Greenleaf used to have a fantastic C, then later C++ based suite of libraries. Many of them were cross platform. Greenleaf Database worked with many different XBase file formats. DataWindows came out long before Microsoft Windows, but the evil empire managed to get a Windows trademark and sent weekly threatening letters to the small company in Texas. (I think they were in Texas.)
From here: https://www.drdobbs.com/new-products/184402584
Greenleaf Software, Inc. began shipping Greenleaf DataWindows v3.0, bundled with the screen designer, Greenleaf Make-Form. Single-user licenses for Greenleaf DataWindows for DOS, UNIX, or 386, are available for $549, and include source code and MakeForm. DataWindows v3.0 adds translucent shadows, device independence for many video modes (including VESA support) and mouse use in data entry and menus. The 386 version adds support for Rational Systems' 386 DOS/4G and the DOS version adds support for Phar Lap's 286 DOS extender. Dialog boxes now have controls including push buttons, check boxes, and radio buttons. New data types, mouse support and more flexible entry and exit points support data entry design. MakeForm allows programmers to "paint" a form and define its attributes in a WYSIWYG fashion.
I used to own this. Threw it out a few years ago during an office cleaning binge. Kind of wish I had it now.