std::strstreambuf::seekpos(3)

NAME
std::strstreambuf::seekpos - std::strstreambuf::seekpos

Synopsis
   protected:
   virtual pos_type seekpos( pos_type sp, (deprecated in C++98)
                        std::ios_base::openmode which = (removed in C++26)
                        std::ios_base::in |
   std::ios_base::out );
   Repositions std::basic_streambuf::gptr and/or std::basic_streambuf::pptr, if
   possible, to the position indicated by sp.
   If std::ios_base::in is set in which, attempts to reposition gptr() (the next
   pointer in the get area). If std::ios_base::out is set in which, attempts to
   reposition pptr() (the next pointer in the put area). If neither bit is set in
   which, the operation fails.
   Each next pointer is repositioned as follows:
     * If the next pointer is null, the operation fails.
     * Otherwise, the new offset newoff (of type off_type) is determined by calling
       sp.offset(). If newoff is negative, out of bounds of the buffer, or invalid, the
       operation fails.
     * Otherwise, the next pointer is assigned as if by gptr() = eback() + newoff or
       pptr() = pbase() + newoff.

Parameters
   sp - stream position, such as one obtained by seekoff() or seekpos()
           defines whether the input sequences, the output sequence, or both are
           affected. 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
   The resultant offset converted to pos_type 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().

Example
// Run this code
 #include <cstring>
 #include <iostream>
 #include <strstream>
 struct mybuf : std::strstreambuf
 {
     mybuf(const char* str) : std::strstreambuf(str, std::strlen(str)) {}
     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::strstreambuf::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";
         return rc;
     }
 };
 int main()
 {
     mybuf buf("12345");
     std::iostream stream(&buf);
     stream.seekg(2);
 }

Output:
 Before seekpos(2), size of the get area is 5 with 5 read positions available.
 seekpos() returns 2.
 After the call, size of the get area is 5 with 3 read positions available.
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
     DR Applied to Behavior as published Correct behavior
   LWG 55 C++98 seekpos returned an undefined pos_type(off_type(-1))
                     invalid stream position on failure is returned on failure

See also
   seekoff repositions the next pointer in the input sequence, output sequence, or
   [virtual] both, using relative addressing
             (virtual protected member function)
             repositions the next pointer in the input sequence, output sequence, or
   seekpos both using absolute addressing
   [virtual] (virtual protected member function of std::basic_streambuf<CharT,Traits>)
             repositions the next pointer in the input sequence, output sequence, or
   seekpos both using absolute addressing
   [virtual] (virtual protected member function of
             std::basic_stringbuf<CharT,Traits,Allocator>)
   seekpos repositions the file position, using absolute addressing
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)