On 1/29/13 6:56 PM, Rob Stewart wrote:
On Jan 28, 2013, at 5:12 PM, Joel de Guzman <djowel@gmail.com> wrote:
On 1/29/13 5:58 AM, Gottlob Frege wrote:
How do I know if is it singular valued?
>
How do you know if an iterator is singular valued? You can't. Same here.
it == container.end()?
Or am I confused?
Nope. A singular valued iterator may not be compared. A singular valued iterator is not associated with any sequence. E.g default constructed iterator pointing to nowhere.
Singular iterators are default constructed instances that represent a universal end iterator. They are not associated with any particular container, but would be returned by end(), and can be obtained by advancing a non-end iterator far enough. Thus, Tony's snippet is a means to discover a singular instance, as would comparing with a default constructed iterator. Is the same now to be possible with recursive_wrapper?
Singular valued iterators cannot be compared (*). The only valid operations are assignment, destruction and move. There is no way to detect a singular valued iterator. (* Iterators can also have singular values that are not associated with any sequence. [ Example: After the declaration of an uninitialized pointer x (as with int* x;), x must always be assumed to have a singular value of a pointer. —end example ] Results of most expressions are undefined for singular values; the only exceptions are destroying an iterator that holds a singular value, the assignment of a non-singular value to an iterator that holds a singular value, and, for iterators that satisfy the DefaultConstructible requirements, using a value-initialized iterator as the source of a copy or move operation.) Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com