Inqlude provides a comprehensive listing of all existing libraries for developers of applications using the Qt toolkit. Inqlude is run by the community and open for contributions.
Open Collaboration Services API
Attica is a Qt library that implements the Open Collaboration Services API version 1.6. The REST API is defined here: https://www.freedesktop.org/wiki/Specifications/open-collaboration-services/
It grants easy access to the services such as querying information about persons and contents. The library is used in KNewStuff3 as content provider. In order to integrate with KDE’s Plasma Desktop, a platform plugin exists in kdebase.
Your basic entrypoint for using Attica’s functionality is the Attica::ProviderManager class.