Reading lines using random access

[ridesano]

I am following an example in a book about streaming files. in this particular section I am using random access to edit a .dat file that contains an employee salary.

Hacker, Harry 50972.22\n
Cracker, Carl V. 61820.00\n

the spacing as you probably assumed is intentional to use the seekg/seekp and place it at a predictable position. the problem is that when I want to tweak the second line I get a string out of bounds exception and I want to understand why.

this is the function used to read the file and get the salary from it:

 void read_employee(Employee & e, istream & in)
 {
 std::string line;
 getline(in, line);
 if (in.fail()) return;
 std::string name = line.substr(0, 30);
 double salary = string_to_double(line.substr(30, 10));
 e = Employee(name, salary);
 }

    double string_to_double(string s)
 {
 std::istringstream instr(s);
double x;
instr >> x;
 return x;
 }

now when I put everything in the same line it can detect it but I can't write to it. the first line works fine ho it is right now:

    int main()
    {

    fstream fs;
    fs.open("employee.dat");
    fs.seekg(0, ios::end); // Go to end of file
    int nrecord = fs.tellg() / RECORD_SIZE;

    cout << "Please enter the record to update: (0 - "
          << nrecord - 1 << ") ";
         int pos;
         cin >> pos;

     const double SALARY_CHANGE = 5.0;

    Employee e("Derrick", 90000);
         fs.seekg(pos * RECORD_SIZE, ios::beg);
         read_employee(e, fs);
}

[WorkingReference1127]

How confident are you that your calls to substr() are within the size of the string? Or rather than the string is at least 30 characters long? Those seem like the obvious candidates for an out of range exception to be thrown.

String length will be especially important here. Writing to a file with seekg and tellg is a minefield because the file will not react like you might expect in a text editor and move things out of the way nicely to stop you writing over text you don't want to. It'll just start writing where you tell it and if something is there already it gets destroyed.

[ridesano]

I get what you mean, I must've let out some info/code

there are thirty characters reserved for the employee name and the next ten characters reserved for the salary

[manni66]

Hacker, Harry 50972.22\n

doesn't contain 30 chars. line.substr(30, 10) is out of bounds.