Wdl-ce is a project mainly written in C and C++, it's free.
WDL Community Edition
This project is a community maintained version of WDL/IPlug (originally by Cockos).
WDL "is a free and open source C++ library that is designed to be easily used by other code". For more info see the Cockos WDL Home.
IPlug (part of WDL) is a cross platform framework for developing VST and Audiounit plugins. The WDL/IPlug Community Edition is an initiative started by a group of plugin developers with the goal of improving the IPlug framework. This means...
Since Cockos are updating and maintaining their version of WDL, which is used in their products, it is likely that this community version will avoid making significant changes to code that is not directly part of IPlug itself, in order to maintain compatibility with the Cockos WDL core.
Discuss WDL and IPlug on the WDL forum
WDL-CE shares the same license as the Cockos edition (see below). You must be comfortable with this if you plan to contribute. Please also see individual header files for any additional license information.
Copyright (C) 2005 and later Cockos Incorporated
Portions copyright other contributors, see each source file for more information
This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.
Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
WDL includes the following 3rd party libraries (which are all similarly licensed):