WxWidget

WxWidgets is a free and open source cross-platform C++ framework for writing advanced GUI applications using native controls.

WxWidgets allows you to write native-looking GUI applications for all the major desktop platforms and also helps with abstracting the differences in the non-GUI aspects between them. It is free for the use in both open source and commercial applications, comes with the full, easy to read and modify, source and extensive documentation and a collection of more than a hundred examples. You can learn more about wxWidgets at https://www.wxwidgets.org/ and read its documentation online at https://docs.wxwidgets.org/