Some mentioned Qt, some wxWidgets.
A quick summary of gui cross-platform kits that you could try:
- Qt : It looks appropriate on the platform on which you use it. However, he has his own build system, which does not always like to play well with others. If you want newbies to have experience creating a GUI with Qt, I recommend Qt Creator . This is a standalone IDE created by Qt developers.
- wxWidgets : While Qt looks native (it tries to best mimic the appearance of the OS you are running on). wxWidgets is native. That is, it uses the GUI elements provided by the operating system. Recently, I looked at wxWidgets, as the C ++ library lagged behind modern development methods. Perhaps you will probably learn about the bad programming habits from this library.
- gtkmm : These are the same guys that do GIMP and Gnome, which, as a MacOS user, probably don't mean much to you. They don't really try to fit into wxWidgets and Qt. However, they probably have the most modern C ++ library. They did a great job of using modern development methods in C ++. This may be useful for a new programmer, as you are less likely to learn bad habits from them. On the other hand, you will fall into the land of templates and function pointers, etc.
Those are the largest with the greatest momentum behind them. There are many others that should also be considered.