How to save all data in the file? C++

Question

I'm having a problem, I want to save all ID numbers in the text file, but it only saves the last ID number that user input.

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

using namespace std;

int main()
{

    string line;
    int idnum;
    ofstream IdData ("data.txt");

    cout<<" Enter your ID number: "<<endl;
    cin>>idnum;

    if(IdData.is_open())
    {
        IdData<< "ID number of voter is: ";
        IdData << idnum << endl;
        IdData.close();
    }
    else 
        cout<<" Unable to open file";


    ifstream Data ("data.txt");

    if(Data.is_open())
    {
        while (getline (Data, line))
        {
            cout<< line << endl;
        }

        Data.close();
    }
    else
        cout<<" Unable to open file";
}

When you say the 'last' ID number, what do you mean? You are only asking for one ID number! — Adrian Mole, Oct 15 '19 at 18:11
Yes, but in the text file I want to see all ID numbers that were typed by different users — Anastasia M., Oct 15 '19 at 18:13
BTW, if `IdData` fails to open, your program continues after printing the messsage. You may want to use `return EXIT_FAILURE;` after printing the message. — Thomas Matthews, Oct 15 '19 at 18:25

Remy Lebeau · Answer 1 · 2019-10-15T18:22:30.203

You are asking the user for only 1 ID, and then you are overwriting the existing file with a new file that contains that 1 ID.

std::ofstream destroys the contents of an existing file unless you explicitly request it not to do so. So, for what you are attempting to do, to append a new ID to the end of the existing file, you need to include the app or ate flag when opening the std::ofstream (see C++ Filehandling: Difference between ios::app and ios::ate?), eg:

ofstream IdData ("data.txt", ios::app);

Or:

ofstream IdData ("data.txt", ios::ate);

How to save all data in the file? C++

1 Answers1