26 Ranges library [ranges]

26.3 Range access [range.access]

26.3.12 ranges​::​empty [range.prim.empty]

The name ranges​::​empty denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
  • If T is an array of unknown bound ([dcl.array]), ranges​::​empty(E) is ill-formed.
  • Otherwise, if bool(t.empty()) is a valid expression, ranges​::​empty(E) is expression-equivalent to bool(t.empty()).
  • Otherwise, if (ranges​::​size(t) == 0) is a valid expression, ranges​::​empty(E) is expression-equivalent to (ranges​::​size(t) == 0).
  • Otherwise, if bool(ranges​::​begin(t) == ranges​::​end(t)) is a valid expression and the type of ranges​::​begin(t) models forward_­iterator, ranges​::​empty(E) is expression-equivalent to bool(​ranges​::​begin(t) == ranges​::​end(t)).
  • Otherwise, ranges​::​empty(E) is ill-formed.
[Note 1:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​empty(E) appears in the immediate context of a template instantiation.
— end note]
[Note 2:
Whenever ranges​::​empty(E) is a valid expression, it has type bool.
— end note]