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

Question

I'm trying to create an istream that reads directly from a raw memory buffer.

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

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

Then I create my istream using this membuf:

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

I then read using getline() and >> operators, and everything is wonderful. However, I can't seem to use seekg() to rewind back to the beginning of my buffer, and istream::tellg() always returns -1.

Do I need to write some more code to get these to work, or is this doomed to failure?

Answer 1

The functions tellg and seekg depends on protected virtual functions seekoff and seekpos, that you would have to implement in your membuf class.

The defaults in basic_streambuf just returns pos_type(off_type(-1)) for all calls (which might be equal to -1 for many implementaions).