Editing a data file

okay, I've been trying this all day and I can't get it to work for the life of me. I'm trying to fill an object from a data file then possibly make changes to it and put it back.

the data file looks like this:

Battlefield Earth //title of a book
L. Ron Hubbard //author of the book
3 //copies of the book in the database
15.95 //price of the book

here's the class headers:

Code:

---

class Book
{
  public:
    void operator = (const Book& b1);
    Book();              //sets title and author to empty strings
                            //copies to 0 and price to 0.0
    string get_title(); //returns title
    string get_author(); //returns author
    void set_title(string str); //changes value of title
    void set_author(string str); //changes value of author
    void set_copies(int num); //changes value of copies
    void set_price(double p); //changes value of price
    int get_copies(); //returns copies
    double get_price(); //returns price
    void print(ostream& outs); //outputs all the data in the format
                                              //I showed above
    void fill(istream& ins); //fills the values
  private:
    string title;
    string author;
    int copies;
    double price;
};

//here's the second class

typedef Book* Bookptr;

class Library
{
  public:
    ~Library();
    Library(const Library& l1);
    void operator = (const Library& l1);
    Library(); //sets size to 1 and allocates memory for books
    void search (); //searches by author or title
    void  print(ostream& outs); //prints books in format above
    void add_new(const string& s1, fstream& file); //adds a new
//book to the library or increments the number of copies if it is
//already in the database. this is the function I've been having
//trouble with
    void fill(istream& ins, int& num); //input function
    void total(); //adds up price of all the books and prints it
    void delete_book(const string& s1); //deletes a book from the
//library, another one I'm having trouble with
    void check_out(const string& s1); //checks a book out
    void random(); //prints a random book
  private:
    int size;
    Bookptr books;
    void resize(Bookptr& b1, int& num);
};

//all of these functions work really well, the only problem I'm
//having is that my data file never gets permanently edited even
//when I want to. here's the definitions of the functions that
//matter

void add_new(const string& s1, fstream& file)
{
  Book b1;
  string str;
  int temp;
  double temp_price;

  for (int i = 0; i < size; i++)
  {
    if (new_title == books[i].get_title())
    {
      temp = books[i].get_copies() + 1;
      books[i].set_copies(temp);
    }
    return;
  }
//if the book isn't in the database already, the rest of this
//function gets its info from the keyboard, puts it into the books
//array, and prints it into the data file. that works fine, the part
//above is what I need help with. I need to know how to scrap
//the old value of copies and print the new one out to the same
//place. I was thinking of something like this:

  file.open("database.dat",ios::out|ios::trunc);
  print(file);
  file.close();

  //but every time I try that it just deletes the data file and
  //doesn't fill it again

---

ultimately what I'm going to need to be able to is open the file and fill a Library object from it. then I need to be able to edit it. and I need to be able to change individual lines of it. I can't figure out how for the life of me. the easiest way I think would be to delete the file then reprint the new data

if you need more code I can provide it, but I can't think of any that you'd really need. none of the other functions are called in this piece of code

How is print defined?

Code:

---

void Book::print(ostream& outs)
{
  outs << title << endl << author << endl << copies << endl << price << endl;
}

void Library::print(ostream& outs)
{
  for (int i = 0; i < size; i++)
    books[i].print(outs);
}

//sorry, I meant to include that in the original post, but I forgot

---

Is this a text file? Or a binary file with fixed length records?

If the former, you can't simply edit one and place it back in the middle. You have to reconstruct the entire file again from that record onward.

If the latter, you can simply seek to the position of the record and write it back to that spot (assuming you've opened the file correctly).

Quzah.

yeah it's a text file. I'm not very familiar with binary files

so back to my original point, how to I rewrite the file once I've inputted from it?

You should try taking a look at the FAQ

The C++ answer is also in the FAQ

After reading those if you have questions feel free to ask.

there's nothing in there that has anything to do with my problem except I learned that ios::trunc doesn't really delete, it just sets file length to 0. I think that explains why my other idea wasn't working

but, I still have no idea how to do this

That's funny because when I do the below code it erases whatever I have in the file and then replaces it with the letter z.

Code:

---

#include <iostream>
#include <fstream>

using namespace std;


int main() {

        fstream fout;

        fout.open("mytext.txt", ios_base::out | ios_base::trunc);

        if(fout.is_open()) {
                fout.put('z');
        }else {
                cout <<"error opening file";
        }

        fout.close();
        return 0;
}

---

Are you sure you were using the right combinations for the writing flags?