Boost C++ Libraries: Ticket #13094: boost::adaptors::transform fails to preserve bidirectional behavior of range

Sat, 24 Jun 2017 00:52:08 GMT

By the way I see this with both clang and Visual Studio libraries. It seems that std::advance, std::prev, etc are relying on std::iterator_traits<Iter>::iterator_category tag dispatch, which for std::vector yields:

std::random_access_iterator_tag

But for boost::adaptors::transform yields:

boost::iterators::detail::iterator_category_with_traversal<std::input_iterator_tag, boost::iterators::random_access_traversal_tag>

It looks like there may be code for boost to understand the standard library iterator_category tags in terms of traversals, but the opposite does not work; this causes behavior changes when passing to code using standard library conventions when the code is modified from passing standard types to boost adaptor types.

status changed; resolution set

Michel Morin — Sat, 24 Jun 2017 02:50:36 GMT

status new → closed
resolution → invalid

Random Access Traversal Iterator (in the Boost iterator traversal concepts) is not the same thing as Random Access Iterator (in the Standard iterator categories).

The former just specifies traversal requirements, but the latter also specifies value access requirements. (For more details, please consult to the documentation of Boost.Iterator.) For example, Random Access Iterator requires that iterator's reference type is a real reference. The iterator in the test case fails to satisfy this requirement.

That being said, I think it would be useful to have boost::advance, which is parameterized by the Boost iterator traversal concepts. I'm going to ask about it on the developer ML.