add move assignment operator for Json::Value class.

include/json/value.h

@@ -23,7 +23,7 @@
 #endif
 
 //Conditional NORETURN attribute on the throw functions would:
-// a) suppress false positives from static code analysis 
+// a) suppress false positives from static code analysis
 // b) possibly improve optimization opportunities.
 #if !defined(JSONCPP_NORETURN)
 #  if defined(_MSC_VER)
@@ -64,7 +64,7 @@ protected:
 /** Exceptions which the user cannot easily avoid.
  *
  * E.g. out-of-memory (when we use malloc), stack-overflow, malicious input
- * 
+ *
  * \remark derived from Json::Exception
  */
 class JSON_API RuntimeError : public Exception {
@@ -75,7 +75,7 @@ public:
 /** Exceptions thrown by JSON_ASSERT/JSON_FAIL macros.
  *
  * These are precondition-violations (user bugs) and internal errors (our bugs).
- * 
+ *
  * \remark derived from Json::Exception
  */
 class JSON_API LogicError : public Exception {
@@ -322,12 +322,21 @@ Json::Value obj_value(Json::objectValue); // {}
 
   /// Deep copy, then swap(other).
   /// \note Over-write existing comments. To preserve comments, use #swapPayload().
-  Value& operator=(Value other);
+  Value& operator=(const Value& other);
+#if JSON_HAS_RVALUE_REFERENCES
+  Value& operator=(Value&& other);
+#endif
+
   /// Swap everything.
   void swap(Value& other);
   /// Swap values but leave comments and source offsets in place.
   void swapPayload(Value& other);
 
+  /// copy everything.
+  void copy(const Value& other);
+  /// copy values but leave comments and source offsets in place.
+  void copyPayload(const Value& other);
+
   ValueType type() const;
 
   /// Compare payload only, not comments etc.