Boost C++ Libraries: Ticket #8617: Surprising/wrong semantics of counting_range

Nathan Ridge — Thu, 30 May 2013 22:41:42 GMT

I confess I find it surprising that an overload of counting_range() that takes a range exists at all.

I agree that constructing a counting range from the first and last values of a range seems arbitrary, but constructing a counting range from the begin and end *iterator* of a range seems pointless (how would the resulting range be different from the original one?).

Neil, what use cases did you have in mind for this overload of counting_range()?

status changed

Neil Groves — Fri, 21 Feb 2014 20:37:27 GMT

status new → assigned

I believe this is just very wrong. I'll sort this out. I'm inclined to remove counting_range to avoid a surprising change of semantic and replace it with something that is less bonkers.

marc.glisse@… — Fri, 21 Feb 2014 23:25:09 GMT

Replying to nathanridge:

I confess I find it surprising that an overload of counting_range() that takes a range exists at all.

I agree that constructing a counting range from the first and last values of a range seems arbitrary, but constructing a counting range from the begin and end *iterator* of a range seems pointless (how would the resulting range be different from the original one?).

It should be the reciprocal of adaptors::indirect. In a range for loop, there is an implicit iterator iter going from begin to end, and you get *iter for each element. With indirect, you get **iter. With counting_range (constructed from iterators, because the constructor from a range is too weird), you get iter (not dereferenced). It can be extremely useful and should be available in boost adaptors with the usual | syntax and a clearer name. This is a documented use of counting_iterator.

status changed; resolution set

Neil Groves — Thu, 27 Feb 2014 17:41:23 GMT

status assigned → closed
resolution → fixed

The counting_range is now just a dumb wrapper for a range of counting_iterator with entirely consistent semantics to the underlying iterator. I agree that other counting type ranges and adapters are useful but that's not this ticket. This ticket is about the defect of the counting_range function that takes ranges.

The adaptor that has been mentioned may be useful, but I don't want to make it a counted adaptor. This is just too bonkers even for my code! I am happy to create something else for doing the opposite of indirecting (directing?!). I shall consider adding a directed adaptor.

Nathan Ridge — Thu, 27 Feb 2014 17:52:56 GMT

Replying to neilgroves:

I am happy to create something else for doing the opposite of indirecting (directing?!). I shall consider adding a directed adaptor.

In P-Stade Oven it is called 'outdirected': http://p-stade.sourceforge.net/oven/doc/html/oven/range_adaptors.html#oven.range_adaptors.outdirected. (I'm not necessarily saying we should call it the same, just pointing out the "prior art".)