25 Iterators library [iterators]

25.5 Iterator adaptors [predef.iterators]

25.5.5 Common iterators [iterators.common]

25.5.5.1 Class template common_iterator [common.iterator]

Class template common_iterator is an iterator/sentinel adaptor that is capable of representing a non-common range of elements (where the types of the iterator and sentinel differ) as a common range (where they are the same).

It does this by holding either an iterator or a sentinel, and implementing the equality comparison operators appropriately.

[Note 1:

The common_iterator type is useful for interfacing with legacy code that expects the begin and end of a range to have the same type.

— end note]

[Example 1: template<class ForwardIterator> void fun(ForwardIterator begin, ForwardIterator end); list<int> s; // populate the list s using CI = common_iterator<counted_iterator<list<int>::iterator>, default_sentinel_t>; // call fun on a range of 10 ints fun(CI(counted_iterator(s.begin(), 10)), CI(default_sentinel)); — end example]

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namespace std { template<input_or_output_iterator I, sentinel_for S> requires (!same_as<I, S> && copyable) class common_iterator { public: constexpr common_iterator() requires default_initializable = 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 auto operator->() const requires see below; constexpr common_iterator& operator++(); constexpr decltype(auto) operator++(int); template<class I2, sentinel_for 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 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 I2, sized_sentinel_for 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 iter_move(const common_iterator& i) noexcept(noexcept(ranges::iter_move(declval<const I&>()))) requires input_iterator; template<indirectly_swappable 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 }; template<class I, class S> struct incrementable_traits<common_iterator<I, S>> { using difference_type = iter_difference_t; }; template<input_iterator I, class S> struct iterator_traits<common_iterator<I, S>> { using iterator_concept = see below; using iterator_category = see below; using value_type = iter_value_t; using difference_type = iter_difference_t; using pointer = see below; using reference = iter_reference_t; }; }

25.5.5.2 Associated types [common.iter.types]

The nested typedef-names of the specialization of iterator_traits for common_iterator<I, S> are defined as follows.

(1.1)
iterator_concept denotes forward_iterator_tag if I models forward_iterator; otherwise it denotes input_iterator_tag.
(1.2)
iterator_category denotes forward_iterator_tag if the qualified-id iterator_traits::iterator_category is valid and denotes a type that models derived_from<forward_iterator_tag>; otherwise it denotes input_iterator_tag.
(1.3)
Let a denote an lvalue of type const common_iterator<I, S>.

If the expression a.operator->() is well-formed, then pointer denotes decltype(a.operator->()).

Otherwise, pointer denotes void.

25.5.5.3 Constructors and conversions [common.iter.const]

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constexpr common_iterator(I i);

Effects: Initializes v_ as if by v_{in_place_type, std::move(i)}.

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constexpr common_iterator(S s);

Effects: Initializes v_ as if by v_{in_place_type<S>, std::move(s)}.

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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);

Preconditions: x.v_.valueless_by_exception() is false.

Effects: Initializes v_ as if by v_{in_place_index, get(x.v_)}, where i is x.v_.index().

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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);

Preconditions: x.v_.valueless_by_exception() is false.

Effects: Equivalent to:

(6.1)
If v_.index() == x.v_.index(), then get(v_) = get(x.v_).
(6.2)
Otherwise, v_.emplace(get(x.v_)).

where i is x.v_.index().

Returns: *this

25.5.5.4 Accessors [common.iter.access]

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constexpr decltype(auto) operator*();
constexpr decltype(auto) operator*() const
  requires dereferenceable<const I>;

Preconditions: holds_alternative(v_) is true.

Effects: Equivalent to: return *get(v_);

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constexpr auto operator->() const
  requires see below;

The expression in the requires-clause is equivalent to: indirectly_readable<const I> && (requires(const I& i) { i.operator->(); } || is_reference_v<iter_reference_t> || constructible_from<iter_value_t, iter_reference_t>)

Preconditions: holds_alternative(v_) is true.

Effects:

(5.1)
If I is a pointer type or if the expression get(v_).operator->() is well-formed, equivalent to: return get(v_);
(5.2)
Otherwise, if iter_reference_t is a reference type, equivalent to: auto&& tmp = *get(v_); return addressof(tmp);
(5.3)
Otherwise, equivalent to: return proxy(*get(v_)); where proxy is the exposition-only class: class proxy { iter_value_t keep_; constexpr proxy(iter_reference_t&& x) : keep_(std::move(x)) {} public: constexpr const iter_value_t* operator->() const noexcept { return addressof(keep_); } };

25.5.5.5 Navigation [common.iter.nav]

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constexpr common_iterator& operator++();

Preconditions: holds_alternative(v_) is true.

Effects: Equivalent to ++get(v_).

Returns: *this.

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constexpr decltype(auto) operator++(int);

Preconditions: holds_alternative(v_) is true.

Effects: If I models forward_iterator, equivalent to: common_iterator tmp = *this; ++*this; return tmp;

Otherwise, if requires(I& i) { { *i++ } -> can-reference; } is true or indirectly_readable && constructible_from<iter_value_t, iter_reference_t> && move_constructible<iter_value_t> is false, equivalent to: return get(v_)++;

Otherwise, equivalent to: postfix-proxy p(**this); ++*this; return p; where postfix-proxy is the exposition-only class: class postfix-proxy { iter_value_t keep_; constexpr postfix-proxy(iter_reference_t&& x) : keep_(std::forward<iter_reference_t>(x)) {} public: constexpr const iter_value_t& operator*() const noexcept { return keep_; } };

25.5.5.6 Comparisons [common.iter.cmp]

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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);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: true if i == j, and otherwise get(x.v_) == get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

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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);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: true if i and j are each 1, and otherwise get(x.v_) == get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

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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);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: 0 if i and j are each 1, and otherwise get(x.v_) - get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

25.5.5.7 Customizations [common.iter.cust]

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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>;

Preconditions: holds_alternative(i.v_) is true.

Effects: Equivalent to: return ranges::iter_move(get(i.v_));

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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&>())));

Preconditions: holds_alternative(x.v_) and holds_alternative<I2>(y.v_) are each true.

Effects: Equivalent to ranges::iter_swap(get(x.v_), get<I2>(y.v_)).