Opportunistically take advantage of C++20 move-in/out-of stringstream (#1457)

* Opportunistically take advantage of C++20 move-out-of stringstream

* Opportunistically take advantage of C++20 move-in/out-of stringstream

---------

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

Author: beevvy

src/lib_json/json_reader.cpp

@@ -587,8 +587,7 @@ bool Reader::decodeDouble(Token& token) {
 
 bool Reader::decodeDouble(Token& token, Value& decoded) {
   double value = 0;
-  String buffer(token.start_, token.end_);
-  IStringStream is(buffer);
+  IStringStream is(String(token.start_, token.end_));
   if (!(is >> value)) {
     if (value == std::numeric_limits<double>::max())
       value = std::numeric_limits<double>::infinity();
@@ -1622,8 +1621,7 @@ bool OurReader::decodeDouble(Token& token) {
 
 bool OurReader::decodeDouble(Token& token, Value& decoded) {
   double value = 0;
-  const String buffer(token.start_, token.end_);
-  IStringStream is(buffer);
+  IStringStream is(String(token.start_, token.end_));
   if (!(is >> value)) {
     if (value == std::numeric_limits<double>::max())
       value = std::numeric_limits<double>::infinity();
@@ -1981,7 +1979,7 @@ bool parseFromStream(CharReader::Factory const& fact, IStream& sin, Value* root,
                      String* errs) {
   OStringStream ssin;
   ssin << sin.rdbuf();
-  String doc = ssin.str();
+  String doc = std::move(ssin).str();
   char const* begin = doc.data();
   char const* end = begin + doc.size();
   // Note that we do not actually need a null-terminator.

src/lib_json/json_writer.cpp

@@ -1251,7 +1251,7 @@ String writeString(StreamWriter::Factory const& factory, Value const& root) {
   OStringStream sout;
   StreamWriterPtr const writer(factory.newStreamWriter());
   writer->write(root, &sout);
-  return sout.str();
+  return std::move(sout).str();
 }
 
 OStream& operator<<(OStream& sout, Value const& root) {