File handlling in C++: updating a line - c++

Question

I am writing program for Library Management. I have a file Student.dat which have four columns. Initially when no book is issued it looks like this.
---------------Students List ----------------
Roll No. Name Book Issued Issued Book No.
001 Abhi 0 No
002 Ashu 0 No
After issuing book to '001'.
---------------Students List ----------------
Roll No. Name Book Issued Issued Book No.
001 Abhi 1 1001
02 Ashu 0 No
The roll number of second student becomes '02'.
This is complete issue function in library.cpp
void Library::book_issue()
{
//Some code
fp.open("Students.dat", std::ios::in | std::ios::out);
fp1.open("Books.dat", std::ios::in | std::ios::out);
//////////////////////////////////////////////////////////////////////
int oldPos = fp.tellg();
while (std::getline(fp, line) && !found_stu)
{
std::stringstream ss(line);
ss >> roll_n >> s_name >> tkn >> issued_b_num;
////////////
std::getline(ss, line);
if (boost::iequals(roll_n, r_num))
{
found_stu = true;
if (tkn == 0)
{
std::cout << "Enter Book No. : ";
std::getline(std::cin, b_num);
while (fp1 >> book_n >> b_name >> a_name && !found_book)
{
if (boost::iequals(book_n, b_num))
{
Book::show_book(book_n, b_name, a_name);
found_book = true;
tkn = 1;
Student::reset_issued_book_num();
issued_b_num = book_n;
//////////////////////////////////////////////////////////////////
fp.seekg(oldPos);
fp << roll_n << " " << s_name << " " << tkn << " " << issued_b_num << '\n';
std::cout << "Book Issued Successfully\n";
break;
}
}
if (!found_book)
{
std::cerr << "Book does not exist\n";
}
}
}
}
if (!found_stu)
{
std::cout << "Student record does not exist\n";
}
fp.close();
fp1.close();
}
I want to know whether I have used oldPos variable correctly?
Edit:
After assigning length of Issued Book No. as length of book number, I get repeated record.
---------------Students List ----------------
Roll No. Name Book Issued Issued Book No.
001 Abhi 1 1001
001 Abhi 1 1001
002 Ashu 0 No

Answer 1

The problem is that you overwrite the file that you read. So if one line would become longer, you'd overwrite characters of the next line(s).
As 002 becomes 02 and not 2, I'll assume that No in the file is followed by a whitespace. So if I use to show in a visible manner the LineFeed, the following content of your file:
...NO <LF>002...
will be overwriten with:
...1001<LF>02...
^ (end of the write, remaining chars unchanged)
So the 3 chars No are overwritten with 100, the LineFeed is overwritten with 1 and the 0 is overwritten with the new LineFeed.
If you want to write in-place like you try here, you must ensure that the size of each line remains fixed in all circumstances. So "No" should be followed by the number of space needed to match the length of a book number.
Other remarks
It's not the cause of the error, but tellg() returns a std::streampos, which can be much larger than an int. So I'd recommend to prefer:
auto oldPos = fp.tellg(); // here you're sure it's the right type
Note also that tellg()/seekg() are meant for input stream and tellp()/seekp()for output streams. Fortunately, for bidirectional file streams, there is only one position for reading and writing. But for other kind of bidirectional strings, this is not guaranteed (see this question).
Finally, if the goal of your repositionning is to overwrite the last line, read (and found) you should update it from time to time.