Set position of internal position pointer.
  Calls protected virtual member seekoff.
  The standard behavior of function seekoff in derived stream buffer classes is to set a new position value for an internal position pointer. Often, strembuf derived object have independent input and output position pointers, which parameter determines which one is affected.

Parameters.

off

Offset value. This is relative to way parameter.
It is an integer value, an object of type streamoff.

way

It is an object of type ios_base::seekdir. It may take any of the following constant values:

• 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::cur (offset from the end of the stream's buffer).

which

Determines which of the internal position pointers shall be modified, either in, out or both. It is an object of type ios_base::openmode that may take any combination of the following significant constant values:

• ios_base::in Modify input position.
• ios_base::out Modify output position.

Return Value.
  The new position value of the modified position pointer.

Example.

// get file size using pubseekoff
#include <iostream>
#include <fstream>
using namespace std;

int main () {

  long size;
  filebuf* pbuf;

  fstream filestr ("test.txt");
  pbuf = filestr.rdbuf();
  size = pbuf->pubseekoff(0,ios_base::end);
  filestr.close();

  cout << "size of file is " << size << endl;

  return 0;
}

Basic template member declaration ( basic_streambuf<charT,traits> ):

typedef traits::pos_type pos_type; typedef traits::off_type off_type; pos_type pubseekoff (off_type off, ios_base::seekdir way, ios_base which = ios_base::in | ios_base::out );

See also.
  pubseekpos
  streambuf class