C++ Program to Append a String in an Existing File

Appending to a file means adding new data at the end of an existing file without overwriting its current contents. In C++, file appending can be performed using the <fstream> library by opening a file in append mode (ios::app).

ios::app ensures that all new data is written at the end of the file.
Both ofstream and fstream can be used to append data to an existing file.

Suppose the file "Geeks for Geeks.txt" initially contains:

Geeks for Geeks

Append a String Using ofstream

The ofstream class is used for output file operations. Opening a file with the ios::app flag appends new data instead of replacing the existing content.

C++

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main()
{
    ofstream of;
    fstream f;

    // Open file in append mode
    of.open("Geeks for Geeks.txt", ios::app);

    if (!of)
        cout << "No such file found";
    else {
        of << " String";

        cout << "Data appended successfully\n";

        of.close();

        string word;

        // Read and display updated file
        f.open("Geeks for Geeks.txt");

        while (f >> word)
            cout << word << " ";

        f.close();
    }

    return 0;
}

Output

Data appended successfully
String

Explanation

ofstream opens the file in append mode using ios::app.
The string " String" is added to the end of the file.
The file is then reopened using fstream to display the updated contents.

Append a String Using fstream

The fstream class supports both input and output operations. It can also append data when opened with the ios::app flag.

C++

#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main()
{
    fstream f;

    // Open file in append mode
    f.open("Geeks for Geeks.txt", ios::app);

    if (!f)
        cout << "No such file found";
    else {
        f << " String_fstream";

        cout << "Data appended successfully\n";

        f.close();

        string word;

        // Read and display updated file
        f.open("Geeks for Geeks.txt");

        while (f >> word)
            cout << word << " ";

        f.close();
    }

    return 0;
}