Static Member Initialization and Singleton Design, error -> multiple definition of member

Question

I'm trying to implement singielton design, but unfortunately this error of multiple definiton occurrs. Everything seems to be alright I cannot find multiple definiton nowhere. I tried to compile it with gcc on WSL and mingw but the problem stays. I'm following this design https://refactoring.guru/pl/design-patterns/singleton/cpp/example

And member static member initialization from https://www.tutorialspoint.com/how-to-initialize-private-static-members-in-cplusplus

fsc.hpp

class FileSystemController {
    private:
        const static std::vector<std::pair<uint32_t, uint32_t>> testProfileData;

    protected:
        FileSystemController {};
        static FileSystemController* fsc_;
    public:
        FileSystemController(FileSystemController& other) = delete;    
        void operator=(const FileSystemController&) = delete;    
        static FileSystemController* GetInstance();        
    };

    FileSystemController* FileSystemController::fsc_ = nullptr;
    const std::vector<std::pair<uint32_t, uint32_t>> FileSystemController::testProfileData = {{1001, 2001},
                                                                                              {1002, 2002},
                                                                                              {1003, 2003},
                                                                                              {1004, 2004},
                                                                                              {1005, 2005},
                                                                                              {1006, 2006},
                                                                                              {1007, 2007}};

fsc.cpp

FileSystemController* FileSystemController::GetInstance() {
    if (fsc_ == nullptr) {
        fsc_ = new FileSystemController();
    }
    return fsc_;
}

logs

 ====================[ Build | FileSystemController | Debug ]====================
    /usr/bin/cmake --build /mnt/e/Dokumenty/AiR_rok_4/S7/EngineeringThesis/Profilometr/FileSystemController/cmake-build-debug --target FileSystemController -- -j 8
    Scanning dependencies of target FileSystemController
    [ 25%] Building CXX object CMakeFiles/FileSystemController.dir/FileSystem/FileSystemController.cpp.o
    [ 50%] Linking CXX executable FileSystemController
    /usr/bin/ld: CMakeFiles/FileSystemController.dir/FileSystem/FileSystemController.cpp.o:(.bss+0x0): multiple definition of `controllers::FileSystemController::fsc_'; CMakeFiles/FileSystemController.dir/main.cpp.o:(.bss+0x0): first defined here
    /usr/bin/ld: CMakeFiles/FileSystemController.dir/FileSystem/FileSystemController.cpp.o:(.bss+0x10): multiple definition of `controllers::FileSystemController::testProfileData'; CMakeFiles/FileSystemController.dir/main.cpp.o:(.bss+0x10): first defined here
    collect2: error: ld returned 1 exit status
    make[3]: *** [CMakeFiles/FileSystemController.dir/build.make:114: FileSystemController] Error 1
    make[2]: *** [CMakeFiles/Makefile2:76: CMakeFiles/FileSystemController.dir/all] Error 2
    make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/FileSystemController.dir/rule] Error 2
    make: *** [Makefile:118: FileSystemController] Error 2

Answer 1

The declaration of that static variable should be in the header, the definition in the .cpp.

otherwise if you include the file multiple time, the compiler will find multiple definitions.