LNK2019: unresolved external symbol in VS unit-testing

Question

I get the error as stated in the title. I ensured the following:
- The Include directory, include library and additional include directory are set correctly
- In the pro

Answer 1

I found the answer by myself: I have to include the .cpp file.
So #include "../LifeLib/StornoTafel.cpp" fixes the error. However, I have no idea why.