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
– Adrian Mole, Commented Oct 15, 2019 at 18:11
Yes, but in the text file I want to see all ID numbers that were typed by different users — Anastasia M.
– Anastasia M., Commented Oct 15, 2019 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
– Thomas Matthews, Commented Oct 15, 2019 at 18:25

Remy Lebeau · Accepted Answer · 2019-10-15 18:22:30Z

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);

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