ENH: improve overall consistency of the HashTable and its iterators

- previously had a mismash of const/non-const attributes on iterators
  that were confused with the attributes of the object being accessed.

- use the iterator keys() and object() methods consistently for all
  internal access of the HashTable iterators. This makes the intention
  clearer, the code easier to maintain, and protects against any
  possible changes in the definition of the operators.

- 'operator*': The standard form expected by STL libraries.
  However, for the std::map, this dereferences to a <key,value> pair,
  whereas OpenFOAM dereferences simply to <value>.

- 'operator()': OpenFOAM treats this like the 'operator*'

- adjusted the values of end() and cend() to reinterpret from nullObject
  instead of returning a static iteratorEnd() object.
  This means that C++ templates can now correctly deduce and match
  the return types from begin() and end() consistently.
  So that range-based now works.

  Eg,
      HashTable<label> table1 = ...;
      for (auto i : table1)
      {
          Info<< i << endl;
      }

  Since the 'operator*' returns hash table values, this prints all the
  values in the table.

src/OpenFOAM/containers/HashTables/HashTable/HashTable.C

@@ -76,7 +76,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(const label size)
 
         for (label hashIdx = 0; hashIdx < tableSize_; hashIdx++)
         {
-            table_[hashIdx] = 0;
+            table_[hashIdx] = nullptr;
         }
     }
 }
@@ -89,7 +89,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(const HashTable<T, Key, Hash>& ht)
 {
     for (const_iterator iter = ht.cbegin(); iter != ht.cend(); ++iter)
     {
-        insert(iter.key(), *iter);
+        insert(iter.key(), iter.object());
     }
 }
 
@@ -268,8 +268,8 @@ bool Foam::HashTable<T, Key, Hash>::set
 
     const label hashIdx = hashKeyIndex(key);
 
-    hashedEntry* existing = 0;
-    hashedEntry* prev = 0;
+    hashedEntry* existing = nullptr;
+    hashedEntry* prev = nullptr;
 
     for (hashedEntry* ep = table_[hashIdx]; ep; ep = ep->next_)
     {
@@ -281,10 +281,10 @@ bool Foam::HashTable<T, Key, Hash>::set
         prev = ep;
     }
 
-    // Not found, insert it at the head
     if (!existing)
     {
-        table_[hashIdx] = new hashedEntry(key, table_[hashIdx], newEntry);
+        // Not found, insert it at the head
+        table_[hashIdx] = new hashedEntry(key, newEntry, table_[hashIdx]);
         nElmts_++;
 
         if (double(nElmts_)/tableSize_ > 0.8 && tableSize_ < maxTableSize)
@@ -316,7 +316,7 @@ bool Foam::HashTable<T, Key, Hash>::set
     {
         // Found - overwrite existing entry
         // this corresponds to the Perl convention
-        hashedEntry* ep = new hashedEntry(key, existing->next_, newEntry);
+        hashedEntry* ep = new hashedEntry(key, newEntry, existing->next_);
 
         // Replace existing element - within list or insert at the head
         if (prev)
@@ -342,11 +342,11 @@ bool Foam::HashTable<T, Key, Hash>::iteratorBase::erase()
     if (entryPtr_)
     {
         // Search element before entryPtr_
-        hashedEntry* prev = 0;
+        entry_type* prev = nullptr;
 
         for
         (
-            hashedEntry* ep = hashTable_->table_[hashIndex_];
+            entry_type* ep = hashTable_->table_[hashIndex_];
             ep;
             ep = ep->next_
         )
@@ -371,8 +371,7 @@ bool Foam::HashTable<T, Key, Hash>::iteratorBase::erase()
             hashTable_->table_[hashIndex_] = entryPtr_->next_;
             delete entryPtr_;
 
-            // Assign any non-nullptr value so it doesn't look
-            // like end()/cend()
+            // 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.
@@ -547,7 +546,7 @@ void Foam::HashTable<T, Key, Hash>::clear()
                     ep = next;
                 }
                 delete ep;
-                table_[hashIdx] = 0;
+                table_[hashIdx] = nullptr;
             }
         }
         nElmts_ = 0;
@@ -625,7 +624,7 @@ void Foam::HashTable<T, Key, Hash>::operator=
 
     for (const_iterator iter = rhs.cbegin(); iter != rhs.cend(); ++iter)
     {
-        insert(iter.key(), *iter);
+        insert(iter.key(), iter.object());
     }
 }
 
@@ -667,9 +666,9 @@ bool Foam::HashTable<T, Key, Hash>::operator==
 
     for (const_iterator iter = rhs.cbegin(); iter != rhs.cend(); ++iter)
     {
-        const_iterator fnd = find(iter.key());
+        const_iterator other = find(iter.key());
 
-        if (fnd == cend() || fnd() != iter())
+        if (!other.found() || other.object() != iter.object())
         {
             return false;
         }