ENH: add get() retrieval of a pointer from PtrLists, HashPtrTable

- naming similarity with autoPtr, unique_ptr and other containers.

  For UPtrList derivatives, this is equivalent to the existing
  operator(). The read-only variant is also equivalent to the
  single-parameter 'set(label)' method.

  With PtrList<T> list(...) :

      const T* ptr = list.get(10);
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.set(10))
      {
          list[10].method();
      }

  For HashPtrTable there is only a read-only variant which is equivalent
  to testing for existence and for value.

  With HashPtrTable<T> hash(...) :

      const T* ptr = list.get("key");
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.found("key"))
      {
          // Fails on null pointer!!
          list["key"].method();
      }

Use of get() is largely a matter of taste or local coding requirements

applications/test/HashPtrTable/Test-HashPtrTable.C

@@ -198,6 +198,24 @@ int main()
 
         Info<< "Table: " << tbl << nl;
 
+        Info<< nl << "Check exists, non-null" << nl;
+
+        for (const word& k : { "abc", "foo", "pi" })
+        {
+            Info<< "    " << k << ' ';
+
+            const auto* inspect = tbl.get(k);
+
+            if (inspect)
+            {
+                Info<< *inspect << nl;
+            }
+            else
+            {
+                Info<< "(null)" << nl;
+            }
+        }
+
         Info<< nl << "... overwrite again" << nl;
 
         tbl.set("abc", new Scalar(42.1));

src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.H

@@ -120,6 +120,13 @@ public:
 
     // Member Functions
 
+    // Access
+
+        //- Return const pointer associated with given entry,
+        //- returning a nullptr if the key does not exist in the table.
+        inline const T* get(const Key& key) const;
+
+
     // Edit
 
         //- Remove entry specified by given iterator.

src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTableI.H

@@ -45,6 +45,18 @@ inline Foam::HashPtrTable<T, Key, Hash>::HashPtrTable(const label size)
 
 // * * * * * * * * * * * * * * * Member Functions * * * * * * * * * * * * * //
 
+template<class T, class Key, class Hash>
+inline const T* Foam::HashPtrTable<T, Key, Hash>::get(const Key& key) const
+{
+    const const_iterator iter(this->cfind(key));
+    if (iter.good())
+    {
+        return iter.val();
+    }
+    return nullptr;
+}
+
+
 template<class T, class Key, class Hash>
 template<class... Args>
 inline bool Foam::HashPtrTable<T, Key, Hash>::emplace

src/OpenFOAM/containers/PtrLists/PtrDynList/PtrDynList.H

@@ -108,10 +108,14 @@ public:
         //- Size of the underlying storage.
         inline label capacity() const noexcept;
 
+        //- Return const pointer to element (can be nullptr),
+        //  with bounds checking.
+        inline const T* get(const label i) const;
+
         //- Return const pointer to element (if set) or nullptr,
         //  with bounds checking.
         //  The return value can be tested as a bool.
-        inline const T* set(const label i) const;
+        const T* set(const label i) const { return this->get(i); }
 
 
     // Edit

src/OpenFOAM/containers/PtrLists/PtrDynList/PtrDynListI.H

@@ -100,7 +100,7 @@ inline Foam::label Foam::PtrDynList<T, SizeMin>::capacity() const noexcept
 
 
 template<class T, int SizeMin>
-inline const T* Foam::PtrDynList<T, SizeMin>::set(const label i) const
+inline const T* Foam::PtrDynList<T, SizeMin>::get(const label i) const
 {
     return (i >= 0 && i < PtrList<T>::size()) ? PtrList<T>::get(i) : nullptr;
 }

src/OpenFOAM/containers/PtrLists/PtrList/PtrList.H

@@ -134,9 +134,9 @@ public:
     // Access
 
         //- Return const pointer to element (can be nullptr),
-        //  without bounds checking.
+        //  without bounds checking - same as get().
         //  The return value can also be tested as a bool.
-        const T* set(const label i) const { return UPtrList<T>::set(i); }
+        const T* set(const label i) const { return this->get(i); }
 
 
     // Edit

src/OpenFOAM/containers/PtrLists/UPtrList/UPtrList.H

@@ -157,10 +157,18 @@ public:
         //- Return reference to the last element of the list
         inline const T& last() const;
 
+        //- Return pointer to element (can be nullptr),
+        //  without bounds checking.
+        inline T* get(const label i);
+
         //- Return const pointer to element (can be nullptr),
         //  without bounds checking.
+        inline const T* get(const label i) const;
+
+        //- Return const pointer to element (can be nullptr),
+        //  without bounds checking - same as get().
         //  The return value can also be tested as a bool.
-        const T* set(const label i) const { return ptrs_[i]; }
+        const T* set(const label i) const { return this->get(i); }
 
 
     // Edit
@@ -212,7 +220,7 @@ public:
         //- Return reference to the element
         inline T& operator[](const label i);
 
-        //- Return const pointer to the element.
+        //- Return const pointer to the element - same as get().
         inline const T* operator()(const label i) const;
 
         //- Copy assignment (shallow copies addresses)

src/OpenFOAM/containers/PtrLists/UPtrList/UPtrListI.H

@@ -107,6 +107,20 @@ inline bool Foam::UPtrList<T>::empty() const noexcept
 }
 
 
+template<class T>
+inline T* Foam::UPtrList<T>::get(const label i)
+{
+    return ptrs_[i];
+}
+
+
+template<class T>
+inline const T* Foam::UPtrList<T>::get(const label i) const
+{
+    return ptrs_[i];
+}
+
+
 template<class T>
 inline void Foam::UPtrList<T>::clear()
 {