@ public:
         inline bool empty() const;
 
         //- Return true if hashed entry is found in table
-        bool found(const Key& key) const;
+        inline bool found(const Key& key) const;
 
         //- Find and return an iterator set at the hashed entry
         //  If not found iterator = end()
-        iterator find(const Key& key);
+        inline iterator find(const Key& key);
 
         //- Find and return an const_iterator set at the hashed entry
         //  If not found iterator = end()
-        const_iterator find(const Key& key) const;
+        inline const_iterator find(const Key& key) const;
 
         //- Find and return an const_iterator set at the hashed entry
         //  If not found iterator = end()
-        const_iterator cfind(const Key& key) const;
+        inline const_iterator cfind(const Key& key) const;
 
         //- Return hashed entry if it exists, or return the given default
         inline const T& lookup(const Key& key, const T& deflt) const;
@@ -512,149 +635,157 @@ protected:
         //  and prevent most external usage.
         //  iterator and const_iterator have the same size, allowing
         //  us to reinterpret_cast between them (if desired)
-        class iterator_base
+
+        template<bool Const>
+        class Iterator
         {
         public:
-            // Public typedefs
-            using table_type = this_type;
-            using key_type   = this_type::key_type;
+          // Typedefs
             using iterator_category = std::forward_iterator_tag;
-            using difference_type  = this_type::difference_type;
+            using difference_type = this_type::difference_type;
 
-        private:
-            using entry_type = hashedEntry;
+            //- The HashTable container type
+            using table_type = typename std::conditional
+            <
+                Const,
+                const this_type,
+                this_type
+            >::type;
 
-          // Private Data
+            //- The node-type being addressed
+            using node_type = typename std::conditional
+            <
+                Const,
+                const this_type::node_type,
+                this_type::node_type
+            >::type;
 
-            //- Currently selected entry.
-            //  MUST be the first member for easy comparison between iterators
-            //  and for reinterpret_cast from nullObject
-            entry_type* entryPtr_;
+            //- The key type
+            using key_type = this_type::key_type;
 
-            //- Pointer to the hash-table for which this is an iterator
-            //  This allows use of the default bitwise copy/assignment
-            table_type* hashTable_;
+            //- The object type being addressed
+            using mapped_type = typename std::conditional
+            <
+                Const,
+                const this_type::mapped_type,
+                this_type::mapped_type
+            >::type;
+
+
+          // Member Functions
+
+            //- True if iterator points to an entry
+            //  This can be used directly instead of comparing to end()
+            inline bool found() const;
+
+            //- The key associated with the iterator
+            inline const Key& key() const;
+
+          // Member Operators
+
+            //- Compare hash-entry element pointers.
+            //  Independent of const/non-const access
+            inline bool operator==(const Iterator<true>& iter) const;
+            inline bool operator!=(const Iterator<true>& iter) const;
+
+            inline bool operator==(const Iterator<false>& iter) const;
+            inline bool operator!=(const Iterator<false>& iter) const;
 
-            //- Current hash index within the hash-table data.
-            //  A signed value, since erase() uses a negative value to signal
-            //  the erasure state.
-            label hashIndex_;
 
         protected:
+            friend class HashTable;  // For begin/find constructors
+
+          // Protected Data
+
+            //- The selected entry.
+            //  MUST be the first member for easy comparison between iterators
+            //  and for reinterpret_cast from nullObject
+            node_type* entry_;
+
+            //- The hash-table container being iterated on.
+            //  Using a pointer allows default bitwise copy/assignment
+            table_type* container_;
+
+            //- Index within the hash-table data.
+            //  A signed value, since iterator_erase() needs a negative value
+            //  to mark the position.
+            label index_;
+
+
+          // Protected Constructors
+
+            //- Construct null (end iterator)
+            inline Iterator();
+
+            //- Construct from begin of hash-table
+            inline Iterator(bool, table_type* tbl);
+
+            //- Construct by finding key in hash table
+            inline Iterator(table_type* tbl, const Key& key);
+
 
           // Protected Member Functions
 
             //- Increment to the next position
             inline void increment();
 
-            //- The referenced object/value element
-            inline T& element() const;
+            //- The object associated with the iterator
+            inline mapped_type& object() const
+            {
+                return entry_->mapped();
+            }
 
-            //- Erase the entry at the current position
-            bool erase();
-
-
-        public:
-
-          // Constructors
-
-            //- Construct null (end iterator)
-            inline iterator_base();
-
-            //- Construct from begin of hash-table
-            inline explicit iterator_base(const table_type* hashTbl);
-
-            //- Construct from hash table, element and hash index
-            inline iterator_base
-            (
-                const table_type* hashTbl,
-                const entry_type* elmt,
-                const label hashIndex
-            );
-
-          // Member functions/operators
-
-            //- True if iterator points to an entry
-            //  This can be used directly instead of comparing to end()
-            inline bool found() const;
-
-            //- Return the Key corresponding to the iterator
-            inline const Key& key() const;
-
-            //- Compare hash-entry element pointers
-            inline bool operator==(const iterator_base& iter) const;
-            inline bool operator!=(const iterator_base& iter) const;
+            //- Permit an explicit cast to the other (const/non-const) searcher
+            inline explicit operator const Iterator<!Const>&() const
+            {
+                return *reinterpret_cast<const Iterator<!Const>*>(this);
+            }
         };
 
 
+        //- Low-level entry erasure using iterator internals.
+        //  This invalidates the iterator until the next ++ operation.
+        //  \return True if the corresponding entry existed and was removed
+        bool iterator_erase(node_type*& entry, label& index);
+
 public:
 
-        //- An iterator wrapper for returning a reference to the key
-        template<class WrappedIterator>
-        class key_iterator_base
-        :
-            public WrappedIterator
-        {
-        public:
-            using value_type = this_type::key_type;
-            using pointer    = const Key*;
-            using reference  = const Key&;
-
-            //- 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
-
         //- Forward iterator with non-const access
         class iterator
         :
-            public iterator_base
+            public Iterator<false>
         {
-            friend class HashTable;  // Uses iterator::erase() method
-            using entry_type = hashedEntry;
-
         public:
+          // Typedefs
+            using iterator_category = std::forward_iterator_tag;
+            using difference_type   = this_type::difference_type;
 
-          // Public typedefs
-            using table_type = this_type;
-            using value_type = this_type::value_type;
-            using pointer    = this_type::pointer;
-            using reference  = this_type::reference;
+            using key_type    = this_type::key_type;
+            using mapped_type = this_type::mapped_type;
+            using value_type  = this_type::value_type;
+            using pointer     = this_type::pointer;
+            using reference   = this_type::reference;
 
           // Constructors
 
             //- Construct null (end iterator)
-            inline iterator();
+            inline iterator() {}
 
-            //- Construct from begin of hash-table
-            inline explicit iterator(table_type* hashTbl);
+            //- Copy construct from similar access type
+            inline explicit iterator(const Iterator<false>& iter)
+            :
+                Iterator<false>(iter)
+            {}
 
-            //- Construct from hash table, element and hash index
-            //  Used by the hash-table find() method.
-            inline iterator
-            (
-                table_type* hashTbl,
-                entry_type* elmt,
-                const label hashIndex
-            );
 
           // Member functions/operators
 
-            //- Return non-const access to referenced object
-            inline reference object() const;
+            //- Non-const access to referenced object
+            using Iterator<false>::object;
 
-            //- Return non-const access to referenced object
-            inline reference operator*() const;
-            inline reference operator()() const;
+            //- Non-const access to referenced object
+            inline reference operator*() const  { return this->object(); }
+            inline reference operator()() const { return this->object(); }
 
             inline iterator& operator++();
             inline iterator operator++(int);
@@ -666,54 +797,111 @@ public:
         //- Forward iterator with const access
         class const_iterator
         :
-            public iterator_base
+            public Iterator<true>
         {
-            using entry_type = const hashedEntry;
-
         public:
+          // Typedefs
+            using iterator_category = std::forward_iterator_tag;
+            using difference_type   = this_type::difference_type;
 
-          // Public typedefs
-            using table_type = const this_type;
-            using value_type = const this_type::value_type;
-            using pointer    = this_type::const_pointer;
-            using reference  = this_type::const_reference;
+            using key_type    = this_type::key_type;
+            using mapped_type = const this_type::mapped_type;
+            using value_type  = const this_type::value_type;
+            using pointer     = this_type::const_pointer;
+            using reference   = this_type::const_reference;
 
           // Constructors
 
             //- Construct null (end iterator)
-            inline const_iterator();
+            inline const_iterator() {}
 
-            //- Construct from begin of hash-table
-            inline explicit const_iterator(table_type* hashTbl);
+            //- Copy construct from similar access type
+            inline explicit const_iterator(const Iterator<true>& iter)
+            :
+                Iterator<true>(iter)
+            {}
 
-            //- Construct from hash table, element and hash index.
-            //  Used by the hash-table find() method.
-            inline const_iterator
-            (
-                table_type* hashTbl,
-                entry_type* elmt,
-                const label hashIndex
-            );
+            //- Copy construct from dissimilar access type
+            inline explicit const_iterator(const Iterator<false>& iter)
+            :
+                Iterator<true>
+                (
+                    static_cast<const Iterator<true>&>(iter)
+                )
+            {}
+
+            //- Implicit conversion from dissimilar access type
+            inline const_iterator(const iterator& iter)
+            :
+                const_iterator(reinterpret_cast<const const_iterator&>(iter))
+            {}
 
-            //- Copy construct from iterator
-            inline const_iterator(const iterator& iter);
 
           // Member functions/operators
 
-            //- Return const access to referenced object
-            inline reference object() const;
+            //- Const access to referenced object
+            using Iterator<true>::object;
 
-            //- Return const access to referenced object
-            inline reference operator*() const;
-            inline reference operator()() const;
+            //- Const access to referenced object
+            inline reference operator*() const  { return this->object(); }
+            inline reference operator()() const { return this->object(); }
 
             inline const_iterator& operator++();
             inline const_iterator operator++(int);
+
+          // Assignment
+
+            const_iterator& operator=(const const_iterator&) = default;
+
+            // Allow assign from iterator to const_iterator
+            const_iterator& operator=(const iterator& iter)
+            {
+                return this->operator=
+                (
+                    reinterpret_cast<const const_iterator&>(iter)
+                );
+            }
         };
 
 
     //- Iterating over keys only
 
+        //- An iterator wrapper for returning a reference to the key
+        template<class Iter>
+        class key_iterator_base
+        :
+            public Iter
+        {
+        public:
+            using value_type = this_type::key_type;
+            using pointer    = const Key*;
+            using reference  = const Key&;
+
+            //- Implicit conversion
+            inline key_iterator_base(const Iter& iter)
+            :
+                Iter(iter)
+            {}
+
+            //- Return the key
+            inline reference operator*() const  { return this->key(); }
+            inline reference operator()() const { return this->key(); }
+
+            inline key_iterator_base& operator++()
+            {
+                this->increment();
+                return *this;
+            }
+
+            inline key_iterator_base operator++(int)
+            {
+                key_iterator_base iter(*this);
+                this->increment();
+                return iter;
+            }
+        };
+
+
         //- Forward iterator returning the key
         using key_iterator = key_iterator_base<iterator>;
 
@@ -793,6 +981,7 @@ inline void Swap
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 #include "HashTableI.H"
+#include "HashTableIterI.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.C
index d6d7bef31d..059be27e3e 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.C
@@ -36,6 +36,8 @@ defineTypeNameAndDebug(HashTableCore, 0);
 // Approximately labelMax/4
 const Foam::label Foam::HashTableCore::maxTableSize(1L << (sizeof(label)*8-3));
 
+Foam::zero::null Foam::HashTableCore::zeroNullElement;
+
 
 // * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * * //
 
@@ -50,7 +52,7 @@ Foam::label Foam::HashTableCore::canonicalSize(const label requested_size)
         return maxTableSize;
     }
 
-    // Enforce power of two - makes for a very fast modulus.
+    // Enforce power of two for fast modulus in hash index calculations.
     // Use unsigned for these calculations.
     //
     // - The lower limit (8) is somewhat arbitrary, but if the hash table
@@ -65,7 +67,8 @@ Foam::label Foam::HashTableCore::canonicalSize(const label requested_size)
     {
         return powerOfTwo;
     }
-    else if (size & (size-1))  // <- Modulus of i^2
+
+    if (size & (size-1))  // <- Modulus of i^2
     {
         // Determine power-of-two. Brute-force is fast enough.
         while (powerOfTwo < size)
@@ -75,10 +78,8 @@ Foam::label Foam::HashTableCore::canonicalSize(const label requested_size)
 
         return powerOfTwo;
     }
-    else
-    {
-        return size;
-    }
+
+    return size;
 }
 
 
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.H
index 46cc0c3f46..76feeb2105 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCore.H
@@ -40,6 +40,7 @@ SourceFiles
 #include "uLabel.H"
 #include "className.H"
 #include "nullObject.H"
+#include "zero.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -77,20 +78,16 @@ struct HashTableCore
     template<class IteratorType, class TableType>
     inline static IteratorType iterator_begin(TableType& table);
 
-    //- Factory method to create a const iterator begin
-    template<class IteratorType, class TableType>
-    inline static IteratorType iterator_begin(const TableType& table);
-
     //- Factory method to create a const iterator begin
     template<class IteratorType, class TableType>
     inline static IteratorType iterator_cbegin(const TableType& table);
 
-    //- Factory method to create a non-const iterator end
+    //- Factory method to return an iterator end
     //  Simply reinterprets a NullObject as a hash-table iterator.
     template<class IteratorType>
     inline static const IteratorType& iterator_end();
 
-    //- Factory method to create a const iterator cend
+    //- Factory method to return an iterator cend
     //  Simply reinterprets a NullObject as a hash-table iterator.
     template<class IteratorType>
     inline static const IteratorType& iterator_cend();
@@ -116,6 +113,13 @@ struct HashTableCore
         inline const IteratorType& end() const;
         inline const IteratorType& cend() const;
     };
+
+
+protected:
+
+    //- A static zero::null for dereferencing as a dummy HashSet element
+    static zero::null zeroNullElement;
+
 };
 
 
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCoreI.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCoreI.H
index 8198e2e4c3..1e59a0baa5 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableCoreI.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableCoreI.H
@@ -35,16 +35,6 @@ inline IteratorType Foam::HashTableCore::iterator_begin
 }
 
 
-template<class IteratorType, class TableType>
-inline IteratorType Foam::HashTableCore::iterator_begin
-(
-    const TableType& table
-)
-{
-    return IteratorType(table.begin());
-}
-
-
 template<class IteratorType, class TableType>
 inline IteratorType Foam::HashTableCore::iterator_cbegin
 (
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
index 50f0c42d1e..27ac134006 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableI.H
@@ -25,30 +25,14 @@ License
 
 #include "error.H"
 
-// * * * * * * * * * * * * * Private Member Classes * * * * * * * * * * * * //
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::hashedEntry::hashedEntry
-(
-    const Key& key,
-    const T& obj,
-    hashedEntry* next
-)
-:
-    key_(key),
-    obj_(obj),
-    next_(next)
-{}
-
-
 // * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>
 inline Foam::label
 Foam::HashTable<T, Key, Hash>::hashKeyIndex(const Key& key) const
 {
-    // size is power of two - this is the modulus
-    return Hash()(key) & (tableSize_ - 1);
+    // capacity is always a power of two - this is the modulus
+    return Hash()(key) & (capacity_ - 1);
 }
 
 
@@ -57,21 +41,76 @@ Foam::HashTable<T, Key, Hash>::hashKeyIndex(const Key& key) const
 template<class T, class Key, class Hash>
 inline Foam::label Foam::HashTable<T, Key, Hash>::capacity() const
 {
-    return tableSize_;
+    return capacity_;
 }
 
 
 template<class T, class Key, class Hash>
 inline Foam::label Foam::HashTable<T, Key, Hash>::size() const
 {
-    return nElmts_;
+    return size_;
 }
 
 
 template<class T, class Key, class Hash>
 inline bool Foam::HashTable<T, Key, Hash>::empty() const
 {
-    return !nElmts_;
+    return !size_;
+}
+
+
+template<class T, class Key, class Hash>
+bool Foam::HashTable<T, Key, Hash>::found(const Key& key) const
+{
+    if (size_)
+    {
+        return Iterator<true>(this, key).found();
+    }
+
+    return false;
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::iterator
+Foam::HashTable<T, Key, Hash>::find
+(
+    const Key& key
+)
+{
+    if (size_)
+    {
+        return iterator(Iterator<false>(this, key));
+    }
+
+    return iterator();
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::find
+(
+    const Key& key
+) const
+{
+    return this->cfind(key);
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::cfind
+(
+    const Key& key
+) const
+{
+    if (size_)
+    {
+        return const_iterator(Iterator<true>(this, key));
+    }
+
+    return const_iterator();
 }
 
 
@@ -82,7 +121,7 @@ inline bool Foam::HashTable<T, Key, Hash>::insert
     const T& obj
 )
 {
-    return this->set(key, obj, true);
+    return this->set(key, obj, false);  // No overwrite
 }
 
 
@@ -93,7 +132,7 @@ inline bool Foam::HashTable<T, Key, Hash>::set
     const T& obj
 )
 {
-    return this->set(key, obj, false);
+    return this->set(key, obj, true);   // Overwrite
 }
 
 
@@ -112,7 +151,7 @@ inline const T& Foam::HashTable<T, Key, Hash>::lookup
     const T& deflt
 ) const
 {
-    const_iterator iter = this->find(key);
+    const const_iterator iter(this->cfind(key));
     return iter.found() ? iter.object() : deflt;
 }
 
@@ -122,7 +161,7 @@ inline const T& Foam::HashTable<T, Key, Hash>::lookup
 template<class T, class Key, class Hash>
 inline T& Foam::HashTable<T, Key, Hash>::operator[](const Key& key)
 {
-    iterator iter = this->find(key);
+    const iterator iter(this->find(key));
 
     if (!iter.found())
     {
@@ -139,7 +178,7 @@ inline T& Foam::HashTable<T, Key, Hash>::operator[](const Key& key)
 template<class T, class Key, class Hash>
 inline const T& Foam::HashTable<T, Key, Hash>::operator[](const Key& key) const
 {
-    const_iterator iter = this->find(key);
+    const const_iterator iter(this->cfind(key));
 
     if (!iter.found())
     {
@@ -156,14 +195,14 @@ inline const T& Foam::HashTable<T, Key, Hash>::operator[](const Key& key) const
 template<class T, class Key, class Hash>
 inline T& Foam::HashTable<T, Key, Hash>::operator()(const Key& key)
 {
-    iterator iter = this->find(key);
+    const iterator iter(this->find(key));
 
     if (iter.found())
     {
         return iter.object();
     }
 
-    this->insert(key, T());
+    this->insert(key, mapped_type());
     return find(key).object();
 }
 
@@ -175,7 +214,7 @@ inline T& Foam::HashTable<T, Key, Hash>::operator()
     const T& deflt
 )
 {
-    iterator iter = this->find(key);
+    const iterator iter(this->find(key));
 
     if (iter.found())
     {
@@ -198,412 +237,6 @@ inline const T& Foam::HashTable<T, Key, Hash>::operator()
 }
 
 
-// * * * * * * * * * * * * * * * iterator base * * * * * * * * * * * * * * * //
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::iterator_base::iterator_base()
-:
-    entryPtr_(nullptr),
-    hashTable_(nullptr),
-    hashIndex_(0)
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::iterator_base::iterator_base
-(
-    const table_type* hashTbl,
-    const entry_type* elmt,
-    const label hashIndex
-)
-:
-    entryPtr_(const_cast<entry_type*>(elmt)),
-    hashTable_(const_cast<table_type*>(hashTbl)),
-    hashIndex_(hashIndex)
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::iterator_base::iterator_base
-(
-    const table_type* hashTbl
-)
-:
-    entryPtr_(nullptr),
-    hashTable_(const_cast<table_type*>(hashTbl)),
-    hashIndex_(0)
-{
-    if (hashTable_ && hashTable_->nElmts_)
-    {
-        // find first non-nullptr table entry
-        while
-        (
-            !(entryPtr_ = hashTable_->table_[hashIndex_])
-         && ++hashIndex_ < hashTable_->tableSize_
-        )
-        {}
-
-        if (hashIndex_ >= hashTable_->tableSize_)
-        {
-            // make into an end iterator
-            entryPtr_ = nullptr;
-            hashIndex_ = 0;
-        }
-    }
-}
-
-
-template<class T, class Key, class Hash>
-inline void
-Foam::HashTable<T, Key, Hash>::iterator_base::increment()
-{
-    // A negative index is a special value from erase
-    if (hashIndex_ < 0)
-    {
-        // the markPos='-curPos-1', but we wish to continue at 'curPos-1'
-        // thus use '-(markPos+1) -1'
-        hashIndex_ = -(hashIndex_+1) - 1;
-    }
-    else if (entryPtr_)
-    {
-        if (entryPtr_->next_)
-        {
-            // Move to next element on the SLList
-            entryPtr_ = entryPtr_->next_;
-            return;
-        }
-    }
-    // else
-    // {
-    //     // if we reach here (entryPtr_ is nullptr) it is already at the end()
-    //     // we should probably stop
-    // }
-
-
-    // Step to the next table entry
-    while
-    (
-        ++hashIndex_ < hashTable_->tableSize_
-     && !(entryPtr_ = hashTable_->table_[hashIndex_])
-    )
-    {}
-
-    if (hashIndex_ >= hashTable_->tableSize_)
-    {
-        // make into an end iterator
-        entryPtr_ = nullptr;
-        hashIndex_ = 0;
-    }
-}
-
-
-template<class T, class Key, class Hash>
-inline bool
-Foam::HashTable<T, Key, Hash>::iterator_base::found() const
-{
-    return entryPtr_;
-}
-
-
-template<class T, class Key, class Hash>
-inline const Key& Foam::HashTable<T, Key, Hash>::iterator_base::key() const
-{
-    return entryPtr_->key_;
-}
-
-
-template<class T, class Key, class Hash>
-inline T& Foam::HashTable<T, Key, Hash>::iterator_base::element() const
-{
-    return entryPtr_->obj_;
-}
-
-
-template<class T, class Key, class Hash>
-inline bool Foam::HashTable<T, Key, Hash>::iterator_base::operator==
-(
-    const iterator_base& iter
-) const
-{
-    return entryPtr_ == iter.entryPtr_;
-}
-
-
-template<class T, class Key, class Hash>
-inline bool Foam::HashTable<T, Key, Hash>::iterator_base::operator!=
-(
-    const iterator_base& iter
-) const
-{
-    return entryPtr_ != iter.entryPtr_;
-}
-
-
-// * * * * * * * * * * * * * * key iterator base * * * * * * * * * * * * * * //
-
-template<class T, class Key, class Hash>
-template<class WrappedIterator>
-inline Foam::HashTable<T, Key, Hash>::key_iterator_base<WrappedIterator>
-::key_iterator_base
-(
-    const WrappedIterator& iter
-)
-:
-    WrappedIterator(iter)
-{}
-
-
-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 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>
-inline Foam::HashTable<T, Key, Hash>::iterator::iterator()
-:
-    iterator_base()
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::iterator::iterator
-(
-    table_type* hashTbl
-)
-:
-    iterator_base(hashTbl)
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::iterator::iterator
-(
-    table_type* hashTbl,
-    entry_type* elmt,
-    const label hashIndex
-)
-:
-    iterator_base(hashTbl, elmt, hashIndex)
-{}
-
-
-template<class T, class Key, class Hash>
-inline T&
-Foam::HashTable<T, Key, Hash>::iterator::object() const
-{
-    return this->element();
-}
-
-
-template<class T, class Key, class Hash>
-inline T&
-Foam::HashTable<T, Key, Hash>::iterator::operator*() const
-{
-    return this->object();
-}
-
-
-template<class T, class Key, class Hash>
-inline T&
-Foam::HashTable<T, Key, Hash>::iterator::operator()() const
-{
-    return this->object();
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::iterator&
-Foam::HashTable<T, Key, Hash>::iterator::operator++()
-{
-    this->increment();
-    return *this;
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::iterator
-Foam::HashTable<T, Key, Hash>::iterator::operator++(int)
-{
-    iterator old = *this;
-    this->increment();
-    return old;
-}
-
-
-// * * * * * * * * * * * * * * * STL const_iterator  * * * * * * * * * * * * //
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::const_iterator::const_iterator()
-:
-    iterator_base()
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::const_iterator::const_iterator
-(
-    const HashTable<T, Key, Hash>::iterator& iter
-)
-:
-    iterator_base(iter)
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::const_iterator::const_iterator
-(
-    table_type* hashTbl
-)
-:
-    iterator_base(hashTbl)
-{}
-
-
-template<class T, class Key, class Hash>
-inline Foam::HashTable<T, Key, Hash>::const_iterator::const_iterator
-(
-    table_type* hashTbl,
-    entry_type* elmt,
-    const label hashIndex
-)
-:
-    iterator_base(hashTbl, elmt, hashIndex)
-{}
-
-
-template<class T, class Key, class Hash>
-inline const T&
-Foam::HashTable<T, Key, Hash>::const_iterator::object() const
-{
-    return this->element();
-}
-
-
-template<class T, class Key, class Hash>
-inline const T&
-Foam::HashTable<T, Key, Hash>::const_iterator::operator*() const
-{
-    return this->object();
-}
-
-
-template<class T, class Key, class Hash>
-inline const T&
-Foam::HashTable<T, Key, Hash>::const_iterator::operator()() const
-{
-    return this->object();
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::const_iterator&
-Foam::HashTable<T, Key, Hash>::const_iterator::operator++()
-{
-    this->increment();
-    return *this;
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::const_iterator
-Foam::HashTable<T, Key, Hash>::const_iterator::operator++(int)
-{
-    const_iterator old = *this;
-    this->increment();
-    return old;
-}
-
-
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::iterator
-Foam::HashTable<T, Key, Hash>::begin()
-{
-    return iterator(this);
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::const_iterator
-Foam::HashTable<T, Key, Hash>::begin() const
-{
-    return const_iterator(this);
-}
-
-
-template<class T, class Key, class Hash>
-inline typename Foam::HashTable<T, Key, Hash>::const_iterator
-Foam::HashTable<T, Key, Hash>::cbegin() const
-{
-    return const_iterator(this);
-}
-
-
-template<class T, class Key, class Hash>
-inline const typename Foam::HashTable<T, Key, Hash>::iterator&
-Foam::HashTable<T, Key, Hash>::end()
-{
-    return iterator_end<iterator>();
-}
-
-
-template<class T, class Key, class Hash>
-inline const typename Foam::HashTable<T, Key, Hash>::const_iterator&
-Foam::HashTable<T, Key, Hash>::end() const
-{
-    return iterator_end<const_iterator>();
-}
-
-
-template<class T, class Key, class Hash>
-inline const typename Foam::HashTable<T, Key, Hash>::const_iterator&
-Foam::HashTable<T, Key, Hash>::cend() const
-{
-    return iterator_cend<const_iterator>();
-}
-
-
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
 
 template<class T, class Key, class Hash>
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
index 54cd7ed922..34392da5ee 100644
--- a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIO.C
@@ -33,17 +33,17 @@ template<class T, class Key, class Hash>
 Foam::HashTable<T, Key, Hash>::HashTable(Istream& is, const label size)
 :
     HashTableCore(),
-    nElmts_(0),
-    tableSize_(HashTableCore::canonicalSize(size)),
+    size_(0),
+    capacity_(HashTableCore::canonicalSize(size)),
     table_(nullptr)
 {
-    if (tableSize_)
+    if (capacity_)
     {
-        table_ = new hashedEntry*[tableSize_];
+        table_ = new node_type*[capacity_];
 
-        for (label hashIdx = 0; hashIdx < tableSize_; ++hashIdx)
+        for (label i=0; i < capacity_; ++i)
         {
-            table_[hashIdx] = nullptr;
+            table_[i] = nullptr;
         }
     }
 
@@ -60,10 +60,10 @@ Foam::Ostream& Foam::HashTable<T, Key, Hash>::printInfo(Ostream& os) const
     label maxChain = 0;
     unsigned avgChain = 0;
 
-    for (label hashIdx = 0; hashIdx < tableSize_; ++hashIdx)
+    for (label i=0; i < capacity_; ++i)
     {
         label count = 0;
-        for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
+        for (node_type* ep = table_[i]; ep; ep = ep->next_)
         {
             ++count;
         }
@@ -81,7 +81,7 @@ Foam::Ostream& Foam::HashTable<T, Key, Hash>::printInfo(Ostream& os) const
     }
 
     os  << "HashTable<T,Key,Hash>"
-        << " elements:" << size() << " slots:" << used << "/" << tableSize_
+        << " elements:" << size() << " slots:" << used << "/" << capacity_
         << " chaining(avg/max):" << (used ? (float(avgChain)/used) : 0)
         << "/" << maxChain << endl;
 
@@ -145,7 +145,7 @@ Foam::Istream& Foam::operator>>
 {
     is.fatalCheck(FUNCTION_NAME);
 
-    // Anull list
+    // Anull existing table
     L.clear();
 
     is.fatalCheck(FUNCTION_NAME);
@@ -154,7 +154,7 @@ Foam::Istream& Foam::operator>>
 
     is.fatalCheck
     (
-        "operator>>(Istream&, HashTable<T, Key, Hash>&) : "
+        "operator>>(Istream&, HashTable&) : "
         "reading first token"
     );
 
@@ -163,11 +163,11 @@ Foam::Istream& Foam::operator>>
         const label s = firstToken.labelToken();
 
         // Read beginning of contents
-        const char delimiter = is.readBeginList("HashTable<T, Key, Hash>");
+        const char delimiter = is.readBeginList("HashTable");
 
         if (s)
         {
-            if (2*s > L.tableSize_)
+            if (2*s > L.capacity_)
             {
                 L.resize(2*s);
             }
@@ -182,7 +182,7 @@ Foam::Istream& Foam::operator>>
 
                     is.fatalCheck
                     (
-                        "operator>>(Istream&, HashTable<T, Key, Hash>&) : "
+                        "operator>>(Istream&, HashTable&) : "
                         "reading entry"
                     );
                 }
@@ -224,15 +224,11 @@ Foam::Istream& Foam::operator>>
 
             Key key;
             is >> key;
-
-            T element;
-            is >> element;
-
-            L.insert(key, element);
+            L.insert(key, pTraits<T>(is));
 
             is.fatalCheck
             (
-                "operator>>(Istream&, HashTable<T, Key, Hash>&) : "
+                "operator>>(Istream&, HashTable&) : "
                 "reading entry"
             );
 
@@ -262,19 +258,26 @@ Foam::Ostream& Foam::operator<<
     const HashTable<T, Key, Hash>& tbl
 )
 {
-    using const_iterator = typename HashTable<T, Key, Hash>::const_iterator;
+    const label sz = tbl.size();
 
-    // Write size and start delimiter
-    os << nl << tbl.size() << nl << token::BEGIN_LIST << nl;
-
-    // Write contents
-    for (const_iterator iter = tbl.cbegin(); iter != tbl.cend(); ++iter)
+    if (sz)
     {
-        os << iter.key() << token::SPACE << iter.object() << nl;
-    }
+        // Size and start list delimiter
+        os << nl << sz << nl << token::BEGIN_LIST << nl;
 
-    // Write end delimiter
-    os << token::END_LIST;
+        // Contents
+        for (auto iter = tbl.cbegin(); iter != tbl.cend(); ++iter)
+        {
+            os << iter.key() << token::SPACE << iter.object() << nl;
+        }
+
+        os << token::END_LIST;    // End list delimiter
+    }
+    else
+    {
+        // Empty hash table
+        os << sz << token::BEGIN_LIST << token::END_LIST;
+    }
 
     os.check(FUNCTION_NAME);
     return os;
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C
new file mode 100644
index 0000000000..9a1b792fca
--- /dev/null
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIter.C
@@ -0,0 +1,123 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+template<class T, class Key, class Hash>
+template<bool Const>
+Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator
+(
+    table_type* tbl,
+    const Key& key
+)
+:
+    entry_(nullptr),
+    container_(tbl),
+    index_(0)
+{
+    if (tbl->size())
+    {
+        const label index = container_->hashKeyIndex(key);
+
+        for (node_type* ep = container_->table_[index]; ep; ep = ep->next_)
+        {
+            if (key == ep->key())
+            {
+                entry_ = ep;
+                index_ = index;
+                break;
+            }
+        }
+    }
+}
+
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+//
+// Any changes here may need changes in the iterator increment() method
+//
+template<class T, class Key, class Hash>
+bool Foam::HashTable<T, Key, Hash>::iterator_erase
+(
+    node_type*& entry,
+    label& index
+)
+{
+    // Safeguard against the following:
+    // - empty table
+    // - nullptr entry
+    // - end iterator (which is also a nullptr)
+    // - negative index from a previous erase. See comment below.
+    if (!size_ || !entry || index < 0)
+    {
+        return false;
+    }
+
+    // Decrease count
+    size_--;
+
+    // The previous element in the singly linked list
+    node_type* prev = nullptr;
+
+    for (node_type* ep = table_[index]; ep; ep = ep->next_)
+    {
+        if (ep == entry)
+        {
+            break;
+        }
+        prev = ep;
+    }
+
+    if (prev)
+    {
+        // Had previous element in linked list - reposition to there
+        prev->next_ = entry->next_;
+        delete entry;
+        entry = prev;
+
+        return true;
+    }
+
+    // Was first element on linked list
+    table_[index] = entry->next_;
+    delete entry;
+
+    // Assign any non-nullptr value so it doesn't look like end()
+    entry = reinterpret_cast<node_type*>(this);
+
+    // Mark the present index to continue and bring it back to the present
+    // location with the next index.
+    //
+    // Save: (-index-1), which has no ambiguity for index 0.
+    // Retrieve:  (-(index+1))
+
+    index = (-index - 1);
+
+    return true;
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H
new file mode 100644
index 0000000000..cc85a8b914
--- /dev/null
+++ b/src/OpenFOAM/containers/HashTables/HashTable/HashTableIterI.H
@@ -0,0 +1,263 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+// * * * * * * * * * * * * * * * iterator base * * * * * * * * * * * * * * * //
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator()
+:
+    entry_(nullptr),
+    container_(nullptr),
+    index_(0)
+{}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline Foam::HashTable<T, Key, Hash>::Iterator<Const>::Iterator
+(
+    bool, // Future use and to avoid implicit construct
+    table_type* tbl
+)
+:
+    entry_(nullptr),
+    container_(tbl),
+    index_(0)
+{
+    if (container_ && container_->size_)
+    {
+        // Locate the first non-nullptr table entry
+        while
+        (
+            !(entry_ = container_->table_[index_])
+         && ++index_ < container_->capacity_
+        )
+        {}
+
+        if (index_ >= container_->capacity_)
+        {
+            // Nothing found - make it an end iterator
+            entry_ = nullptr;
+            index_ = 0;
+        }
+    }
+}
+
+
+//
+// Any changes here may need changes in iterator_erase() method too
+//
+template<class T, class Key, class Hash>
+template<bool Const>
+inline void
+Foam::HashTable<T, Key, Hash>::Iterator<Const>::increment()
+{
+    if (index_ < 0)
+    {
+        // Negative index is a special value from erase
+        //
+        // Saved as (-index-1), retrieved as (-(index-1)) but to with an
+        // extra (-1) to compensate for the ++ in the following while loop
+        index_ = -(index_+1) - 1;
+    }
+    else if (index_ < container_->capacity_ && entry_ && entry_->next_)
+    {
+        // Move to next element on the linked-list
+        entry_ = entry_->next_;
+        return;
+    }
+
+    // Move to the next non-nullptr table entry
+    while
+    (
+        ++index_ < container_->capacity_
+     && !(entry_ = container_->table_[index_])
+    )
+    {}
+
+    if (index_ >= container_->capacity_)
+    {
+        // Nothing found - make it an end iterator
+        entry_ = nullptr;
+        index_ = 0;
+    }
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline bool
+Foam::HashTable<T, Key, Hash>::Iterator<Const>::found() const
+{
+    return entry_;
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline const Key& Foam::HashTable<T, Key, Hash>::Iterator<Const>::key() const
+{
+    return entry_->key();
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline bool Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator==
+(
+    const Iterator<true>& iter
+) const
+{
+    return entry_ == iter.entry_;
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline bool Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator!=
+(
+    const Iterator<true>& iter
+) const
+{
+    return entry_ != iter.entry_;
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline bool Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator==
+(
+    const Iterator<false>& iter
+) const
+{
+    return entry_ == iter.entry_;
+}
+
+
+template<class T, class Key, class Hash>
+template<bool Const>
+inline bool Foam::HashTable<T, Key, Hash>::Iterator<Const>::operator!=
+(
+    const Iterator<false>& iter
+) const
+{
+    return entry_ != iter.entry_;
+}
+
+
+// * * * * * * * * * * * * * * * * STL iterator  * * * * * * * * * * * * * * //
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::iterator&
+Foam::HashTable<T, Key, Hash>::iterator::operator++()
+{
+    this->increment();
+    return *this;
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::iterator
+Foam::HashTable<T, Key, Hash>::iterator::operator++(int)
+{
+    iterator iter(*this);
+    this->increment();
+    return iter;
+}
+
+
+// * * * * * * * * * * * * * * * STL const_iterator  * * * * * * * * * * * * //
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator&
+Foam::HashTable<T, Key, Hash>::const_iterator::operator++()
+{
+    this->increment();
+    return *this;
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::const_iterator::operator++(int)
+{
+    const_iterator iter(*this);
+    this->increment();
+    return iter;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::iterator
+Foam::HashTable<T, Key, Hash>::begin()
+{
+    return iterator(Iterator<false>(true, this));
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::begin() const
+{
+    return const_iterator(Iterator<true>(true, this));
+}
+
+
+template<class T, class Key, class Hash>
+inline typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::cbegin() const
+{
+    return const_iterator(Iterator<true>(true, this));
+}
+
+
+template<class T, class Key, class Hash>
+inline const typename Foam::HashTable<T, Key, Hash>::iterator&
+Foam::HashTable<T, Key, Hash>::end()
+{
+    return iterator_end<iterator>();
+}
+
+
+template<class T, class Key, class Hash>
+inline const typename Foam::HashTable<T, Key, Hash>::const_iterator&
+Foam::HashTable<T, Key, Hash>::end() const
+{
+    return iterator_cend<const_iterator>();
+}
+
+
+template<class T, class Key, class Hash>
+inline const typename Foam::HashTable<T, Key, Hash>::const_iterator&
+Foam::HashTable<T, Key, Hash>::cend() const
+{
+    return iterator_cend<const_iterator>();
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/containers/HashTables/Map/Map.H b/src/OpenFOAM/containers/HashTables/Map/Map.H
index e862ff97e3..784a123a74 100644
--- a/src/OpenFOAM/containers/HashTables/Map/Map.H
+++ b/src/OpenFOAM/containers/HashTables/Map/Map.H
@@ -27,6 +27,11 @@ Class
 Description
     A HashTable to objects of type \<T\> with a label key.
 
+Note
+    The Map contents are unordered.
+    When the key order is important, use the sortedToc() method to obtain
+    a list of sorted keys and use that for further access.
+
 See also
     PtrMap
 
@@ -65,13 +70,19 @@ public:
 
     // Constructors
 
-        //- Construct given initial size
-        Map(const label size = 128)
+        //- Construct null with default table size
+        Map()
+        :
+            parent_type()
+        {}
+
+        //- Construct with given table size
+        explicit Map(const label size)
         :
             parent_type(size)
         {}
 
-        //- Construct from Istream
+        //- Construct from Istream with default table size
         Map(Istream& is)
         :
             parent_type(is)
@@ -96,7 +107,7 @@ public:
         {}
 
         //- Construct by transferring the parameter contents
-        Map(const Xfer<HashTable<T, label, Hash<label>>>& map)
+        Map(const Xfer<parent_type>& map)
         :
             parent_type(map)
         {}
diff --git a/src/OpenFOAM/containers/HashTables/PtrMap/PtrMap.H b/src/OpenFOAM/containers/HashTables/PtrMap/PtrMap.H
index 178bc67098..4e86e1aae9 100644
--- a/src/OpenFOAM/containers/HashTables/PtrMap/PtrMap.H
+++ b/src/OpenFOAM/containers/HashTables/PtrMap/PtrMap.H
@@ -62,8 +62,14 @@ public:
 
     // Constructors
 
+        //- Construct null with default table size
+        PtrMap()
+        :
+            parent_type()
+        {}
+
         //- Construct given initial map size
-        PtrMap(const label size = 128)
+        explicit PtrMap(const label size)
         :
             parent_type(size)
         {}