on Sun Feb 10 2013, Evgeny Panasyuk <evgeny.panasyuk-AT-gmail.com> wrote:
10.02.2013 17:10, Ian Hobson:
Is there any interest in a version of for_each that operates on each adjacent pair of elements in a range? [...] Initial implementation available at http://github.com/irh/adjacent_for_each
I have implemented for_each_adjacent on project I worked on (sorry, can't share source). I had two versions: for InputIterator and ForwardIterator. Selection was implemented as tag dispatching of iterator_category.
You need special version for InputIterator which maintains temporary copy of value_type, due to 24.1.1/3:
[Note: For input iterators, a == b does not imply ++a == ++b. (Equality does not guarantee the substitution property or referential transparency.) Algorithms on input iterators should never attempt to pass through the same iterator twice. They should be single pass algorithms.]
In this regard your version at https://github.com/irh/adjacent_for_each/blob/master/boost/algorithm/adjacen... is broken. Because it advertises to work on InputIterator, but it dereferences iterator at same position several times. As the result current implementation is for ForwardIterator.
That's incorrect. The mental model for InputIterator is an ephemeral input stream *with a one-element backing buffer*. You are allowed to dereference the same InputIterator multiple times at the same position until that iterator or a copy of it is incremented. Table 107 spells this out: +---------------+----------------+------------+-------------------------------+ |Expression |Return Type |Operational | Assertion/note | | | |Semantics |pre-/post-condition | +===============+================+============+===============================+ |*a |convertible to T| |pre: a is dereferenceable. | | | | |**The expression (void)*a, *a | | | | |is equivalent to *a.** | | | | |(emphasis mine). | +---------------+----------------+------------+-------------------------------+ -- Dave Abrahams