22 Iterators library [iterators]

22.5 Iterator adaptors [predef.iterators]

22.5.7 Unreachable sentinel [unreachable.sentinels]

22.5.7.1 Class unreachable_­sentinel_­t [unreachable.sentinel]

1
#
Class unreachable_­sentinel_­t can be used with any WeaklyIncrementable 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<WeaklyIncrementable I>
      friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept;
    template<WeaklyIncrementable I>
      friend constexpr bool operator==(const I&, unreachable_sentinel_t) noexcept;
    template<WeaklyIncrementable I>
      friend constexpr bool operator!=(unreachable_sentinel_t, const I&) noexcept;
    template<WeaklyIncrementable I>
      friend constexpr bool operator!=(const I&, unreachable_sentinel_t) noexcept;
  };
}

22.5.7.2 Comparisons [unreachable.sentinel.cmp]

🔗
template<WeaklyIncrementable I> friend constexpr bool operator==(unreachable_sentinel_t, const I&) noexcept; template<WeaklyIncrementable I> friend constexpr bool operator==(const I&, unreachable_sentinel_t) noexcept;
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Returns: false.
🔗
template<WeaklyIncrementable I> friend constexpr bool operator!=(unreachable_sentinel_t, const I&) noexcept; template<WeaklyIncrementable I> friend constexpr bool operator!=(const I&, unreachable_sentinel_t) noexcept;
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Returns: true.