I created a static member m_info_fileclass:

class FileOperations
{
private:
    static std::fstream m_info_file;
    static std::string m_index_date; // May be volatile to further changes
    static void InitializeFile(bool file_was_created_before);
    static void MoveFilePtr(uint32_t p_point);
    static std::string GetTimeFromFile();
    static bool m_file_was_created_before;
    static bool m_file_initialized;
    static bool m_class_initialized;
    static SearchIndex m_index;
    static std::mutex m_index_mutex;    
public:
    static void Start();
} 

and defined it in the source file:

void FileOperations::Start()
{
#pragma region MEMBERS_INNIT
    bool m_file_was_created_before;
    bool m_file_initialized;
    bool m_class_initialized;
    std::string m_index_date; // May be volatile to further changes
    std::fstream m_info_file;
    SearchIndex m_index;
    std::mutex m_index_mutex;
#pragma endregion
}       

But I am still getting the error. It is worth noting that I’ve done the exact same for other members and it worked perfectly fine.

Here is the error:

Severity    Code    Description Project File    Line    Suppression State   Details
Error   LNK2001 unresolved external symbol "private: static class std::basic_fstream<char,struct std::char_traits<char> > FileOperations::m_info_file" (?m_info_file@FileOperations@@0V?$basic_fstream@DU?$char_traits@D@std@@@std@@A)  FileSearchEngine    C:UsersbahgesourcereposFileSearchEngineFileOperations.obj 1