24 Ranges library [ranges]

24.6 Range factories [range.factories]

24.6.2 Single view [range.single] Overview [range.single.overview]

single_­view produces a view that contains exactly one element of a specified value.
The name views​::​single denotes a customization point object ([customization.point.object]).
Given a subexpression E, the expression views​::​single(E) is expression-equivalent to single_­view{E}.
single_view s{4};
for (int i : s)
  cout << i;        // prints 4
— end example