File Handling with LinkList in C++

[deepecstasy]

PROBLEM : I am trying to write (binary writing) an object of Doubly Linked List onto File & write from it too. I have to write object's complete contents, then load it from file, and store it into new object to re-create the list in FIFO order. I thinking I am writing correctly, but I seriously have no idea how to load (read) it from file. REMEMBER: I am just trying to save and read CONTENTS of a node, & NOT POINTERS.

CODE::

//template type BOOK class

template<class mytype>
class BOOK      
{
private:
    static int count;   //declaration of static variable to set ID for books
public:
    BOOK<mytype> *next, *prev;  //BOOK type pointers; 'next' to store address of 
next BOOK & 'prev' to store address of previous BOOK
    int ID;         //variable to store ID of a book
    string bookName;//string to store name of a book
    string author;  //string to store name of author of book
    string book_type;//string to store type of a book
    long copies;    //variable to store no. of copies a book
    long price;     //variable to store price of a book
    string status;  //to store status of a book, either its in stock or not
    dynamicQueue<string> book_queue;    //created an object of queueClass as data member of each Book

    BOOK()  //Constructor 0 argument to initialize everything
    {
        count++;    //increment counter
        ID=count;   //assign counter to ID to be ID of newly added book

        next = prev = 0;        //Initializing both pointers to 0

        bookName = "\0";
        author = "\0";
        book_type = "\0";
        copies = price = 0;
        status= "InStock";
    }

    BOOK(BOOK *n =  0, BOOK *p = 0, string book = "\0", string athr = "\0", string buk_type = "\0", long cp=0, long pr=0) //Constructor multiple arguments, to store information about a book
    {
        next = n;       //store contents of user-given value n into next
        prev = p;       //store contents of user-given value p into previous

        bookName = book;//store contents of user-given value book into bookName
        author = athr;  //store contents of user-given value athr into author
        book_type = buk_type;//store contents of user-given value buk_type into book_type
        copies = cp;    //store contents of user-given value cp into copies
        price = pr;     //store contents of user-given value pr into price
        status= "InStock";
        count++;        //increment counter
        ID=count;       //assign counter to ID to be ID of newly added book
    }
};

template <class mytype>    // declaration of
int BOOK<mytype>::count=0; // static variable to set ID for books
//-------------------- 

Main Part for Adding a new Book.

BookStoreDataBase<char> obj;    //created object of Doubly linked list
string Book, Author, Booktype;
long Copies=1, Price=0;
cout<<"Enter Name of Book = ";  cin>>Book;
cout<<"Enter Author = ";        cin>>Author;
cout<<"Enter Type of Book = ";  cin>>Booktype;
cout<<"Enter Number of Copies = ";  cin>>Copies;
cout<<"Enter Price (PKR) = ";   cin>>Price;

obj.addBook(Book, Author, Booktype, Copies, Price);

Saving function to save all data to file

template <class mytype>
void DoublyLinkedList<mytype>::save_data()
{
    NODE<mytype> * temp = head; //made copy of head
    fstream file;     //created new file
    file.open("mydata.txt", ios::binary | ios::out | ios::app);

    while(temp->next!=0) //Until link list end
    {
          file.write(reinterpret_cast<char*>(&temp), sizeof(temp));
          temp = temp - > next; //move temp to next node
    }
    file.write(reinterpret_cast<char*>(&temp), sizeof(temp)); //write again for last  
                                                              //book's data
    file.close();
}

Now I have practically no idea on how to read the list from file, store contents into each node & with disturbing the saved arrangement, re-create the list in FIFO order. So I can print it later. I have practiced alot, gone to forums etc etc but not found any concrete Solution. Please help me out. Thanks in advance

A sample of my effort

template <class mytype>
void DoublyLinkedList<mytype>::load_data()
{
    fstream file;
    file.open("mydata.txt", ios::binary | ios::in);
    while(!file.eof())
    {
        NODE<mytype> *temp = new NODE<mytype>;
        file.read(reinterpret_cast<char*>(&temp), sizeof(temp));
        if(is_Empty())
        {
            head = tail = temp;
        }
        else
        {
            temp->prev->next = temp;
            temp->next=0;
        }
    }
    file.close();
}
//------------------- 

NO COMPILE TIME ERROR.
RUNTIME ERROR : Unhandled exception at 0x00CD8391 in DobulyList.exe: 0xC0000005: Access violation writing location 0x00000004.

[cire]

REMEMBER: I am just trying to save and read CONTENTS of a node, & NOT POINTERS.

Then you should probably stop attempting to read in the pointers which ARE the contents of a node.

You've combined the book type with the node type which makes for some rather confusing code. A book is not a node and a node is not a book.


file.write(reinterpret_cast<char*>(&temp), sizeof(temp));

This is wrong. First, you're writing the pointers to next and previous, which you don't want to do, second, this isn't going to work with std::string types as members of your class (where sizeof will only get the size of the pointers to dynamic memory in the string object.)

Reading is obviously wrong as well. I would suggest you begin by writing a std::string to file and successfully read it back before you move on to a more complex data structure.

[deepecstasy]

Ok what if I go for arrays[size] ?

[cire]

That would simplify things.