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?

Do you _know_ that `seekg()` isn't rewinding? Have you proven that `tellg()` works properly? — Lightness Races in Orbit, Jul 20 '11 at 15:02
Yes, after calling seekg(0), getline() doesn't read my first line. The tellg() function definitely doesn't work, as it always returns -1, (even when I just started reading from my buffer). I believe Bo's answer is the way to go here... — EdSanville, Jul 20 '11 at 15:17

score 7 · Accepted Answer · edited Jan 28 '16 at 15:11

7

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).

edited Jan 28 '16 at 15:11

Patryk

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answered Jul 20 '11 at 14:53

Bo Persson

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Thank you very much! I will implement those right away. I'm curious though, how does istream::tellg() use those functions? Does it call streambuf::seekoff(0, ios_base::cur) and get the return value? – EdSanville Jul 20 '11 at 15:02
In effect, yes. It actually calls pubseekoff, which then calls seekoff, but that's an interface design detail. :-) – Bo Persson Jul 20 '11 at 15:06
2

I am trying to do this... implement `seekoff` & `seekpos` on a custom membuf class, except there's a problem in my code: `seekoff` is not being called consistently by `tellg` and `seekpos` is not called at all. Any suggestions or working examples? – Brian McFarland Jun 30 '15 at 20:16

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

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