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std::sentinel_for

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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 [i, s) 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 [++i, s) 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 [i, s) denotes a range and i != s, [i, s) 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

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