25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts]

25.3.4.7 Concept sentinel_for [iterator.concept.sentinel]

The sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range.

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template<class S, class I>
  concept sentinel_for =
    semiregular<S> &&
    input_or_output_iterator<I> &&
    weakly-equality-comparable-with<S, I>;      // see [concept.equalitycomparable]

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

Types S and I model sentinel_for<S, I> only if

(2.1)
i == s is well-defined.
(2.2)
If bool(i != s) then i is dereferenceable and [++i, s) denotes a range.
(2.3)
assignable_from<I&, S> is either modeled or not satisfied.

The domain of == is not static.

Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, i and s are not required to continue to denote a range after incrementing any other iterator equal to i.

Consequently, i == s is no longer required to be well-defined.