1 Revision History

R0 [P1871R0] was presented in Belfast and originally proposed making all of the variable template opt-ins for concepts named enable_NAME. LEWG rejected this direction. However, one of the other changes in the paper was to ensure at least that the traits were named the same as the concepts. This used to be the case, except when we renamed the concept to sized_sentinel_for we kept the trait named disable_sized_sentinel, so this paper proposes simply updating that trait to have the same name - so that all such traits are either enable_NAME or disable_NAME.

2 Motivation

The concept sized_sentinel_for, from [iterator.concept.sizedsentinel], is defined as:

template<class S, class I>
  concept sized_sentinel_for =
    sentinel_for<S, I> &&
    !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&
    requires(const I& i, const S& s) {
      { s - i } -> same_as<iter_difference_t<I>>;
      { i - s } -> same_as<iter_difference_t<I>>;
    };

Note that the concept is named sized_sentinel_for but the trait is named disable_sized_sentinel. These used to be closely related because the concept used to be named SizedSentinel and got the suffix _for added as part of [P1754R1].

This paper proposes that the trait and the concept should have the same name.

3 Wording

Change 23.2 [iterator.synopsis]:

  // [iterator.concept.sizedsentinel], concept sized_sentinel_for
  template<class S, class I>
-   inline constexpr bool disable_sized_sentinel = false;
+   inline constexpr bool disable_sized_sentinel_for = false;

and later:

  template<class Iterator1, class Iterator2>
      requires (!sized_sentinel_for<Iterator1, Iterator2>)
-   inline constexpr bool disable_sized_sentinel<reverse_iterator<Iterator1>,
-                                                reverse_iterator<Iterator2>> = true;
+   inline constexpr bool disable_sized_sentinel_for<reverse_iterator<Iterator1>,
+                                                    reverse_iterator<Iterator2>> = true;

Change 23.3.4.8 [iterator.concept.sizedsentinel]:

1 The sized_sentinel_for concept specifies requirements on an input_or_output_iterator and a corresponding sentinel_for that allow the use of the - operator to compute the distance between them in constant time.

template<class S, class I>
  concept sized_sentinel_for =
    sentinel_for<S, I> &&
-   !disable_sized_sentinel<remove_cv_t<S>, remove_cv_t<I>> &&
+   !disable_sized_sentinel_for<remove_cv_t<S>, remove_cv_t<I>> &&
    requires(const I& i, const S& s) {
      { s - i } -> same_as<iter_difference_t<I>>;
      { i - s } -> same_as<iter_difference_t<I>>;
    };

2 Let i be an iterator of type I, and s a sentinel of type S such that [i, s) denotes a range. Let N be the smallest number of applications of ++i necessary to make bool(i == s) be true. S and I model sized_sentinel_for<S, I> only if

- [2.1]{.pnum} If `N` is representable by `iter_difference_t<I>`, then `s - i` is well-defined and equals `N`.
- [2.2]{.pnum} If `−N` is representable by `iter_difference_t<I>`, then `i - s` is well-defined and equals `−N`.

  template<class S, class I>
-   inline constexpr bool disable_sized_sentinel = false;
+   inline constexpr bool disable_sized_sentinel_for = false;

3 Remarks: Pursuant to [namespace.std], users may specialize disable_sized_sentinel disable_sized_sentinel_for for cv-unqualified non-array object types S and I if S and/or I is a program-defined type. Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

4 [ Note: disable_sized_sentinel disable_sized_sentinel_for allows use of sentinels and iterators with the library that satisfy but do not in fact model sized_sentinel_for. — end note ]

4 References