cpp/experimental/ranges/iterator/SizedSentinel

The concept specifies that an object of the iterator type  and an object of the sentinel type  can be subtracted to compute the distance between them in constant time.

Let be an iterator of type, and  a sentinel of type  such that  denotes a range. Let be the smallest number of applications of  necessary to make  be true. Then is satisfied only if:


 * If is representable by, then  is well-defined and equals ; and
 * If is representable by, then  is well-defined and equals.

The variable template provides a mechanism for iterators and sentinels that can be subtracted but do not meet the semantic requirements of  to opt out of the concept by specializing the variable template to have the value.