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

https://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix

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.