Question

Assuming that if the error’s spotted in the .o file that it’s the linker’s problem…

Anyway, I’m writing a program to organize the courses I want to take in college by outputting them into a nicely formatted HTML file. To do this, I’m going to throw a bunch of objects of my Course class into a list (the data structure I picked, unless someone warns otherwise), organize them by course code, and output them to the aforementioned HTML file, placing horizontal line breaks between each different major/minor/etc. At the top of each line-broken segment will be the name of the major, reading like;

AMS (Applied Mathematics and Statistics)

Whenever the user enters the course code (just the three letters, not the number), I want the class to check whether the code is on a .txt I provide with the program. Because if I’m going to have dozens of Courses in the end, why bloat my code with an fstream or a member function for each object? So I figured using some static magic might be a good idea. Then I get this error message, using g++;

(Sorry it looks messy, by the way, I’m not sure how to format this neatly here.)

In function `std::basic_ifstream >::open(char const*, std::_Ios_Openmode)’:

/usr/include/c++/4.5/fstream| 528 | undefined reference to Course::courses'
/usr/include/c++/4.5/fstream|533|undefined reference toCourse::courses’
/usr/include/c++/4.5/fstream|533|undefined reference to Course::courses'
obj/Debug/main.o In functionstd::basic_filebuf >::is_open() const’:

/usr/include/c++/4.5/fstream|223|undefined reference to Course::courses'
obj/Debug/main.oIn functionstd::basic_ifstream >::close()’:
/usr/include/c++/4.5/fstream|566|undefined reference to Course::courses'
obj/Debug/main.o:/usr/include/c++/4.5/fstream|529|more undefined references toCourse::courses’ follow

I’m then directed to this segment of the fstream header;

void open(const char* __s, ios_base::openmode __mode = ios_base::in)
  {
if (!_M_filebuf.open(__s, __mode | ios_base::in))
  this->setstate(ios_base::failbit);
else
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 409. Closing an fstream should clear error state
  this->clear();
  }

The two static members I have (open and close the stream, at the start and end of the whole collection process for the courses) as well as the member itself are declared like so;

class Course
{
    public:
        Course();
        void setup();
        bool confirm();
        stringstream entry;
        static void openCodeList() { courses.open("codes"); if (!courses.is_open()) exit(1); }
        static void closeCodeList() { courses.close(); }
    private:
        //Irrelevant strings and bits, etc, etc.
        static ifstream courses;
}

I do have other .cpp and .h files involved (namely a File class to handle actually outputting everything in the end), but I have not even mentioned these fstream-related members in any of them but my obviously WIP main.cpp;

int main()
{
    cout << "Welcome to the Stony Brook Course Organizer!  This program will help you\n"
            "organize the courses that you wish to take at Stony Brook University.\n\n";
    Course thefirst;
    Course::openCodeList();
    thefirst.setup();
    thefirst.confirm();
    Course::closeCodeList();
}

Any help in dealing with my static issues and advice for the future (even if not directly related to that) would greatly be appreciated. Thank you!

Answer 1

You need to define the static member in a source file of your program, outside of the class definition.

E.g. in a .cpp:

std::ifstream Course::courses;