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File Positions

Introduction to File Positions in C++

In C++, file handling is a critical feature that allows us to work with files. It provides us with the ability to create, write, read, and update files. An essential aspect of file handling is file positioning, which helps us navigate through a file. This tutorial will focus on understanding the concept of file positions and how to work with them in C++.

Understanding File Positions​

When a file is opened in C++, an internal pointer is used to keep track of where we are in the file. This pointer is known as the 'file position pointer.' This pointer moves as we read or write data in the file. The position of this pointer is crucial as it determines from where the reading or writing operation will take place in a file.

File Position Functions​

C++ provides several functions to control the position of the file position pointer. These functions are defined in the fstream library. They include:

  1. seekg(): This function is used with input streams. It changes the location of the get pointer. The get pointer determines the next location to be read in the file.

  2. seekp(): This function is used with output streams. It changes the location of the put pointer. The put pointer determines the next location to be written in the file.

  3. tellg(): This function is used with input streams. It gives the current position of the get pointer.

  4. tellp(): This function is used with output streams. It gives the current position of the put pointer.

Using seekg() and seekp()​

The seekg() and seekp() functions take two arguments:

  1. The offset: This is the number of positions the pointer should move from the position specified by the second argument.

  2. The direction: This determines from where the offset should be applied. It can be ios::beg(from the beginning), ios::cur(from current position), or ios::end(from the end).

Here's an example of using seekg():

ifstream file;
file.open("example.txt");
file.seekg(10, ios::beg); // Moves the get pointer 10 positions from the beginning

And here's an example of using seekp():

ofstream file;
file.open("example.txt");
file.seekp(0, ios::end); // Moves the put pointer to the end of the file

Using tellg() and tellp()​

The tellg() and tellp() functions do not take any arguments. They return the current position of the get pointer and put pointer, respectively.

Here's an example of using tellg():

ifstream file;
file.open("example.txt");
cout << "Current position is: " << file.tellg();

And here's an example of using tellp():

ofstream file;
file.open("example.txt");
cout << "Current position is: " << file.tellp();

Conclusion​

Understanding file positions is crucial when working with files in C++. You can navigate through files, read from specific positions, or write at specific positions using the file position functions. This allows for more flexibility and control over file handling in your C++ programs.