AES hosted our first plugin development workshop this semester, led by Jon Soifer in Design Lab 1. He covered the C++ library wdl-ol, which he uses to interface between his custom C++ algorithms and various digital audio workstations. Wdl-ol is open source and can be downloaded from its github repo. It also has fairly active forums here.
IPlug is a simple-to-use C++ framework for developing cross platform audio plugins and targeting multiple plugin APIs with the same code. Originally developed by Schwa/Cockos, IPlug has been enhanced by various contributors. IPlug depends on WDL, and that is why this project is called WDL-OL, although most of the differences from Cockos' WDL are to do with IPlug. This version of IPlug targets VST2, VST3, AudioUnit RTAS and AAX (Native) APIs. It can also produce standalone Windows/OSX audio/midi apps and apps for Apple's IOS devices.
Thanks to Jon for hosting this incredible event!