Reference not found for xxx

 In Gtest

If main is not found, you might missed something in cmake, such as not didn’t add executable.

In polymorphic classes

not define/override some virtual function

Multiple inherentence (in the case of Operator and Project in BuzzDB)

Actually the problem caused when I change the base class signature and not recompile from scratch. Solved it by make clean, cmake .., and make.