Fathom is written in Visual Studio C++ and is built on the JUCE audio framework.
JUCE is a class library which covers three main areas of software development, Audio Processing, Software Infrastructure and Graphics.
JUCE is hands down the best class library I’ve used in my career, and I can say this based on five years of experience with .NET and over fifteen year’s experience with MFC.
My personal preference for JUCE over other cross platform solutions is that you can develop with a single code base and cross compile natively for all the major target operating systems currently in use. This include at the very least, Windows, Mac OSX, Linux, Android and iPhone. Because the code is compiled natively for the target OS, your end users do not have to contend with an intermediate execution layer.
This library is run-time efficient and based on extremely high quality code. In the three years I’ve been using JUCE I have yet to find a single bug in their code base.
JUCE is also a great option for developers who prefer a ramp up time that can be measured in hours, rather than weeks, months or even years, as is the case with other libraries.
JUCE has become my first choice for a class library even for applications not related to audio.
You can read more about JUCE here.