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In our last article, we have explained how to use the a very important file output/write function - put() function, used to write a character to the file. In this tutorial, we are going to explain how modify the content of an existing file, but before we do that, it is critical to understand the pointer associated with each file, which allow to read the content of a file on disk. This pointer is called get pointer.

File Pointer	Description
get	The get pointer allows us to read the content of a file when we open the file in read-only mode. It automatically points at the beginning of file, allowing us to read the file from the beginning.

Before we show you how to access and manipulate with the get pointer to read the content of a file on the disk, we must show you how to open the file. For this, the C++ language provides us stream classes used to perform file input/read operation.

File I/O Stream Classes

In order to perform a file input operation using the get pointer, C++ provides us a few file stream classes, such as -

File I/O Stream Classes	Description
ifstream	This class is used to create an input stream and gives access to get pointer, to perform the file read operation.
fstream	This class is used to used to create an input-output stream and gives access to both get and put pointers, to perform binary file read/write operations.



These 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 see the file mode that we require to use, in order to use the put() function to perform input/read operation on a file.




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File mode to work with get pointer


Let's look at the combination of modes required to read the content of a file while working with the get pointer.



File Mode	Description
ios::in	Searches for the file and opens it in read mode only and gives access to get pointer, to read the content of a file.








Functions to work with the get pointer


When we open the file in a read-only mode i.e. "ios::in", the get pointer is automatically set the beginning of the file, to allow us to read the file from its start. But we are also provided some functions which allow us to -
• Access the current location of the get pointer.
• Allow us to set the get pointer at the particular location in a file.




Functions to use get pointer	Description
tellg()	Gives us the current location of the get pointer. When the file is opened in a read-only mode, tellg() returns zero i.e. the beginning of the file.
seekg (streampos position );	This function is used to set the location of the get pointer to a desired position/offset. The position variable is the new position in the file i.e. an integer value representing the number of bytes from the beginning of the file.
seekg ( streamoff offset, ios_base::seekdir dir );	This function is used to set the location of the get pointer to a desired position/offset. The position variable is the new position in the file i.e. an integer value representing the number of bytes from the beginning of the file. The position variable is relative to the dir parameter. dir is the seeking direction and it can take any of the following constant values: ios::beg - offset from the beginning of the file. ios::cur - offset from the current position in the file. ios::end - offset from the end of the file.








Example to use tellg() and seekg() functions



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

File1.txt

10
a
9.99
Hello World!
Keep Smiling!


Now let's try to read(twice) the content of this file by working with the get pointer and seekg() function. 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 input stream to read the content of a file
ifstream ifstream_ob;


//Opening a file named country1.txt to read its content
ifstream_ob.open("File1.txt", ios::in);

cout<<"The first location in the file : " <<ifstream_ob.tellg() << "\n";

char ch;


cout<<"\nReading the content of file : \n";

//Read the file until EOF is reached
while(ifstream_ob)
{
ch = ifstream_ob.get(); 
cout<<ch;	
}

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

cout<<"\n\nReading the content of file once again : \n";


//Taking the get pointer at the zero byte location from the beginning of the file
ifstream_ob.seekg(0, ios::beg);

//Reading the content of the file again
while(ifstream_ob)
{
ch = ifstream_ob.get(); 
cout<<ch;
}

return 0;
}








Output




The first location in the file : 0

Reading the content of file :
10
a
9.99
Hello World!
Keep Smiling!

Reading the content of file once again :
10
a
9.99
Hello World!
Keep Smiling!


As you can see in the output, the program has twice read the same file by using the seekg() function to manipulate with the get pointer.






Program Analysis


Let's understand an important line of code -



• ifstream_ob.seekg(0, ios::beg);



This line of code calls the seekg()function which takes the get pointer(used to read the content of a file) to the beginning of the file after we have already read the file once, only to read it for the second time. This has been made possible by -
• 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".





• ofstream_ob.clear();


Once the reading of content of a file completes, the EOF(End of file) is reached and the EOF flag is set on which does not allow us to access of file again for reading it again. Hence, a call to clear() function is very critically important because it sets the EOF flag off, which allows us to re-read the content of file.

In the next article, we are going to show you how to work and manipulate with the put pointer to write the content of a file on disk. So, let's turn the page then!




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