C++ streams from memory buffer

membuf.cpp

  struct membuf : std::streambuf
  {
    membuf(char *begin, char *end) : begin(begin), end(end)
    {
      this->setg(begin, begin, end);
    }
    
    virtual pos_type seekoff(off_type off, std::ios_base::seekdir dir, std::ios_base::openmode which = std::ios_base::in) override
    {
      if(dir == std::ios_base::cur)
        gbump(off);
      else if(dir == std::ios_base::end)
        setg(begin, end+off, end);
      else if(dir == std::ios_base::beg)
        setg(begin, begin+off, end);
      
      return gptr() - eback();
    }
    
    virtual pos_type seekpos(streampos pos, std::ios_base::openmode mode) override
    {
      return seekoff(pos - pos_type(off_type(0)), std::ios_base::beg, mode);
    }
    
    char *begin, *end;
  };
  
  static istream *currentReadStream;
  
  /* from memory.. */
  
  membuf sbuf(buf, buf+dataSz);
  std::istream in(&sbuf);
  currentReadStream = ∈

  /* or from file */
  std::ifstream file(filename, ios::binary);
  currentReadStream = &file;