Make the special constructor private.

Author: hchunhui

include/json/value.h

@@ -233,9 +233,12 @@ class JSON_API Value {
 #endif // ifndef JSON_USE_CPPTL_SMALLMAP
 #endif // ifndef JSONCPP_DOC_EXCLUDE_IMPLEMENTATION
 
-public:
+private:
   struct StaticInitTag {};
-  Value(StaticInitTag); ///< A special constructor for Value::null.
+  Value(StaticInitTag); ///< A special constructor for Value::kNull.
+  static const Value kNull;
+
+public:
   /** \brief Create a default Value of the given type.
 
     This is a very useful constructor.

src/lib_json/json_value.cpp

@@ -23,9 +23,9 @@
 
 namespace Json {
 
-static const Value kNull((Value::StaticInitTag()));
-const Value& Value::null = kNull;
-const Value& Value::nullRef = kNull;
+const Value  Value::kNull((Value::StaticInitTag()));
+const Value& Value::null = Value::kNull;
+const Value& Value::nullRef = Value::kNull;
 
 const Int Value::minInt = Int(~(UInt(-1) / 2));
 const Int Value::maxInt = Int(UInt(-1) / 2);