std::strstreambuf::seekoff(3)

NAME
std::strstreambuf::seekoff - std::strstreambuf::seekoff

Synopsis
   protected:
   virtual pos_type seekoff( off_type off, (deprecated in C++98)
                        ios_base::seekdir way, (removed in C++26)
                        ios_base::openmode which =
   ios_base::in | ios_base::out );
   Repositions std::basic_streambuf::gptr and/or std::basic_streambuf::pptr, if
   possible, to the position that corresponds to exactly off characters from beginning,
   end, or current position of the get and/or put area of the buffer.
     * If which includes ios_base::in and this buffer is open for reading, then
       repositions the read pointer std::basic_streambuf::gptr inside the get area as
       described below.
     * If which includes ios_base::out and this buffer is open for writing, then
       repositions the write pointer std::basic_streambuf::pptr inside the put area as
       described below.
     * If which includes both ios_base::in and ios_base::out and the buffer is open for
       both reading and writing, and way is either ios_base::beg or ios_base::end, then
       repositions both read and write pointers as described below.
     * Otherwise, this function fails.
   If the pointer (either gptr or pptr or both) is repositioned, it is done as follows:
   1) If the pointer to be repositioned is a null pointer and the new offset newoff
   would be non-zero, this function fails.
   2) The new pointer offset newoff of type off_type is determined
   a) if way == ios_base::beg, then newoff is zero
   b) if way == ios_base::cur, then newoff is the current position of the pointer
   (gptr() - eback() or pptr() - pbase())
   c) if way == ios_base::end, then newoff is the length of the entire initialized part
   of the buffer (if overallocation is used, the high watermark pointer minus the
   beginning pointer)
   3) If newoff + off is negative or out of bounds of the initialized part of the
   buffer, the function fails
   4) Otherwise, the pointer is assigned as if by gptr() = eback() + newoff + off or
   pptr() = pbase() + newoff + off

Parameters
   off - relative position to set the next pointer(s) to
           defines base position to apply the relative offset to. It can be one of the
           following constants:
   way - Constant Explanation
           beg the beginning of a stream
           end the ending of a stream
           cur the current position of stream position indicator
           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
   pos_type(newoff) on success, pos_type(off_type(-1)) on failure and if pos_type
   cannot represent the resulting stream position.

Example
// Run this code
 #include <iostream>
 #include <strstream>
 int main()
 {
     char a[] = "123";
     std::strstream ss(a, sizeof a); // in/out
     std::cout << "put pos = " << ss.tellp()
               << " get pos = " << ss.tellg() << '\n';
     // absolute positioning both pointers
     ss.rdbuf()->pubseekoff(1, std::ios_base::beg); // move both forward
     std::cout << "put pos = " << ss.tellp()
               << " get pos = " << ss.tellg() << '\n';
     // try to move both pointers 1 forward from current position
     if (-1 == ss.rdbuf()->pubseekoff(1, std::ios_base::cur))
         std::cout << "moving both pointers from current position failed\n";
     std::cout << "put pos = " << ss.tellp()
               << " get pos = " << ss.tellg() << '\n';
     // move the write pointer 1 forward, but not the read pointer
     // can also be called as ss.seekp(1, std::ios_base::cur);
     ss.rdbuf()->pubseekoff(1, std::ios_base::cur, std::ios_base::out);
     std::cout << "put pos = " << ss.tellp()
               << " get pos = " << ss.tellg() << '\n';
     ss << 'a'; // write at put position
     std::cout << "Wrote 'a' at put position, the buffer is now: '";
     std::cout.write(a, sizeof a);
     std::cout << "'\n";
     char ch;
     ss >> ch;
     std::cout << "reading at get position gives '" << ch << "'\n";
 }

Output:
 put pos = 0 get pos = 0
 put pos = 1 get pos = 1
 moving both pointers from current position failed
 put pos = 1 get pos = 1
 put pos = 2 get pos = 1
 Wrote 'a' at put position, the buffer is now: '12a'
 reading at get position gives '2'
   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 seekoff returned an undefined pos_type(off_type(-1))
                     invalid stream position on failure is returned on failure

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