< cpp‎ | iterator
Iterator library
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Defined in header <iterator>
template< class S, class I >

    concept sentinel_for =
        std::semiregular<S> &&
        std::input_or_output_iterator<I> &&

        __WeaklyEqualityComparableWith<S, I>;
(since C++20)

The sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range. The exposition-only concept __WeaklyEqualityComparableWith is described in equality_comparable.

[edit] Semantic requirements

Let s and i be values of type S and I, respectively, such that [is) denotes a range. sentinel_for<S, I> is modeled only if:

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

The domain of == can change over time. Given an iterator i and sentinel s such that [is) denotes a range and i != s, [is) is not required to continue to denote a range after incrementing any iterator equal to i (and so i == s is no longer required to be well-defined after such an increment).

[edit] Notes

A sentinel type and its corresponding iterator type are not required to model equality_comparable_with, because the sentinel type may not be comparable with itself, and they are not required to have a common reference type.

It has been permitted to use a sentinel type different from the iterator type in the range-based for loop since C++17.

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3453 C++20 semantic requirements for sentinel_for were too loose for ranges::advance strengthened