std::basic_stringbuf::seekoff(3)

NAME
std::basic_stringbuf::seekoff - std::basic_stringbuf::seekoff

Synopsis
   protected:
   virtual pos_type seekoff(off_type off,
   ios_base::seekdir dir,
   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 (that is, if
       ((which & ios_base::in) == ios_base::in), 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 (that is,
       (which & ios_base::out) == ios_base::out), 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 ((which & (ios_base::in|ios_base::out))
       ==(ios_base::in|ios_base::out)), and dir 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 dir == ios_base::beg, then newoff is zero
   b) if dir == ios_base::cur, then newoff is the current position of the pointer
   (gptr()-eback() or pptr()-pbase())
   c) if dir == 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 < 0 (the repositioning would move the pointer to before the
   beginning of the buffer) or if newoff + off would point past the end of the buffer
   (or past the last initialized character in the buffer if overallocation is used),
   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:
   dir - 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 <sstream>
 int main()
 {
     std::stringstream ss("123"); // 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 1 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 " << ss.str() << '\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'

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