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common_iterator
should be completely constexpr
-ableSection: 24.5.5.1 [common.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2021-07-21 Last modified: 2023-11-22
Priority: 3
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Discussion:
After P2231R1, variant
becomes completely constexpr
-able, which means that
common_iterator
can also be completely constexpr
-able.
[2021-08-20; Reflector poll]
Set priority to 3 after reflector poll.
[2021-08-23; Reflector poll]
Set status to Tentatively Ready after five votes in favour during reflector poll.
[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4892.
Modify 24.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: constexpr common_iterator() requires default_initializable<I> = default; constexpr common_iterator(I i); constexpr common_iterator(S s); template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> constexpr common_iterator(const common_iterator<I2, S2>& x); template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x); constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>; constexpr decltype(auto) operator->() const requires see below; constexpr common_iterator& operator++(); constexpr decltype(auto) operator++(int); template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y); template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y); template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y); friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>; template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>()))); private: variant<I, S> v_; // exposition only }; […] }
Modify 24.5.5.3 [common.iter.const] as indicated:
template<class I2, class S2> requires convertible_to<const I2&, I> && convertible_to<const S2&, S> && assignable_from<I&, const I2&> && assignable_from<S&, const S2&> constexpr common_iterator& operator=(const common_iterator<I2, S2>& x);-5- Preconditions:
x.v_.valueless_by_exception()
isfalse
.[…]
Modify 24.5.5.4 [common.iter.access] as indicated:
constexpr decltype(auto) operator*(); constexpr decltype(auto) operator*() const requires dereferenceable<const I>;-1- Preconditions:
holds_alternative<I>(v_)
istrue
.[…]
constexpr decltype(auto) operator->() const requires see below;-3- The expression in the requires-clause is equivalent to:
[…]
Modify 24.5.5.5 [common.iter.nav] as indicated:
constexpr common_iterator& operator++();-1- Preconditions:
holds_alternative<I>(v_)
istrue
.[…]
constexpr decltype(auto) operator++(int);-4- Preconditions:
holds_alternative<I>(v_)
istrue
.[…]
Modify 24.5.5.6 [common.iter.cmp] as indicated:
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-1- Preconditions: […]
template<class I2, sentinel_for<I> S2> requires sentinel_for<S, I2> && equality_comparable_with<I, I2> friend constexpr bool operator==( const common_iterator& x, const common_iterator<I2, S2>& y);-3- Preconditions: […]
template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2> requires sized_sentinel_for<S, I2> friend constexpr iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y);-5- Preconditions: […]
Modify 24.5.5.7 [common.iter.cust] as indicated:
friend constexpr iter_rvalue_reference_t<I> iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator<I>;-1- Preconditions: […]
template<indirectly_swappable<I> I2, class S2> friend constexpr void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y) noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));-3- Preconditions: […]