How to implement custom std :: streambuf seekoff ()?

Question

I have the following implementation based on, for example, this question and answer

struct membuf : std::streambuf
{
  membuf(char* begin, char* end)
  {
    this->setg(begin, begin, end);
  }

protected:
  virtual pos_type seekoff(off_type off,
                           std::ios_base::seekdir dir,
                           std::ios_base::openmode which = std::ios_base::in)
  {
    std::istream::pos_type ret;
    if(dir == std::ios_base::cur)
    {
      this->gbump(off);
    }
    // something is missing here...
  }
};

I would like to use it in my methods as follows:

  char buffer[] = { 0x01, 0x0a };
  membuf sbuf(buffer, buffer + sizeof(buffer));
  std::istream in(&sbuf);

and then call for example. tellg()on inand get the correct result.

While this is almost perfect - it does not stop at the end of the stream.

How do I update it so that it works correctly?

My main motivation is to simulate behavior std::ifstream, but with the binary char[]filed in them in the tests (instead of relying on binary files).

Answer 1

, . , seekoff :

  pos_type seekoff(off_type off,
                   std::ios_base::seekdir dir,
                   std::ios_base::openmode which = std::ios_base::in)
  {
    if (dir == std::ios_base::cur) gbump(off);

    return gptr() - eback();
  }

Answer 2

, std::ios_base::beg, std::ios_base::end. , :

pos_type seekoff(off_type off,
                 std::ios_base::seekdir dir,
                 std::ios_base::openmode which = std::ios_base::in) {
  if (dir == std::ios_base::cur)
    gbump(off);
  else if (dir == std::ios_base::end)
    setg(eback(), egptr() + off, egptr());
  else if (dir == std::ios_base::beg)
    setg(eback(), eback() + off, egptr());
  return gptr() - eback();
}