Class templates use the Grantlee library for rendering templates. It has more features than KMacroExpander, the most important being loops and the ability to expose custom data types from C++ code to templates. The use of Grantlee makes templates more powerful, but also more difficult to write.
For information regarding Grantlee templates in general, refer to its documentation. Since Grantlee's template language is based on Django's, it might be useful to read the Django template documentation as well.
Like project templates, class templates are compressed directories, and they too contain a special description file and any number of content files. It is recommended that the description file has a .desktop extension instead of .kdevtemplate, although both are supported.
Unlike with project templates, not all files from the archive are copied to the output directory. Because Grantlee supports template inheritance and inclusions, class templates may contain helper files that only serve for convenience and produce no actual output. The actual output files have to be specified in the template description.
The variables which are passed to class templates are described in the TemplateClassGenerator API documentation.