22 Iterators library [iterators]

22.3 Iterator requirements [iterator.requirements]

22.3.4 Iterator concepts [iterator.concepts]

22.3.4.8 Concept SizedSentinel [iterator.concept.sizedsentinel]

The SizedSentinel concept specifies requirements on an Iterator and a Sentinel that allow the use of the - operator to compute the distance between them in constant time.

🔗

template<class S, class I>
  concept SizedSentinel =
    Sentinel<S, I> &&
    !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&
    requires(const I& i, const S& s) {
      { s - i } -> Same<iter_difference_t<I>>;
      { i - s } -> Same<iter_difference_t<I>>;
    };

Let i be an iterator of type I, and s a sentinel of type S such that [i, s) denotes a range.

Let N be the smallest number of applications of ++i necessary to make bool(i == s) be true.

S and I model SizedSentinel<S, I> only if

(2.1)
If N is representable by iter_difference_t<I>, then s - i is well-defined and equals N.
(2.2)
If $- N$ is representable by iter_difference_t<I>, then i - s is well-defined and equals $- N$ .

[ Note

disable_sized_sentinel allows use of sentinels and iterators with the library that satisfy but do not in fact model SizedSentinel.

— end note

]

[ Example

The SizedSentinel concept is modeled by pairs of RandomAccessIterators ([iterator.concept.random.access]) and by counted iterators and their sentinels ([counted.iterator]).

— end example

]