Read access violation, error code 0xFFFFFFFFFFFFFFFF

Question

I'm sorry this is a repeat question, but no solutions seem to work for my code.

This is for an assignment in school on reading from a file and copying the data to an array. An exception is thrown every time I try to edit the array "arr" in main.

Here's my code:

#include <iostream>
#include <fstream>

using namespace std;

struct Student {
    string name;
    float gpa;
    int id;
};

void PrintStudents(Student arr[], int nstudents) {
    for (int i = 0; i < nstudents; i++) {
        cout << "Student name: " << arr[i].name << endl;
        cout << "Student GPA: " << arr[i].gpa << endl;
        cout << "Student ID: " << arr[i].id << endl;
    }
}

int ReadStudents(string fname, Student arr[]) {
    ifstream file;
    file.open(fname);

    int counter = 0;
    string name_local;
    float gpa_local;
    int id_local;

    int index = 0;

    while (!file.eof()) {
        if (counter == 0) {
            file >> name_local;
        }
        else if (counter == 1) {
            file >> gpa_local;
        }
        else if (counter == 2) {
            file >> id_local;
        }

        counter++;

        if (counter == 3) {
            counter = 0;
            Student newStudent = { name_local, gpa_local, id_local };
            arr[index] = newStudent;
            index++;
        }
    }

    file.close();
    return index;
}

void fillStudentArray(Student array[], int array_size) {
    Student temp = { "", 0, 0 };
    for (int i = 0; i < array_size; i++) {
        array[i] = temp;
    }
    return;
}

int main() {
    Student arr[128];
    fillStudentArray(arr, 128); // exception thrown here??
    cout << "Array filled." << endl;

    cout << "Reading students" << endl;
    int nstudents = ReadStudents("csci10.hw8.students.txt", arr);
    PrintStudents(arr, nstudents);

    return 0;
}

Thanks for any help! I'm totally stumped.

Edit: Woah, I left for a 30 minute coffee break and came back to a ton of answers! I'll try to respond to all of them.

Edit 2: Just got a solution! I was working in VS 2019, switched to old school terminal G++ and it worked! Thanks everyone for all the answers :)

Answer 1

1. You do not check the file was successfully opened. Try this:

   ifstream file( fname );
   if ( !file )
     return -1;
   

2. You do not need local variables for reading. Read directly in your array elements:

   file >> arr[index].name
   

3. ReadStudents ignores the size of the passed array: you might get in trouble if you read more than the allocated size (read this again). You might use std::vector, if allowed. Alternatively, pass the size, too – the same way you did for fill.

4. The way you are trying to read from file is overly complicated. Try a more c++ approach:

   • Define an extraction operator for Student:

     std::istream& operator>>( std::istream& is, Student& s )
     {
       return is >> s.name >> s.gpa >> s.id;
     }
     

   • Use it like you would use it for reading an integer:

     file >> arr[ index ]
     

     Alternatively you could use:

     is >> arr[ index ].name >> arr[ index ].gpa >> arr[ index ].id
     

You will get something like this:

int ReadStudents( string fname, Student arr[]/*how large is the array?*/ )
{
  ifstream file( fname );
  if ( !file )
    return -1;

  int index = 0;
  while ( file >> arr[ index ].name >> arr[ index ].gpa >> arr[ index ].id )
    index++;
  return index;
}

Answer 2

Explanation

If the stream does not get opened successfull, you get an endless loop. You never check if the reading operations are successfull. If they are not you never reach eof but keep incrementing index and write to array indices out of bounds. Also the counter is superfluos here.

Furthermore I would suggest to use a std::vector instead of an array and use push_back(). This makes sure, you don't write out of bounds.

(Possible) solution

This loop:

while (!file.eof()) {
    if (counter == 0) {
         file >> name_local;
    }
    else if (counter == 1) {
        file >> gpa_local;
    }
    else if (counter == 2) {
        file >> id_local;
    }
    counter++;

    if (counter == 3) {
        counter = 0;
        Student newStudent = { name_local, gpa_local, id_local };
        arr[index] = newStudent;
        index++;
    }
}

should be changed (with the function definition) to:

int ReadStudents(string fname, std::vector<Student> &vec)
{
    // open stream, etc.

    while (file >> name_local >> gpa_local >> id_local) {
        Student newStudent = { name_local, gpa_local, id_local };
        vec.push_back(newStudent);
    }

    // cleanup
}

To explain a bit further what the while (file >> name_local >> gpa_local >> id_local) does:

Since std::ifstream::operator>> returns a reference to the stream itself, you can chain those statements together.

The last reference gets implicitly converted to bool (or void* in c++11 or earlier) as seen here. This evaluates true if the last reading operations where successfull (so name_local, gpa_local and id_local now have valid values) and the stream is ready for IO-operations (so it didn't reach eof while reading). This implies that it's also checking if the stream was opened at all.

Once those conditions are met you can create a new element and push it into the vector.