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In our last article, we have explained how to write to a file on the disk. In this tutorial, we are going to explain how to append data at the end of an existing file on disk. But before we peform the append operation on a file, we must open it. For this, the C++ language provides us stream classes which are used to perform file output/write/append operations.

In order to perform a file output/write/append operations, C++ provides us a few file stream classes, such as -

• ofstream, to perform the file output operations.
• fstream, to perform the any file input and output operation.

These two file stream classes provides us a function named open(), using which we could open a file by providing its location on disk, and perform a specific input/output file operation on it by specifying a specific mode in which we open this file. Let us take a look at the syntax of the open() function.

• Syntax of open() function




file-stream-object.open("filename", mode);




• file-stream-object, is an object of a file stream class used to perform a file output/write operation.
• filename, is the name of a file on which we are going to perform file operations.
• mode, is a single or multiple file modes in which we are going to open a file.

Let's read the file mode to use to append new data at the end of a file without deleting its old content.

Mode to append data to the end of a file

Let's look at the mode required to append new data to the end of a file.

File Mode	Description
ios::app	This mode searches for a file and opens it in the append mode i.e. this mode allows you to append new data to the end of a file. If file is not found, a new file is created.

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Appending data a file by taking input from a char[] array

Let's say that we already have a file on disk named File1.txt, with content :

Greetings! Hope you are doing well. 

Now let's try to write and append data to this file by taking input from an already initialized char[] array, This file will be searched in the current directory(the same directory where we are going to store the next program).

//C++ Appending the data to a file using fstream class and modes ios::app 
 
#include<iostream>
#include<fstream>

using namespace std;

int main()
{
	int size =50;

	//Creating and initializing a char array
	char str[size] = "Best wishes for all your endeavours!";
	char ch;


	//Creating an output stream to append new data to a file
	fstream fstream_ob;


	//Opening a file named File.txt to append new content at its end, by using the mode - ios::app
	fstream_ob.open("File.txt", ios::app);


	//Appending the char array to the end of the file
	fstream_ob<< str << "\n";


	//closing the output stream after completing the write operation on the file
	fstream_ob.close();

	return 0;
}

Output

Executing this program will append the content of our char[] at the end of a file named File2.txt in the current directory, with its content:

Greetings! Hope you are doing well. Best wishes for all your endeavours!

Appending data a file by taking input from a user at the console

Let's say that we already have a file on disk named File2.txt, with content :

Too much of work and no play made Jack a dull boy. 

Let us append data to this file by taking inputs from a user at the console, This file will be searched in the current directory(the same directory where we are going to store the upcoming C++ program.).

//C++ Appending the data to a file using fstream class and modes ios::app 
 
#include<iostream>
#include<fstream>

using namespace std;

int main()
{

	//Creating an output stream to append new data to a file
	fstream ofstream_ob;


	//Opening a file named to append new content at its end
	ofstream_ob.open("File2.txt", ios::app);


	int i; 
	cout<<"Enter an int : ";
	cin>>i;


	//writing an int value to the file
	ofstream_ob<< i << "\n";


	char ch;
	cout<<"Enter a char value : ";
	cin>>ch;

	//writing an char value to the file
	ofstream_ob<< ch << "\n";



	float f;
	cout<<"Enter a float value : ";
	cin>>f;


	//Writing a float value to the file
	ofstream_ob<< f << "\n";

	char newline_chr;
	cin.get(newline_chr);


	char arr[40];
	cout<<"Enter the content of char array : ";
	cin.getline(arr,40);


	//writing a char array to the file
	ofstream_ob<< arr << "\n";

	
	string str;
	cout<<"Enter the content of a string : ";
	getline(cin, str);

	//writing a string to the file
	ofstream_ob<< str<< "\n";


	//closing the output stream after completing the write operation on the file
	ofstream_ob.close();

	return 0;
}

Output

Enter an int : 1000
Enter a char value : B
Enter a float value : 15.4
Enter the content of char array : Blue Sky
Enter the content of a string : Always Shine

Executing this program will append the content of an int, char, float, char[] and a string to the end of a file named File2.txt in the current directory, with its content :

Too much of work and no play made Jack a dull boy.1000
B
15.4
Blue Sky
Always Shine

In the next tutorial, we are going to explain how to modify the content of an existing file in C++.

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