Reading from file returns a result I didn't expect, trying to understand why

Question

I have this code which contains a class and a main function:

class Employee {
    int m_id;
    string m_name;
    int m_age; public:
    Employee(int id, string name, int age) :m_id(id), m_name(name), m_age(age) {}
    friend ostream& operator<<(ostream& os, const Employee& emp)
    {
        os << emp.m_id << " " << emp.m_name << " " << emp.m_age
           << endl;     
        return os;
    }
};

int main() {
    const int Emp_Num = 3;
    fstream fs("dataBase.txt", ios::out);
    if (!fs) {
        cerr << "Failed opening file. Aborting.\n";
        return -1;
    }

    Employee* list[Emp_Num] = 
        { new Employee(1234, "Avi", 34),
          new Employee(11111, "Beni", 24),
          new Employee(5621, "Reut", 26) };
    for (int i = 0; i < Emp_Num; i++) 
    {
        fs << (*list[i]);
        delete list[i];
    }
    fs.close();

    fs.open("dataBase.txt");
    if (!fs) {
        cerr << "Failed opening file. Aborting.\n";
        return -1;
    }
    fs.seekg(4);
    string strRead;
    fs >> strRead;
    cout << strRead << endl;
    fs.seekg(6, ios::cur);
    fs >> strRead;
    cout << strRead << endl;
    fs.seekg(-9, ios::end);
    fs >> strRead;
    cout << strRead << endl;
}

Here is how I understand it, after the first file open and close, the file dataBase.txt should look like this:

1234 Avi 34

11111 Beni 24

5621 Reut 26

My problem is with the reading and output to the console. After I open the file, the pointer of my current position is at the first byte, which is the 1 before the 1234.

I seek 4 from the beginning of the file, so my pointer should be at (before) the space between the 1234 and Avi.

Now I get the next string into my string variable strRead, now strRead contains "Avi" and the pointer should be between the i of Avi and the space after it.

Now i seek 6 from my current position, by my count those are the 6 bytes i pass through:

1. Space

2. 3

3. 4

4. Line break (return)

5. 1

6. 1

So my pointer should be at the second line, after the two first ones.

I mean like this:

11|111 Beni 24

Now I get a string to strRead, by my understanding of the code strRead should now contain "111", instead, for some reason, it contains and output later "1111".

Could someone explain me why does it work that way? There is no character between the first line drop and the first letter of the second line, so it should count as only 1 byte...

Answer 1

I did the following test:

I have run the second part of your code (which read from file) on a file with the text:

1234 Avi 34 11111 Beni 24 5621 Reut 26

So, I have replaced end of lines with spaces, and the code print to console output the expected result 111. I then began to be suspicious about seek skipping end of lines.

Then I changed the code instead (without file modifications) and worked with the file in binary mode:

//...
fstream fs("dataBase.txt", ios::out | ios::binary);
//...
fs.open("dataBase.txt",  ios::in | ios::binary );
//...

Again the result was the expected: 111.

What change in both cases?

Well, in plain text (not in binary mode) the end of line are actually 2 chars (this can vary for other platforms, I'm reproducing this on Windows): \r and \n. Thats why you're reading four ones (1111) instead three (111).

Counting 6 positions from the space after Avi:

  A v i _ 3 4 \r \n 1 1 1 1 1
                    ^
        1 2 3  4  5 6 7 8

In the first test I performed, the a space (only one character) replaced two of them.

  A v i _ 3 4 _ 1 1 1 1 1
                  ^
        1 2 3 4 5 6 7 8

And in binary mode both of characters are represented as a single one unit to read(I don't have investigated if this is platform dependant).

A v i _ 3 4 B 1 1 1 1 1
                ^
      1 2 3 4 5 6 7 8

B stands here for some binary code.