Append Content of One Text File to Another in C++

In C++, we can work with files to read, write, or even combine(append) their contents. Here, our task is to append the content of one text file to another.

Imagine a scenario where we have two txt files which is named as 'source.txt' and 'destination.txt' files. So, with the help of these files we need to copy everything from the source file and append it at the end of the destination file.

Example

Here, we are giving an input as two text files as source.txt and destination.txt to append content in C++:

source.txt file contains "Tutorials"
destination.txt file contains "point"

After appending the two files with each other, the output will be:

Tutorialspoint

Algorithm

Following is the algorithm is as follows:

Begin
   Define a fstream class object as fin.
   Open a input file a.txt with input file stream class object fin.
   Open a output file a1.txt with output file stream class object fout
   in append mode.
   Check if the file is not existing then
      Print "File not found".
   Else append content from fin to fout.
   Open the destination file in read mode.
   Display its content as output. 
End.

To achieve this, below are the different approaches we can use in C++:

Appending with fstream and open()

In this approach, we use fstream where the destination file is opened in 'ios::app' mode, allowing content to be appended directly.

Syntax

Following is the syntax is as follows:

fstream file;
file.open("destination.txt", ios::app);
file<<data;

Example

This program reads each line from "source.txt" and appends (adds) it to the end of "destination.txt".

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main() {
   fstream source("source.txt", ios::in);
   fstream dest("destination.txt", ios::app);
   if (!source || !dest) {
      cerr<<"Error opening files!"<<endl;
      return 1;
   }
   string line;
   while (getline(source, line)) {
      dest<<line<<endl;
   }
   source.close();
   dest.close();
   cout<<"Content appended successfully."<<endl;
   return 0;
}

If the file is opened, the output will be:

Content appended successfully.

If the file is not opened, the output will be:

Error opening files!

Manual File I/O with ifstream & ofstream

This approach uses 'ifstream' for reading from the source and 'ofstream' to append for writing to the destination.

Syntax

Following is the syntax is as follows:

ifstream source("source.txt");
ofstream dest("destination.txt", ios::app);
while (getline(source, line))
   dest<<line<<endl;

Example

This program reads each line from source.txt and adds it to the end of destination.txt without deleting its existing content.

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main() {
   ifstream src("source.txt");
   ofstream dest("destination.txt", ios::app);
   if (!src.is_open() || !dest.is_open()) {
      cerr<<"Unable to open one or both files."<<endl;
      return 1;
   }
   string line;
   while (getline(src, line)) {
      dest<<line<<endl;
   }  
   src.close();
   dest.close();
   cout<<"Data has been appended to destination.txt"<<endl;
   return 0;
}

If the file is opened, the output will be:

Data has been appended to destination.txt

If the file is not opened, the output will be:

Unable to open one or both files.

Appending Using File Buffers (rdbuf)

The rdbuf() is used to copy the entire buffers from the source file into the destination file.

Syntax

Following is the syntax is as follows:

ifstream source("source.txt", ios::binary);     
ofstream dest("destination.txt", ios::binary | ios::app);
dest<<source.rdbuf();

Example

This program appends the entire content of source.txt to destination.txt in binary mode using a buffer.

#include<iostream>
#include<fstream>
using namespace std;
int main() {
   ifstream src("source.txt", ios::binary);
   ofstream dest("destination.txt", ios::binary | ios::app);
   if (!src || !dest) {
      cerr<<"Failed to open files."<<endl;
      return 1;
   }
   dest<<src.rdbuf();
   src.close();
   dest.close();
   cout<<"File content appended using rdbuf."<<endl;
   return 0;
}

If the file is opened, the output will be:

File content appended using rdbuf.

If the file is not opened, the output will be:

Failed to open files.