# itk::NeighborhoodRange: a new class for efficient modern C++ style iteration

**URL:** <https://discourse.itk.org/t/itk-neighborhoodrange-a-new-class-for-efficient-modern-c-style-iteration/833>\
**Category:** Engineering\
**Created:** [April 10, 2018, 7:37pm UTC](https://discourse.itk.org/t/itk-neighborhoodrange-a-new-class-for-efficient-modern-c-style-iteration/833 "2018-04-10T19:37:56Z")\
**Posts on this page:** 1\
**Showing post:** 37

<div class="post-metadata">

**Author:** ![Niels\_Dekker](https://discourse.itk.org/letter_avatar_proxy/v4/letter/n/9d8465/32.png) [@Niels\_Dekker](https://discourse.itk.org/u/Niels_Dekker)\
**Post date:** [April 25, 2018, 2:18pm UTC](https://discourse.itk.org/t/itk-neighborhoodrange-a-new-class-for-efficient-modern-c-style-iteration/833/37 "2018-04-25T14:18:50Z")

</div>

I’m preparing another patch set which I’ll try to submit later today. It should include renaming the range class, as we discussed here. Hopefully this will be the final patch set before the merge to the master branch of ITK!

But there’s something I need to tell you… While developing the range class, I was informed by two C++ experts, [Anthony Williams](https://github.com/anthonywilliams) and [Jonathan Wakely](https://github.com/jwakely), that the iterators I’m proposing here do not entirely satisfy the iterator requirements as specified by the current C++ Standard! 😮 So what does that mean…?

itk::NeighborhoodRange::iterator supports everything you would expect from a bidirectonal iterator: `++it, --it, it++, it--, *it, auto it2{it1}, it2 = it1, it1 == it2, it1 != it2`. However, it does not satisfy the following requirements (quoted from [the current Working Draft of the C++ Standard](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4741.pdf)):

From section [iterator.requirements.general]:

> An iterator i for which the expression (\*i).m is well-defined supports the expression i-\>m with the same semantics as (\*i).m.

From section [forward.iterators]:

> - if `X` is a mutable iterator, `reference` is a reference to `T`; if `X` is a constant iterator, `reference` is a reference to `const T`

itk::NeighborhoodRange::iterator does not have an `operator->()`, so it does not support it-\>m. Moreover, itk::NeighborhoodRange::iterator::reference (the return type of iterator::operator\*()) is not a real C++ reference type (`PixelType &`), instead, it’s a proxy class (`PixelProxy`).

Strictly speaking, that implies that itk::NeighborhoodRange::iterator is not _guaranteed_ to work well as argument to a C++ Standard Library function that requires a standard compliant iterator.

Fortunately itkNeighborhoodRangeGTest.cxx tests such use cases, and they all pass successfully:

```
std::vector<PixelType>(range.cbegin(), range.cend());
std::reverse_copy(range.begin(), range.end(), stdVector.begin());
std::inner_product(range.begin(), range.end(), range.begin(), 0.0);
std::for_each(range.begin(), range.end(), [](const PixelType&){});

```

Moreover: some iterators in the C++ Standard Library don’t satisfy these particular requirement either! `std::istreambuf_iterator` does not have an `operator->()` (as [reported](https://cplusplus.github.io/LWG/issue2790) by Jonathan Wakely) And `std::vector<bool>::iterator` also has a proxy as `iterator::reference` type.

Even more hopeful: There is a very detailed proposal to get full support for proxy iterators into the C++ Standard Library: [Proxy Iterators for the Ranges Extensions](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0022r2.html) by [Eric Niebler](https://github.com/ericniebler).

Summary: `itk::NeighborhoodRange::iterator` ([patch set 19](http://review.source.kitware.com/#/c/23326/19)) may not yet _officially_ satisfy all iterator requirements from the current C++ Standard, but in practice, it just appears to work well, as argument to an `std` function. And of course, the other advantages are still there as well: a significant performance improvement, support for multi-threading and support for _range-based for loops_. 😃

---

_[View the full topic](https://discourse.itk.org/t/itk-neighborhoodrange-a-new-class-for-efficient-modern-c-style-iteration/833)._