add move assignment operator for Json::Value class.

include/json/value.h

@@ -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.

src/lib_json/json_value.cpp

@@ -508,10 +508,18 @@ Value::~Value() {
   value_.uint_ = 0;
 }
 
-Value& Value::operator=(Value other) {
+Value& Value::operator=(const Value& other) {
+  swap(const_cast<Value&>(other));
+  return *this;
+}
+
+#if JSON_HAS_RVALUE_REFERENCES
+Value& Value::operator=(Value&& other) {
+  initBasic(nullValue);
   swap(other);
   return *this;
 }
+#endif
 
 void Value::swapPayload(Value& other) {
   ValueType temp = type_;
@@ -523,6 +531,12 @@ void Value::swapPayload(Value& other) {
   other.allocated_ = temp2 & 0x1;
 }
 
+void Value::copyPayload(const Value& other) {
+  type_ = other.type_;
+  value_ = other.value_;
+  allocated_ = other.allocated_;
+}
+
 void Value::swap(Value& other) {
   swapPayload(other);
   std::swap(comments_, other.comments_);
@@ -530,6 +544,13 @@ void Value::swap(Value& other) {
   std::swap(limit_, other.limit_);
 }
 
+void Value::copy(const Value& other) {
+  copyPayload(other);
+  comments_ = other.comments_;
+  start_ = other.start_;
+  limit_ = other.limit_;
+}
+
 ValueType Value::type() const { return type_; }
 
 int Value::compare(const Value& other) const {