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.
-Gabe
[1] http://www.boost.org/doc/libs/1_53_0/libs/iterator/doc/iterator_facade.html#...
====================
#include
#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';
}
====================
