WxWidgets
wxWidgets is a widget toolkit and tools library for creating graphical user interfaces (GUIs) for cross-platform applications. wxWidgets enables a program's GUI code to compile and run on several computer platforms with no significant code changes. A wide choice of compilers and other tools to use with wxWidgets facilitates development of sophisticated applications. wxWidgets supports a comprehensive range of popular operating systems and graphical libraries, both proprietary and free.
Source: Wikipedia
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