C++ library for various versions of CIM and CGMES:
https://github.com/cim-iec/libcimpp
Online class documentation of CIM:
https://cim.fein-aachen.org/libcimpp/doc/IEC61970_16v29a/annotated.html
Python library for CGMES (v2.4.15):
https://github.com/sogno-platform/cimpy
Java library for CGMES (v2.4.15):
https://github.com/powsybl/powsybl-core
Online documentation :
https://www.powsybl.org/pages/documentation/grid/formats/cim-cgmes.html