int get_datasize(std::fstream& data) {
    int start;
    int end;
    data.seekg(0, std::ios::beg);
    start = data.tellg(); //sizeof start is output 0
    data.seekg(0, std::ios::end);
    end = data.tellg(); // sizeof end is output 4

    return  ((end - start) / sizeof(Student) ); // sizeof student is output 128
}

The resulting always got 0 for got data size, for more information the Student is struct type

Why the result always 0 for data size? Especially the program can add and writing new data to the file. I think the problem on the last line of the code, but I found not other solutions to solve this 0 data size