C++ Modifying a file on Disk

In our previous articles, we have explained how to perform read, write and append operation on a file on disk. In this tutorial, we are going to explain how to modify the content of a file on disk. But before we modify the content of an existing file present on the disk, we must open it. For this, the C++ language provides us stream classes used to perform file output/write/modify operations.

In order to perform a file output/write/modify 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 operations.

These two file stream classes provides us a function named open(), using which we could provide location of the file stored on the disk to depending on the mode in which we open this file.

Syntax of open() function

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

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

Let's read the file mode to use to modify the content of a file without deleting its old content.

Modes to modify data of a file

Let's look at the combination of modes required to modify the content of a file.

File Mode	Description
ios::in"	Searches for the file and opens it in read mode only(if the file is found).
*ios::out*	Searches for the file and opens it in write mode. If the file is found, its content is overwritten. If file is not found, a new file is created. Allows you to write* to the file.*
*"ios::binary"*	Searches for the file and opens the file(if the file is found) in a binary mode to perform binary input/output file operations. This mode when used with ios::binary, ios::in and ios::out modes, allows you to modify* the content of a file*

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

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

File1.txt

Gender : Male
Age    : 28
City   : Tokyo
Weight : 78.6 Kg

Now let's try to modify the content of this file by replacing the character 'e' to character 'X'. This file will be searched in the current directory(the same directory where we are going to store the upcoming C++ program.).

//Modifying the data to a file using fstream class and modes ios::
 
#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 country1.txt to modify the old content
ofstream_ob.open("File1.txt", ios::in|ios::out|ios::binary);


char ch;

while(ofstream_ob)
{
ch = ofstream_ob.get(); 
	 if(ch =='e')
	{	
		//Going one byte position back from the current put pointer position
		ofstream_ob.seekp(-1, ios::cur);
		ofstream_ob.put('X');
	}
	
}

//Setting the EOF flag off, to allow the access of file again for reading/writing
ofstream_ob.clear();

cout<<"The modified content of the file : \n";

//Taking the get pointer at the beginning of the file
ofstream_ob.seekg(0, ios::beg);

//Reading and displaying the modified file
while(ofstream_ob)
{
ch = ofstream_ob.get(); 
cout<<ch;
}


ofstream_ob.close();

return 0;
}

Output

Executing this program will modify the content of a file named File2.txt in the current directory, by replacing the character 'e' in it with character 'X' and the content of file looks like -

The modified content of the file :
GXndXr : MalX
AgX    : 28
City   : Tokyo
WXight : 78.6 Kg

Note :

Let's understand a few important lines of code -

```
ofstream_ob.seekp(-1, ios::cur);;
```

seekp()

put

used to write the content of a file

put

get()

get

put

By setting the first parameter of seekp() function i.e. offset position to -1.
And by setting the second parameter seekg() function i.e. flag to "ios::cur".

```
ofstream_ob.seekg(0, ios::beg);
```

seekg()

get

used to read the content of a file

By setting the first parameter of seekg() function i.e. offset position to 0.
And by setting the second parameter seekg() function i.e. flag to "ios::beg".