Agar

LibAgar is a cross-platform GUI system. It provides a base framework and a standard toolkit of widgets from which high-performance, portable graphical applications can be built. The toolkit is extensible through class registration. A powerful style engine allows precise control over the appearance of elements. Agar applications work under X Windows, MS Windows, MacOS, SDL (1.2 and 2.0) and others.

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“Agar is a powerful open-source, cross-platform toolkit for graphical applications in C / C++”