basic_istream::seekg

Moves to a variable position in a stream.

  basic_istream<charT, traits>& seekg(pos_type);
  basic_istream<charT, traits>& seekg
  (off_type, ios_base::seekdir dir);  

The function seekg is overloaded to take a pos_type object, or an off_type object (defined in basic_ios class.) The function is used to set the position of the get pointer of a stream to a random location for character extraction.

The this pointer is returned.

basic_streambuf::pubseekoff() and pubseekpos().

// The file ewl-test contains:
// ABCDEFGHIJKLMNOPQRSTUVWXYZ

#include <iostream>
#include <fstream>
#include <cstdlib>

int main()
{
using namespace std;
   ifstream in("ewl-test");
   if(!in.is_open())
   {cout << "could not open file"; exit(1);} 

// note streampos is typedef in iosfwd
   streampos spEnd(5), spStart(5);
   in.seekg(spStart);
   streampos aCheck = in.tellg();
   cout << "The offfset at the start of the reading in bytes is " 
      << aCheck << endl; 

   char ch;
   while(spEnd != spStart+10) 
   {
      in.get(ch);
      cout << ch;
      spEnd = in.tellg();
   }

   aCheck = in.tellg();
   cout << "\nThe current position's offset in bytes now is " 
      << aCheck << endl; 

   streamoff gSet = 0;       
   in.seekg(gSet, ios::beg);
   aCheck = in.tellg();
   cout << "The final position's offset in bytes now is " 
   << aCheck << endl; 

   in.close();
   return 0;
}

Result:

  The offfset at the start of the reading in bytes is 5
  FGHIJKLMNO
  The current position's offset in bytes now is 15
  The final position's offset in bytes now is 0