• William Woodall presents https://github.com/wjwwood/serial
• Manash Kumar Mandal presents https://github.com/manashmandal/SerialPort
• For Qt https://github.com/qextserialport/qextserialport
• From Qt (since 5.1) https://doc.qt.io/qt-5/qserialport.html
• From Boost https://www.boost.org/doc/libs/1_77_0/doc/html/boost_asio/overview/serial_ports.html (examples https://github.com/fedetft/serial-port and article https://gist.github.com/kaliatech/427d57cb1a8e9a8815894413be337cf9)
• Teunis van Beelen presents https://www.teuniz.net/RS-232/
• PJ Naughter presents http://www.naughter.com/serialport.html
• (not C++, but C) sigrok https://sigrok.org/wiki/Libserialport (example https://hackaday.com/2018/09/14/easy-portable-serial-ports/)

Win32 API

• Ramon de Klein presents https://www.codeproject.com/Articles/992/Serial-library-for-C

Linux

• libserial https://github.com/crayzeewulf/libserial (https://libserial.readthedocs.io/en/latest/tutorial.html)
• Poco::Serial from machine.io

Arduino

• https://arduino.stackexchange.com/questions/34126/whats-the-difference-between-all-the-software-serial-libraries-which-one-is-ar/34130#34130