24 Iterators library [iterators]

24.5 Iterator adaptors [predef.iterators]

24.5.8 Unreachable sentinel [unreachable.sentinel]

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Class unreachable_sentinel_t can be used with any weakly_incrementable type to denote the “upper bound” of an unbounded interval.
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[Example 1: char* p; // set p to point to a character buffer containing newlines char* nl = find(p, unreachable_sentinel, '\n');
Provided a newline character really exists in the buffer, the use of unreachable_sentinel above potentially makes the call to find more efficient since the loop test against the sentinel does not require a conditional branch.
— end example]
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namespace std { struct unreachable_sentinel_t { template<weakly_incrementable I> friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept { return false; } }; }