Add an etl::nullptr_t type to <etl/nullptr.h> (#924)

* Add an etl::nullptr_t type

* etlcpp/etl issue #921 (etl::unique_ptr reset): add etl::unique_ptr(...)::reset(ETL_NULLPTR)

Remove default argument for the normal reset method of etl::unique_ptr (sorry, didn't notice 😬)

Silence the unused argument warning

Fix operator =(nullptr)

Replace the nullptr_t enum with a class which acts more similar to C++11 nullptr

* Add member pointer support and delete the addressof operator

* "Delete" etl::addressof(ETL_NULLPTR)

* Ensure compatibility with C++98

* ACTUALLY ensure compatibility with C++98

I'm stupid :/

* Correct definition according to cppreference

include/etl/memory.h

@@ -1395,8 +1395,8 @@ namespace etl
     //*********************************
     void reset(pointer p_ = pointer()) ETL_NOEXCEPT
     {
-      assert(p_ != p);
-
+      assert(p_ == ETL_NULLPTR || p_ != p);
+      
       pointer value = p;
       p = p_;
 
@@ -1420,29 +1420,16 @@ namespace etl
       return (p != ETL_NULLPTR);
     }
 
-#if ETL_USING_STL && ETL_USING_CPP11
     //*********************************
-    unique_ptr&	operator =(std::nullptr_t) ETL_NOEXCEPT
+    unique_ptr&	operator =(etl::nullptr_t) ETL_NOEXCEPT
     {
       if (p)
       {
-        reset(nullptr);
+        reset(ETL_NULLPTR);
       }
 
       return *this;
     }
-#else
-    //*********************************
-    unique_ptr&	operator =(void*) ETL_NOEXCEPT
-    {
-      if (p)
-      {
-        reset(NULL);
-      }
-
-      return *this;
-    }
-#endif
 
 #if ETL_USING_CPP11
     //*********************************
@@ -1622,6 +1609,11 @@ namespace etl
       }
     }
 
+    void reset(etl::nullptr_t = ETL_NULLPTR) ETL_NOEXCEPT
+    {
+      reset(pointer());
+    }
+
     //*********************************
     void swap(unique_ptr& v) ETL_NOEXCEPT
     {
@@ -1636,23 +1628,13 @@ namespace etl
       return (p != ETL_NULLPTR);
     }
 
-#if ETL_USING_STL && ETL_USING_CPP11
-    //*********************************
-    unique_ptr& operator =(std::nullptr_t) ETL_NOEXCEPT
-    {
-      reset(nullptr);
-
-      return *this;
-    }
-#else
     //*********************************
-    unique_ptr& operator =(void*) ETL_NOEXCEPT
+    unique_ptr& operator =(etl::nullptr_t) ETL_NOEXCEPT
     {
-      reset(NULL);
+      reset(ETL_NULLPTR);
 
       return *this;
     }
-#endif
 
 #if ETL_USING_CPP11
     //*********************************

include/etl/nullptr.h

@@ -35,13 +35,33 @@ SOFTWARE.
 
 #include <stddef.h>
 
+namespace etl
+{
 #if ETL_CPP11_NOT_SUPPORTED
-  // Use the old style C++ NULL definition.
-  #define ETL_NULLPTR 0
+  class nullptr_t
+  {
+  public:
+    template <class T>
+    inline operator T*() const { return 0; }
+
+    template <class C, class T>
+    inline operator T C::* () const { return 0; }
+
+    inline bool operator==(nullptr_t) const { return true; }
+    inline bool operator!=(nullptr_t) const { return false; }
+  private:
+    void operator&() const ETL_DELETE; // cannot take the address of ETL_NULLPTR
+  };
+
+  static const nullptr_t _nullptr = nullptr_t();
+
+  #define ETL_NULLPTR (etl::_nullptr)
 #else
   // Use the new style nullptr.
+  typedef decltype(nullptr) nullptr_t;
   #define ETL_NULLPTR nullptr
 #endif
+}
 
 #endif
 

include/etl/private/addressof.h

@@ -32,6 +32,7 @@ SOFTWARE.
 #define ETL_ADDRESSOF_INCLUDED
 
 #include "../platform.h"
+#include "../type_traits.h"
 
 #if defined(ETL_IN_UNIT_TEST) || ETL_USING_STL
   #include <memory>
@@ -48,7 +49,7 @@ namespace etl
   ///\ingroup memory
   //*****************************************************************************
   template <typename T>
-  ETL_CONSTEXPR17 T* addressof(T& t)
+  ETL_CONSTEXPR17 typename etl::enable_if<!etl::is_same<T, etl::nullptr_t>::value, T>::type* addressof(T& t)
   {
 #if ETL_USING_STL && ETL_USING_CPP11
     return std::addressof(t);