std::basic_filebuf::seekpos(3)

NAME
std::basic_filebuf::seekpos - std::basic_filebuf::seekpos

Synopsis
   protected:
   virtual pos_type seekpos( pos_type sp,
   std::ios_base::openmode which = std::ios_base::in | std::ios_base::out );
   Repositions the file pointer, if possible, to the position indicated by sp.
   If the associated file is not open (is_open()==false), fails immediately.
   If the file is open for writing, first writes the put area and any unshift sequences
   required by the currently imbued locale, using overflow()
   Then repositions the file pointer, as if by calling std::fsetpos()
   If the file is open for reading, updates the get area if necessary.
   If sp was not obtained by calling seekoff() or seekpos() on the same file, the
   behavior is undefined.

Parameters
   sp - file position obtained by seekoff() or seekpos() called earlier on the same
           file
           defines which of the input and/or output sequences to affect. It can be one
           or a combination of the following constants:
   which -
           Constant Explanation
           in affect the input sequence
           out affect the output sequence

Return value
   sp on success or pos_type(off_type(-1)) on failure.

Notes
   seekpos() is called by std::basic_streambuf::pubseekpos(), which is called by the
   single-argument versions of std::basic_istream::seekg() and
   std::basic_ostream::seekp()
   Many implementations do not update the get area in seekpos(), delegating to
   underflow() that is called by the next sgetc()

Example
   on some implementations, the get area is emptied by seekpos() and the second
   underflow() is necessary to observe the effects // Run this code
 #include <fstream>
 #include <iostream>
 struct mybuf : std::filebuf
 {
     pos_type seekpos(pos_type sp, std::ios_base::openmode which) {
          std::cout << "Before seekpos(" << sp << "), size of the get area is "
                    << egptr()-eback() << " with "
                    << egptr()-gptr() << " read positions available\n";
          pos_type rc = std::filebuf::seekpos(sp, which);
          std::cout << "seekpos() returns " << rc << ".\nAfter the call, "
                    << "size of the get area is "
                    << egptr()-eback() << " with "
                    << egptr()-gptr() << " read positions available\n";
 // uncomment if get area is emptied by seekpos()
 // std::filebuf::underflow();
 // std::cout << "after forced underflow(), size of the get area is "
 // << egptr()-eback() << " with "
 // << egptr()-gptr() << " read positions available\n";
         return rc;
     }
 };
 int main()
 {
     mybuf buf;
     buf.open("test.txt", std::ios_base::in);
     std::istream stream(&buf);
     stream.get(); // read one char to force underflow()
     stream.seekg(2);
 }

Possible output:
 Before seekpos(2), size of the get area is 110 with 109 read positions available
 seekpos() returns 2.
 After the call, size of the get area is 110 with 108 read positions available

See also
   pubseekpos invokes seekpos()
              (public member function of std::basic_streambuf<CharT,Traits>)
   seekoff repositions the file position, using relative addressing
   [virtual] (virtual protected member function)
   fseek moves the file position indicator to a specific location in a file
              (function)