1. u can ctrl+left click XXX in codelite, normally it will show u the definition file of xxx, then u may find out which file the implementation is in, for example the same or similiar name as the header file.
2. search content xxx in the errored file. Sometimes the implementation file is in the same place as the errored file.
3. google "undefined reference to XXX" or just xxx, u may find some useful infomation there.