Boost C++ Libraries: Ticket #5825: constructing function_input_iterator without consuming an item

charlie@… — Tue, 03 Jan 2012 11:32:01 GMT

I have the same problem - here's a simple example where we try to emulate a boost counting iterator. What's particularly nasty is that the two calls to make_function_input_iterator are called in arbitrary order, so the behaviour isn't even defined, as either of the two iterators could end up with the first element.

#include <boost/iterator/function_input_iterator.hpp>
#include <boost/range/algorithm.hpp>
#include <iostream>
#include <vector>
class Counter
{
public:
  typedef int result_type;
  Counter(int initial)
  : m_val(initial)
  { }
  int operator()()
  {
    std::cout << "Returning: " << m_val << std::endl;
    return m_val++;
  }
private:
  int m_val;
};
int main()
{
  Counter c(10);
  std::vector<int> v;
  std::copy(boost::make_function_input_iterator(c, 0),
            boost::make_function_input_iterator(c, 5),
            std::back_inserter(v));
  boost::copy(v, std::ostream_iterator<int>(std::cout, " "));
  std::cout << std::endl;
  return 0;
}

Here's the output:

Returning: 10
Returning: 11
Returning: 12
Returning: 13
Returning: 14
Returning: 15
Returning: 16
11 12 13 14 15

I believe the iterators should be modified to never invoke the generator initially until they are dereferenced (so in particular the second iterator should never invoke the generator if used as an end iterator).

severity changed

Dave Abrahams — Tue, 03 Jan 2012 11:51:10 GMT

severity Problem → Showstopper

I agree; this looks like a serious problem.

owner changed; cc set

Dean Michael Berris — Mon, 27 Aug 2012 11:39:33 GMT

cc mikhailberis@… added
owner changed from Dave Abrahams to Dean Michael Berris

I wasn't watching this and I've just gotten time to take a look at this now. Let me whip up a patch to fix this.

Dean Michael Berris — Mon, 27 Aug 2012 12:30:09 GMT

Update: I have a patch ready locally, just getting the necessary approvals to release the code under the Boost Software License. If things go well I should have something late Pacific Time, Monday August 27.

attachment set

Dean Michael Berris — Mon, 27 Aug 2012 23:53:42 GMT

attachment → boost-5825.patch

Fix to implementation and docs.

Dean Michael Berris — Mon, 27 Aug 2012 23:54:18 GMT

The attached patch applies to Boost trunk.

status changed

Dean Michael Berris — Mon, 27 Aug 2012 23:55:09 GMT

status new → assigned

status changed; resolution set

Dean Michael Berris — Mon, 27 Aug 2012 23:55:50 GMT

status assigned → closed
resolution → fixed

jeffrey.hellrung — Sun, 09 Sep 2012 15:33:16 GMT

(In [80467]) fix #5825; fix #7194