How to edit and overwrite specific location in file in C++

Question

I am making an airline reservation software and I don't know much about the Visual C++. I am using the simple compiler "TCWIN45". In my program I wish to use file handling and I am succeed to save all the inputs in text file. i need to add search option and modification option. if user choose search and Enter the name then how can I access specific number of lines. because my files contains the record of multiple passengers but I want to show the only one's data. same is the case for modification. I want to access specific location or line and also to overwrite it. please suggest me the most simplest way.

This is my code to save all the record in one text file:

ofstream thefile("ID.txt" , ios::app);
thefile<<"\n\nDay : "<<p1[i].day<<"\nFlight Date : "<<p1[i].date<<"\nFlight Type : "<<p1[i].type<<"\nReturn Date : "<<p1[i].rdate<<"\nDeparture Place : "<<p1[i].from<<"\nDestination : "<<p1[i].to<<"\nClass Type : "<<p1[i].clas<<"\nTime of Flight : "<<p1[i].time<<"\nTitle : "<<p1[i].prefix<<"\nFirst Name : "<<p1[i].fname<<"\nLast Name : "<<p1[i].lname<<"\nDate of Birth : "<<p1[i].dob<<"\nPassport Number : "<<p1[i].ppt_no<<"\nExpiry Date : "<<p1[i].edate<<"\n Contact Number : "<<p1[i].cont<<"\nMeal Type : "<<p1[i].meal<<"\n\n------------------------------";

Answer 1

You'll probably want to define a reservation class that represents a single reservation, and a data class, that holds all your data, as a vector of reservations. The data class will want to have a member function that takes a std::ostream by reference, and saves the reservations to a text file, (easiest is one variable per line). It will also want a member function that takes a std::istream by reference and reads in the data from the text file.

The main part of your program would (I'm making TONS of assumptions here) load the file into the data class with the std::istream member function, and asks the user for some sort of ID. You then call a member function of data that checks all of the elements in datas vector until it finds the matching ID (by reference), and lets the user change some members. Then it calls the std::ostream member function again to save the changes.

Streams are handled like this. In this sample, I do not use the data class or a vector, since this question looks suspiciously like homework, but this shows the tricky parts of file handling.

#include <string>
#include <iostream>
#include <fstream>

class Reservation {
    std::string ID;
    std::string date;
public:
    //default constructor
    Reservation()
    {}
    //helpful constructor
    Reservation(std::string _id, std::string _date)
    :ID(_id), date(_date)
    {}
    //copy constructor
    Reservation(const Reservation& b)
    :ID(b.ID), date(b.date)
    {}
    //move constructor
    Reservation(Reservation&& b)
    :ID(std::move(b.ID)), date(std::move(b.date))
    {}
    //destructor
    ~Reservation() 
    {}
    //assignment operator
    Reservation& operator=(const Reservation& b)
    {
        ID = b.ID;
        date = b.date;
        return *this;
    }
    //move operator
    Reservation& operator=(Reservation&& b)
    {
        ID = std::move(b.ID);
        date = std::move(b.date);
        return *this;
    }
    //save
    std::ostream& save(std::ostream& file) {
        file << ID << '\n';
        file << date << '\n';
        return file; //be in the habit of returning file by reference
    }
    //load
    std::istream& load(std::istream& file) {
        std::getline(file, ID);
        std::getline(file, date);
        return file; //be in the habit of returning file by reference
    }
};

int main() {
    Reservation reserve; //create a Reservation to play with

    {  //load the reservation from loadfile
        std::ifstream loadfile("myfile.txt");
        reserve.load(loadfile);
    }

    //display the reservation
    reserve.save(cout);

    { //save the reservation to a different file
        std::ofstream savefile("myfile2.txt");
        reserve.save(savefile);
    }
    return 0;       
}

Answer 2

From your comments to other answers, it does not seem like the best way for you to do this is to store the data in a text file at all. You will probably want a Reservation class that contains all of the information for the reservation. Then, use some kind of Collection to store all of the reservations. Writing to a text file just adds a huge amount of unnecessary difficulty.

Something like this:

class Reservation
{
    std::string day;
    std::string date;
    std::string flightType;
    std::string meal;
    /* ... */
};

It would be even better if you made separate classes for each one of the class members (like a Day class, a FlightType class, etc.).

You would then use some kind of Map to access a particular reservation and change its members.

Answer 3

Ali, this can be done in a flat file if you really want to not use a database. The trick, is to either: 1.) have all records the same size OR 2.) have a "record header" that provides "enough" information to be able to unserialize the record from the hard disk. If you store different kinds of records, "enough" information could be size of the record or a record type for RTTI purposes. I find it useful to also store an ID for each record so that I can store an index table for record offsets.

If you records have varying sizes, then your record's serialization functions have to be able to handle this. In fact, it is trivial to do this.

The index table is a table of file offsets.

typedef uint16_t record_id;
typedef long offset_t;

offset_t  indices[ MAX_RECORDS ];


typedef struct _record {
     uint16_t type;
     uint16_t id;
     offset_t next;
     offset_t prev;
} record;

typedef struct _header {
   uint32_t count;
   offset_t first_record;
   offset_t deleted_record;
} header;

So to find the position of the record, you find the offset into the file, which is indices[ record_id ]. Adding a record is like adding a node to a linked list, but the nodes are in the file. Deleting records is a little tricky. You have to use "lazy delete" to delete records and later these deleted records get reused. You can even write a shrink function that will remove all deleted records from the file to free up unused space.

The limitations of this technique is that you can only search by record id. If you have other information, you will need to generate additional data structures to support this.

I have code available that does this in C if you would like a working example. However, doing this from scratch is feasible but NOT WORTH THE EFFORT. Just use a database like Sqlite or MySQL--it will save time!

Example Code

• flat-db.c
• flat-db.h
• test-flat-db.c