use SwapPayload() to retain comments

All tests pass, but we might be missing coverage.

issue #47

include/json/value.h

@@ -235,23 +235,26 @@ Json::Value obj_value(Json::objectValue); // {}
   Value(const CppTL::ConstString& value);
 #endif
   Value(bool value);
+  /// Deep copy.
   Value(const Value& other);
   ~Value();
 
+  // Deep copy, then swap(other).
   Value& operator=(Value other);
-  /// Swap values.
+  /// Swap everything.
   void swap(Value& other);
+  /// Swap values but leave comments and source offsets in place.
+  void swapPayload(Value& other);
 
   ValueType type() const;
 
+  /// Compare payload only, not comments etc.
   bool operator<(const Value& other) const;
   bool operator<=(const Value& other) const;
   bool operator>=(const Value& other) const;
   bool operator>(const Value& other) const;
-
   bool operator==(const Value& other) const;
   bool operator!=(const Value& other) const;
-
   int compare(const Value& other) const;
 
   const char* asCString() const;
@@ -442,9 +445,6 @@ private:
 
   Value& resolveReference(const char* key, bool isStatic);
 
-  /// Swap values but leave comments and source offsets in place.
-  void swapPayload(Value& other);
-
 #ifdef JSON_VALUE_USE_INTERNAL_MAP
   inline bool isItemAvailable() const { return itemIsUsed_ == 0; }
 

src/lib_json/json_reader.cpp

@@ -173,9 +173,12 @@ bool Reader::readValue() {
     currentValue().setOffsetLimit(token.end_ - begin_);
     break;
   case tokenNull:
-    currentValue() = Value();
+    {
+    Value v;
+    currentValue().swapPayload(v);
     currentValue().setOffsetStart(token.start_ - begin_);
     currentValue().setOffsetLimit(token.end_ - begin_);
+    }
     break;
   case tokenArraySeparator:
     if (features_.allowDroppedNullPlaceholders_) {
@@ -393,7 +396,8 @@ bool Reader::readString() {
 bool Reader::readObject(Token& tokenStart) {
   Token tokenName;
   std::string name;
-  currentValue() = Value(objectValue);
+  Value init(objectValue);
+  currentValue().swapPayload(init);
   currentValue().setOffsetStart(tokenStart.start_ - begin_);
   while (readToken(tokenName)) {
     bool initialTokenOk = true;
@@ -486,7 +490,7 @@ bool Reader::decodeNumber(Token& token) {
   Value decoded;
   if (!decodeNumber(token, decoded))
     return false;
-  currentValue() = decoded;
+  currentValue().swapPayload(decoded);
   currentValue().setOffsetStart(token.start_ - begin_);
   currentValue().setOffsetLimit(token.end_ - begin_);
   return true;

src/lib_json/json_value.cpp

@@ -406,7 +406,7 @@ Value::~Value() {
 }
 
 Value& Value::operator=(Value other) {
-  swapPayload(other);
+  swap(other);
   return *this;
 }