This linker error happens, if the linker can't find a used symbol. Most of the time, this happens if a used library is not linked against.
qmake:
LIBS += nameOfLib
cmake:
TARGET_LINK_LIBRARIES(target nameOfLib)
g++ call:
g++ -o main main.cpp -Llibrary/dir -lnameOfLib
One might also forget to compile and link all used .cpp
files (functionsModule.cpp defines the needed function):
g++ -o binName main.o functionsModule.o