Reduce string copying and reallocating in Writer::write

Author: mudzot

include/json/writer.h

@@ -29,7 +29,7 @@ class JSON_API Writer {
 public:
   virtual ~Writer();
 
-  virtual std::string write(const Value& root) = 0;
+  virtual const std::string& write(const Value& root) = 0;
 };
 
 /** \brief Outputs a Value in <a HREF="http://www.json.org">JSON</a> format
@@ -57,7 +57,7 @@ class JSON_API FastWriter : public Writer {
   void omitEndingLineFeed();
 
 public: // overridden from Writer
-  virtual std::string write(const Value& root);
+  virtual const std::string& write(const Value& root);
 
 private:
   void writeValue(const Value& value);
@@ -101,7 +101,7 @@ class JSON_API StyledWriter : public Writer {
    * \param root Value to serialize.
    * \return String containing the JSON document that represents the root value.
    */
-  virtual std::string write(const Value& root);
+  virtual const std::string& write(const Value& root);
 
 private:
   void writeValue(const Value& value);

src/lib_json/json_writer.cpp

@@ -187,8 +187,8 @@ void FastWriter::dropNullPlaceholders() { dropNullPlaceholders_ = true; }
 
 void FastWriter::omitEndingLineFeed() { omitEndingLineFeed_ = true; }
 
-std::string FastWriter::write(const Value& root) {
-  document_ = "";
+const std::string& FastWriter::write(const Value& root) {
+  document_.clear();
   writeValue(root);
   if (!omitEndingLineFeed_)
     document_ += "\n";
@@ -249,8 +249,8 @@ void FastWriter::writeValue(const Value& value) {
 StyledWriter::StyledWriter()
     : rightMargin_(74), indentSize_(3), addChildValues_() {}
 
-std::string StyledWriter::write(const Value& root) {
-  document_ = "";
+const std::string& StyledWriter::write(const Value& root) {
+  document_.clear();
   addChildValues_ = false;
   indentString_ = "";
   writeCommentBeforeValue(root);