Set position with seekg() in C++ language file handling

seekg() is a function in the iostream library (part of the standard library) that allows you to seek to an arbitrary position in a file. It is used in file handling to sets the position of the next character to be extracted from the input stream from a given file. For example :

Input : "Hello World" 
Output : World

Syntax – There are two syntax for seekg() in file handling :

  1. istream&seekg(streampos position);
  2. istream&seekg(streamoff offset, ios_base::seekdir dir);

Description –

  • position : is the new position in the stream buffer.
  • offset : is an integer value of type streamoff representing the offset in the stream’s buffer. It is relative to the dir parameter.
  • dir : is the seeking direction. It is an object of type ios_base::seekdir that can take any of the following constant values.

There are 3 direction we use for offset value :

  • ios_base::beg (offset from the beginning of the stream’s buffer).
  • ios_base::cur (offset from the current position in the stream’s buffer).
  • ios_base::end (offset from the end of the stream’s buffer).

Code –



// Code to demonstrate the seekg function in file handling
#include <fstream>
#include <iostream>
  
using namespace std;
   
int main (int argc, char** argv)
{
    // Open a new file for input/output operations
    // discarding any current in the file (assumes 
    // a length of zero on opening)
    fstream myFile("test.txt", ios::in | ios::out | ios::trunc);
      
    // Add the characters "Hello World" to the file
    myFile << "Hello World";
      
    // Seek to 6 characters from the beginning of the file
    myFile.seekg(6, ios::beg);
      
    // Read the next 5 characters from the file into a buffer
    char A[6];
    myFile.read(A, 5);
      
    // End the buffer with a null terminating character
    A[5] = 0;
      
    // Output the contents read from the file and close it 
    cout << buffer << endl;
      
    myFile.close();

Output –

World

Note –
If we previously get an end of file on the stream, seekg will not reset it but will return an error in many implementations. – use the clear() method to clear the end of file bit first. This is a relatively common mistake and if seekg() is not performing as expected.

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