Constrain distance function using is_iterator type trait

[jeremy-murphy]

distance will match anything, which gets annoying when your geometry algorithm for distance is... distance.

[jeremy-murphy]

Looks like this works in principle, now to add a unit test. (haha, so much for TDD...)

[Lastique]

One potential issue is that is_iterator is a heuristic and may return false for unconventional iterators that don't expose their category. Though category/traversal is needed to select distance_impl internally, so maybe that's a moot point.

[Lastique]

We should use std::enable_if since Boost.Iterator requires C++11 now.

[Lastique]

The same restriction should probably be applied to advance.

[jeremy-murphy]

One potential issue is that is_iterator is a heuristic and may return false for unconventional iterators that don't expose their category. Though category/traversal is needed to select distance_impl internally, so maybe that's a moot point.

Yeah, that's what I figured -- if it unconditionally uses iterator_traversal then we should be able to use that expectation to conditionally enable it. However, in the brief testing I did (clang/libc++), it didn't actually disable it, so there might be more to it. In other words, I'm more concerned about false positives than false negatives.

[jeremy-murphy]

We should use std::enable_if since Boost.Iterator requires C++11 now.

OK, sure thing.

[amlel-el-mahrouss]

@jeremy-murphy, hey, I started working on the unit test for #96.