Re: [boost] [iterator] Code/documentation mismatch in iterator_facade

Subject: Re: [boost] [iterator] Code/documentation mismatch in iterator_facade
From: Gabriel Redner (gredner_at_[hidden])
Date: 2013-04-23 14:42:21

Thanks - I feared that that would be the case!  However, as long as
the docs match the code (and preferably call out this unexpected
behavior) I don't think it's a major issue.

Bug filed: https://svn.boost.org/trac/boost/ticket/8488

Thanks,
-Gabe

On Tue, Apr 23, 2013 at 2:22 PM, Jeffrey Lee Hellrung, Jr.
<jeffrey.hellrung_at_[hidden]> wrote:
> On Tue, Apr 23, 2013 at 11:00 AM, Gabriel Redner <gredner_at_[hidden]> wrote:
>
>> Hi folks,
>>
>> I've stumbled upon an issue with iterator_facade.  When creating a
>> random access facade, one must implement the distance_to member
>> function in order for std::distance to work.  However, it looks like
>> the arguments passed to this method are reversed from what the
>> documentation states.
>>
>> Specifically, the docs [1] say (paraphrased):  a.distance_to(b) is
>> equivalent to distance(a, b).
>>
>> However, when I call std::distance(a, b), I find that b.distance_to(a)
>> is invoked in the facade - the mirror image of what the docs say!
>> This is not a problem if just wrapping some other random access
>> iterator, but in my case the calculation of distance is nontrivial and
>> this issue led to a rather tiresome debugging session and the solution
>> (negating before returning from distance_to, knowing that
>> iterator_facade will re-negate my response) is annoying to explain.
>>
>> Should I file a bug?
>>
>> A test case is appended to this message.
>>
>
> This looks like it could potentially be a bug. File a trac ticket. I'll
> have to look at the documentation a bit closer and also peruse the tests,
> but at a first glance it doesn't look good :/
>
> If it is a bug, I'm afraid the only viable thing to do is fix the
> documentation to match the code (rather than fix the code to match the
> documentation).
>
> - Jeff
>
> -Gabe
>>
>> [1]
>> http://www.boost.org/doc/libs/1_53_0/libs/iterator/doc/iterator_facade.html#iterator-facade-requirements
>>
>> ====================
>> #include <boost/iterator/iterator_facade.hpp>
>>
>> #include <vector>
>> #include <iterator>
>> #include <iostream>
>>
>> class vector_iterator
>>   : public boost::iterator_facade<
>>       vector_iterator,
>>       int,
>>       boost::random_access_traversal_tag>
>> {
>> public:
>>
>>   vector_iterator(char tag, std::vector<int>::iterator it) :
>> _tag(tag), _it(it) {}
>>
>> private:
>>   friend class boost::iterator_core_access;
>>
>>   int& dereference() const { return *_it; }
>>   bool equal(vector_iterator const& other) const { return _it ==
>> other._it; }
>>   void increment() { ++_it; }
>>   void decrement() { --_it; }
>>   void advance(int n) { std::advance(_it, n); }
>>
>>   std::vector<int>::iterator::difference_type
>>   distance_to(const vector_iterator& other) const
>>   {
>>     std::cerr << _tag << ".distance_to(" << other._tag << ")\n";
>>     std::vector<int>::iterator::difference_type result =
>> std::distance(_it, other._it);
>>     std::cerr << "result = " << result << '\n';
>>     return result;
>>   }
>>
>> private:
>>   const char _tag;
>>   std::vector<int>::iterator _it;
>> };
>>
>> int main()
>> {
>>   std::vector<int> v;
>>   v.push_back(1);
>>   v.push_back(2);
>>
>>   vector_iterator a('a', v.begin());
>>   vector_iterator b('b', v.end());
>>
>>   std::cerr << std::distance(a, b) << '\n';
>> }
>> ====================
>>
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