A specification for method libraries in XML can be found at http://methods.ringing.org/. This site is also home to a server which serves XML method data in response to HTTP queries. The classes declared in ringing/xmllib.h can not only parse the XML data, but can also automatically download method data from the server.
To use XML method libraries, you need to have the Xerces library from the Apache project, available at http://xml.apache.org/.