ENH: ensure nullObject is large enough for reinterpret

- previously occupied 1 byte. Now occupies enough to hold a pointer,
  which helps if using reinterpret_cast to something that has some
  member content.

applications/test/nullObject/Make/files

@@ -1,3 +1,3 @@
 Test-nullObject.C
 
-EXE = $(FOAM_USER_APPBIN)/nullObject
+EXE = $(FOAM_USER_APPBIN)/Test-nullObject

applications/test/nullObject/Test-nullObject.C

@@ -21,6 +21,17 @@ int main()
     SimpleClass* ptrToClass = new SimpleClass;
     SimpleClass& refToClass(*ptrToClass);
 
+    typedef unsigned long ptrval;
+
+    Info<<"nullObject address=" << ptrval(&(nullObjectPtr)) << endl;
+    Info<<"sizeof(nullObject)" << " == "
+        << sizeof(NullObject::nullObject)
+        << " vs. sizeof(void*)" << " == " << sizeof(void*)
+        << endl;
+
+    Info<<"nullObject pointer:" << ptrval(nullObjectPtr->pointer()) << endl;
+    Info<<"nullObject value:"   << nullObjectPtr->value() << endl;
+
     if (notNull(ptrToClass))
     {
         Info<< "Pass: ptrToClass is not null" << endl;

src/OpenFOAM/primitives/nullObject/nullObject.H

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2014-2016 OpenFOAM Foundation
-     \\/     M anipulation  |
+     \\/     M anipulation  | Copyright (C) 2017 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -25,7 +25,9 @@ Class
     Foam::nullObject
 
 Description
-    Singleton null-object class and instance
+    Singleton null-object class and instance.
+    It occupies enough space to reinterpret its content as a class with
+    a null pointer for its content.
 
 SourceFiles
     nullObjectI.H
@@ -47,20 +49,41 @@ namespace Foam
 
 class NullObject
 {
+    //- Ensure it occupies enough space to reinterpret_cast to a class
+    //  having some member data
+    const union
+    {
+        void* ptr;
+        unsigned long val;
+    } null;
+
     //- Private constructor
     NullObject()
+    :
+        null{nullptr}
     {}
 
     //- Disallow default bitwise copy construct
-    NullObject(const NullObject&);
+    NullObject(const NullObject&) = delete;
 
     //- Disallow default bitwise assignment
-    void operator=(const NullObject&);
+    void operator=(const NullObject&) = delete;
 
 public:
-
     //- The unique null object
     static const NullObject nullObject;
+
+    //- A nullptr pointer content
+    inline const void* pointer() const
+    {
+        return null.ptr;
+    }
+
+    //- A zero value content
+    inline unsigned long value() const
+    {
+        return null.val;
+    }
 };