Frank Mori Hess wrote:
It seems to me the purpose of the cache in the slot_call_iterator is to prevent the slot from being run multiple times if the combiner does multiple dereferences on an iterator. However, since the cache is shared, if a copy of an already dereferenced iterator is made and incremented, then the original iterator will run the slot a second time when dereferenced. So is this a bug, or is it just unspecified what will happen if you dereference a slot iterator more than once?
Such a combiner would violate the single pass traversal limitation of the input iterator concept. The cache solution allows dereferencing the same iterator multiple times to acquire the same value without calling the slot again, which already is a stronger guarantee than an input iterator usually provides. The combiner example in the docs makes use of it. Regards Timmo Stange