XVT
XVT is a software development environment for building cross-platform GUI applications in C or C++. XVT allows developers to graphically lay out an application's GUI, and provides cross-platform libraries to aid development. XVT currently supports the Microsoft Windows, Linux, Mac OS X, Solaris, HP-UX, and AIX platforms. With XVT, applications can be designed and developed once and then subsequently compiled on any of these platforms. XVT also allows programs to be built to run across a network, meaning that the processing would take place on a local host but the user interface would take place on a remote host. XVT has been in development since 1989 and has provided a cross-platform software development toolkit for the majority of that time. XVT software is utilized in a number of mission-critical production sites and applications. Their approach is not as a widget replacement look-alike set, but acts more like a universal API translator. XVT is an "abstraction layer" that removes the developer from being concerned with which OS the application will run on. When your application is compiled, the XVT API points to the native controls/widgets for the OS on which you are compiling your code. This matters to OS user-purists, who can be very fussy when the views "aren't quite right." It also allows you to build applications that will adapt whatever user themes the OS allows users to set.
