Re: [Boost-users] filter iterator example fails concept checks

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2003-10-21 09:15:40

Hi Chris,

It looks like there is a bug in the iterator concept checks. When I first
created the concept checks, I was using the SGI STL web pages as a
reference... and at that time I didn't know there were subtle differences
between the SGI concepts and the C++ standard. Anyways, the C++ standard
does not require an Input Iterator to be default constructible (the SGI
docs do), so the code below should be legal.

Cheers,
Jeremy

On Tue, 21 Oct 2003, Chris Saunders wrote:
ctsa> I've tried to compile one of the examples from the filter_iterator
ctsa> documentation:
ctsa>
ctsa> main()
ctsa> {
ctsa>   int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
ctsa>   const int N = sizeof(numbers)/sizeof(int);
ctsa>
ctsa>   std::copy(boost::make_filter_iterator(numbers, numbers + N,
ctsa>                                         std::bind2nd(std::greater<int>(),
ctsa> -2)),
ctsa>             boost::make_filter_iterator(numbers + N, numbers + N,
ctsa>                                         std::bind2nd(std::greater<int>(),
ctsa> -2)),
ctsa>             std::ostream_iterator<int>(std::cout, " "));
ctsa>   std::cout << std::endl;
ctsa>
ctsa> }
ctsa>
ctsa> using a version of g++ 3.3.2 that has concept-checks enabled. In this case
ctsa> the compiler rejects both of the iterators returned by
ctsa> make_filter_iterator in the above code because they do not fulfill the
ctsa> InputIterator concept required by std::copy. This is because InputIterator
ctsa> requires the DefaultConstructable concept, and the predicate:
ctsa> std::bind2nd(std::greater<int>(),-2) , isn't default constructable.
ctsa>
ctsa> My question is this: is this type of iterator (non-default-constructable)
ctsa> safe to use in general even if it fails a conceptual requirement? Of
ctsa> course the above code runs as expected with a normal g++ build and I could
ctsa> always wrap the predicate in another object if I really wanted to use
ctsa> concept-checks... but I'm interested in whether the above code is
ctsa> considered normal and safe as it stands.

----------------------------------------------------------------------
 Jeremy Siek                          http://php.indiana.edu/~jsiek/
 Ph.D. Student, Indiana Univ. B'ton   email: jsiek_at_[hidden]
 C++ Booster (http://www.boost.org)   office phone: [phone]
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