CMake: undefined reference to

I use CMake 3.5.2. When trying to build my C++ code I get the following error: [100%] Linking CXX executable SomeExecutable CMakeFiles/SomeExecutable.dir/Common/src/FunctionOne.cpp.o: In function `FunctionOne::FunctionOne(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, double, unsigned int, bool)': FunctionOne.cpp:(.text+0x490): undefined reference to `Helper::initLevel(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)' How do I fix it? CMakeLists.txt: (shortened for brevity) cmake_minimum_required(VERSION 3.0) project(ProjectName) add_definitions(-std=c++11) add_definitions(-Wall) add_definitions(-O2) link_directories(/usr/local/lib) add_executable(SomeExecutable SomeExecutable.cpp ${OptFunctionOne} ${OptFunctionTwo}) target_link_libraries(SomeExecutable -pthread -lboost_thread ${Boost_LIBRARIES} myLib armadillo) File structure (shortened for brevity): ├── bin ├── CMakeCache.txt ├── cmake_install.cmake ├── CMakeLists.txt ├── Common │   ├── include │   │   ├── dataContainer.h │   │   ├── OptFunctionOne.h │   │   ├── OptFunctionTwo.h │   └── src │   ├── OptFunctionOne.cpp │   └── OptFunctionTwo.cpp ├── SomeExecutable.cpp ├── myLib │   ├── example.cpp │   ├── include │   │   ├── Config.h │   │   ├── Helper.h │   ├── libmyLib.so