23 Iterators library [iterators]

23.5 Iterator adaptors [predef.iterators]

23.5.7 Unreachable sentinel [unreachable.sentinels]

23.5.7.1 Class unreachable_­sentinel_­t [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
: 
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
]
namespace std {
  struct unreachable_sentinel_t {
    template<weakly_incrementable I>
      friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept
      { return false; }
  };
}