ENH: add HashPtrTable release method

- effectively 'steals' the pointer from the table but leaves its
  name as a placeholder

applications/test/HashPtrTable/Test-HashPtrTable.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011 OpenFOAM Foundation
-    Copyright (C) 2017-2020 OpenCFD Ltd.
+    Copyright (C) 2017-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -125,6 +125,7 @@ int main()
     myTable.set("pi", new double(3.14159));
     myTable.set("natlog", new double(2.718282));
     myTable.insert("sqrt2", autoPtr<double>::New(1.414214));
+    myTable.insert("euler", autoPtr<double>::New(0.577216));
 
     HashTable<std::unique_ptr<double>, word, string::hash> myTable1;
 
@@ -139,14 +140,24 @@ int main()
     myTable2.set("natlog", autoPtr<double>(new double(2.718282)));
     myTable2.insert("sqrt2", autoPtr<double>::New(1.414214));
 
-    // Info<< myTable << endl;
+    Info<< "Initial table" << nl;
     printTable(myTable);
+
+    Info<< "print" << nl;
     Info<< myTable2 << nl;
 
+    {
         auto iter2 = myTable2.find("pi");
-
+        Info<< nl "Got pi=";
-    Info<<"PI: " << **iter2 << nl;
+        if (iter2.good())
-
+        {
+            Info<< **iter2 << nl;
+        }
+        else
+        {
+            Info<< "not-found" << nl;
+        }
+    }
 
     HashPtrTable<double> copy(myTable);
 
@@ -165,6 +176,9 @@ int main()
     myTable.erase("abc");
     myTable.erase("unknownKey");
 
+    myTable.release("euler");
+
+    Info<< "After erasure" << nl;
     printTable(myTable);
 
     HashPtrTable<double> moved(std::move(copy));

src/OpenFOAM/containers/HashTables/HashPtrTable/HashPtrTable.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2020 OpenCFD Ltd.
+    Copyright (C) 2017-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -26,8 +26,9 @@ License
 
 \*---------------------------------------------------------------------------*/
 
-#include "error.H"
 #include "HashPtrTable.H"
+#include "autoPtr.H"
+#include "error.H"
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
@@ -77,6 +78,28 @@ Foam::HashPtrTable<T, Key, Hash>::~HashPtrTable()
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+template<class T, class Key, class Hash>
+Foam::autoPtr<T> Foam::HashPtrTable<T, Key, Hash>::release(iterator& iter)
+{
+    if (iter.good())
+    {
+        autoPtr<T> old(iter.val());
+        iter.val() = nullptr;
+        return old;
+    }
+
+    return nullptr;
+}
+
+
+template<class T, class Key, class Hash>
+Foam::autoPtr<T> Foam::HashPtrTable<T, Key, Hash>::release(const Key& key)
+{
+    iterator iter(this->find(key));
+    return this->release(iter);
+}
+
+
 template<class T, class Key, class Hash>
 Foam::autoPtr<T> Foam::HashPtrTable<T, Key, Hash>::remove(iterator& iter)
 {

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

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2020 OpenCFD Ltd.
+    Copyright (C) 2017-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -93,7 +93,7 @@ public:
     // Constructors
 
         //- Default construct with default table capacity
-        inline HashPtrTable();
+        HashPtrTable() = default;
 
         //- Construct given initial table capacity
         inline explicit HashPtrTable(const label size);
@@ -130,15 +130,26 @@ public:
 
     // Edit
 
+        //- Release ownership of the pointer and replace with a nullptr
+        //  Includes a safeguard against the end-iterator.
+        //
+        //  \return the entry's old value as an encapsulated pointer.
+        autoPtr<T> release(iterator& iter);
+
+        //- Release ownership of the pointer and replace with a nullptr
+        //
+        //  \return the entry's old value as an encapsulated pointer.
+        autoPtr<T> release(const Key& key);
+
         //- Remove entry specified by given iterator.
         //  Includes a safeguard against the end-iterator.
         //
-        //  \return entry as an encapsulated pointer.
+        //  \return the entry's old value as an encapsulated pointer.
         autoPtr<T> remove(iterator& iter);
 
         //- Remove entry specified by given key.
         //
-        //  \return entry as an encapsulated pointer.
+        //  \return the entry's old value as an encapsulated pointer.
         autoPtr<T> remove(const Key& key);
 
         //- Erase entry specified by given iterator and delete the

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

@@ -29,13 +29,6 @@ License
 
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
-template<class T, class Key, class Hash>
-inline Foam::HashPtrTable<T, Key, Hash>::HashPtrTable()
-:
-    parent_type()
-{}
-
-
 template<class T, class Key, class Hash>
 inline Foam::HashPtrTable<T, Key, Hash>::HashPtrTable(const label size)
 :