std:: sentinel_for

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 .

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).

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.

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