Istream :: tellg () returns -1 when used with my custom streambuf class?

Question

I am trying to create istreamone that is read directly from the raw memory buffer.

I found a good way to do this in another post here:

  class membuf : public basic_streambuf<char>
  {
  public:
      membuf(char* p, size_t n) {
          setg(p, p, p + n);
      }
  };

Then I create my own istreamusing this membuf:

    membuf mb(dataPointer, dataLength);
    istream reader(&mb);

Then I read using the getline()and operators >>, and everything is fine. However, I can not use seekg()to rewind back to the beginning of my buffer, but istream::tellg()always returns -1.

Do I need to write some more code to make them work, or is it doomed to failure?

Answer 1

tellg seekg seekoff seekpos, membuf.

basic_streambuf pos_type(off_type(-1)) ( -1 ).