What is it:
JUCE (Jules' Utility Class Extensions) is an all-encompassing C++ class library for developing cross-platform software.
It contains pretty much everything you're likely to need to create most applications, and is particularly well-suited for building highly-customised GUIs, and for handling graphics and sound.
JUCE can target the following platforms:
Mac OSX - Applications and VST/AudioUnit/RTAS/NPAPI plugins can be compiled with Xcode for OSX 10.4 or later.
iOS - Native iPhone and iPad apps can be built with Xcode.
Windows - Applications and VST/RTAS/NPAPI/ActiveX plugins can be built using MS Visual Studio. The results are all fully compatible with Windows XP, Vista or Win7.
Linux - Applications and plugins can be built for any kernel 2.6 or later.
Android - NEW! Android apps can now be built using Ant and Eclipse, with the Android NDK v5 or later. (This is still a work in progress, so some features aren't still to be finished).
For all the platforms above, the code that you write is the same, and you don't need to worry about any platform-specific details. If your C++ is portable, then you should be able to simply re-compile your app to run it on other OSes.
Design and coding style:
In designing JUCE, I've tried to make it:
Literate - class, method and variable names are clear, self-explanatory and consistent. It's vital in such a large library that when you look at a class, it should be obvious how it works; and when you use a class in your own code, the result should be as readable as possible. The best way to achieve this is by using well-written names.
Coherent - a library needs to be predictable enough that once you've got a feel for the way things are arranged, you can navigate around it without any surprises. If you need to do a particular task, you should be able to guess what the classes to do it might be called and find them quickly. (Having a single person write the entire library helps with this!)
Cross-platform - platform-dependent code is all confined to a single area and kept out away from public view. When you include juce.h, you only include pure C++ classes, it won't pull in any platform-dependent headers. Wherever it's possible to use a pure C++ technique instead of native functionality, I've done so.
High Quality C++ - having been a professional C++ coder for 15 years, and having studied all the available C++ guru literature, I think I'm finally starting to get the hang of it! Every class in the library is intended to be a good example of the best possible coding practices - so if you spot anything dodgy in there, don't hesitate to let me know!
Integrating into a project:
Adding JUCE to your app is very simple - the easiest way involves simply including juce.h in your files and adding a single cpp file to your project. No need to compile any libraries or worry about dependencies. Ideally I'd like to have made JUCE an include-only library like the std c++ library.. that's not actually possible because of the platform-specific nastiness that it has to deal with, but to be able to add a complete multi-platform library to your app in only two steps is a pretty good result!
Of course JUCE can also be built as a static library and linked into your application in the traditional way. Or you can use it in its 'amalgamated' form, where the entire library (all 350,000 lines of it!) has been cunningly compressed into just two (large!) source files. Having only two files to deal with means that you can easily add a local copy of them to a project and check them into your source control system, avoiding any external dependencies.
To further simplify the process of building across multiple platforms, the Introjucer will automatically generate all the compiler-specific project files you need to get the same app running in Xcode, Visual Studio, etc. Just use the Introjucer's IDE to build your project, and it'll take care of the hassle involved in keeping several different IDE projects in sync with each other.
JUCE is released under the GNU Public Licence, which means it can be freely copied and distributed, and costs nothing to use in open-source applications.
If you'd like to release a closed-source application that uses JUCE, commercial licences are available for a fee - click here for more information on pricing and terms.