9 Iterators library [iterators]

9.7 Iterator adaptors [iterators.predef]

9.7.8 Unreachable sentinel [unreachable.sentinels]

9.7.8.1 Class unreachable [unreachable.sentinel]

1

Class unreachable is a sentinel type that can be used with any Iterator to denote an infinite range. Comparing an iterator for equality with an object of type unreachable always returns false.

[ Example: 

char* p;
// set p to point to a character buffer containing newlines
char* nl = find(p, unreachable(), '\n');

Provided a newline character really exists in the buffer, the use of unreachable 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 { namespace experimental { namespace ranges { inline namespace v1 {
  class unreachable { };

  template <Iterator I>
    constexpr bool operator==(const I&, unreachable) noexcept;
  template <Iterator I>
    constexpr bool operator==(unreachable, const I&) noexcept;
  template <Iterator I>
    constexpr bool operator!=(const I&, unreachable) noexcept;
  template <Iterator I>
    constexpr bool operator!=(unreachable, const I&) noexcept;
}}}}