This is a full C++ class library that comes with the Zinc Designer (a WYSIWYG GUI builder). Their classes include some graphic capabilities, a rather nice error system, and a portable, if rudimentary, help system. Also included is the source for the library (and for the Designer!).
In addition to the usual GUI stuff, Zinc also provides international character support (an extended version, for extra bux, is in the works), some container classes, and filesystem portability. They also allow you to incorporate platform-specific stuff (e.g., system messages) into their API; but, of course, you're on your own when it comes to porting any additions.
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