ENH: add swallow assignment to nullObject

- similar to the behaviour of std::ignore and consistent with the
  no input / no output nature of nullObject. Similarly accept a
  const reference for its Istream operator.

- make most nullObject methods constexpr

applications/test/nullObject/Test-nullObject.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2014 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -47,9 +47,8 @@ class SimpleClass
 {
 public:
 
-    //- Null constructor
-    SimpleClass()
-    {}
+    //- Default construct
+    SimpleClass() {}
 };
 
 
@@ -149,6 +148,12 @@ int main()
         printInfo(pointField::null());
     }
 
+    // Test swallow assigment (like std::ignore)
+    // Looks pretty ugly though!
+
+    NullObject::nullObject = "hello world";
+    NullObject::nullObject = labelList({1, 2, 3});
+
     Info<< nl;
 
     return 0;

src/OpenFOAM/primitives/nullObject/nullObject.H

@@ -105,31 +105,41 @@ public:
         }
 
         //- Zero valued integer content
-        inline unsigned long value() const
+        inline constexpr unsigned long value() const
         {
-            return data_[0].val;
+            return 0;
         }
 
         //- No elements
-        inline bool empty() const
+        inline constexpr bool empty() const
         {
             return true;
         }
 
         //- Zero elements
-        inline label size() const
+        inline constexpr label size() const
         {
             return 0;
         }
 
         //- No-op method (for HashTable replacement)
-        inline const NullObject& toc() const
+        inline constexpr const NullObject& toc() const
         {
             return *this;
         }
 
         //- No-op method (for HashTable replacement)
-        inline const NullObject& sortedToc() const
+        inline constexpr const NullObject& sortedToc() const
+        {
+            return *this;
+        }
+
+
+    // Member Operators
+
+        //- Swallow assignment (cf, std::ignore)
+        template<class T>
+        inline constexpr const NullObject& operator=(const T&) const
         {
             return *this;
         }
@@ -146,13 +156,13 @@ extern const NullObject* nullObjectPtr;
 
 // IOstream Operators
 
-//- Read from Istream consumes no content.
-inline Istream& operator>>(Istream& is, NullObject&) noexcept
+//- Read from Istream consumes no content
+inline Istream& operator>>(Istream& is, const NullObject&) noexcept
 {
     return is;
 }
 
-//- Write to Ostream emits no content.
+//- Write to Ostream emits no content
 inline Ostream& operator<<(Ostream& os, const NullObject&) noexcept
 {
     return os;