Defined in header
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
i be values of type
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
iis dereferenceable and
[++i, s)denotes a range.
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).
A sentinel type and its corresponding iterator type are not required to model equality_comparable_with, because the sentinel type may be not 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.