23 Iterators library [iterators]

23.5 Iterator adaptors [predef.iterators]

23.5.4 Common iterators [iterators.common]

23.5.4.1 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]
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&> common_iterator& operator=(const common_iterator<I2, S2>& x); decltype(auto) operator*(); decltype(auto) operator*() const requires dereferenceable<const I>; decltype(auto) operator->() const requires see below; common_iterator& operator++(); decltype(auto) operator++(int); template<class I2, sentinel_­for<I> S2> requires sentinel_­for<S, I2> friend 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 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 iter_difference_t<I2> operator-( const common_iterator& x, const common_iterator<I2, S2>& y); friend 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 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<I>; }; 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<I>; using difference_type = iter_difference_t<I>; using pointer = see below; using reference = iter_reference_t<I>; }; }

23.5.4.2 Associated types [common.iter.types]

The nested typedef-names of the specialization of iterator_­traits for common_­iterator<I, S> are defined as follows.
  • iterator_­concept denotes forward_­iterator_­tag if I models forward_­iterator; otherwise it denotes input_­iterator_­tag.
  • iterator_­category denotes forward_­iterator_­tag if the qualified-id iterator_­traits<I>​::​iterator_­category is valid and denotes a type that models derived_­from<forward_­iterator_­tag>; otherwise it denotes input_­iterator_­tag.
  • If the expression a.operator->() is well-formed, where a is an lvalue of type const common_­iterator<I, S>, then pointer denotes the type of that expression.
    Otherwise, pointer denotes void.

23.5.4.3 Constructors and conversions [common.iter.const]

constexpr common_iterator(I i);
Effects: Initializes v_­ as if by v_­{in_­place_­type<I>, std​::​move(i)}.
constexpr common_iterator(S s);
Effects: Initializes v_­ as if by v_­{in_­place_­type<S>, std​::​move(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);
Preconditions: x.v_­.valueless_­by_­exception() is false.
Effects: Initializes v_­ as if by v_­{in_­place_­index<i>, get<i>(x.v_­)}, where i is x.v_­.index().
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&> common_iterator& operator=(const common_iterator<I2, S2>& x);
Preconditions: x.v_­.valueless_­by_­exception() is false.
Effects: Equivalent to:
  • If v_­.index() == x.v_­.index(), then get<i>(v_­) = get<i>(x.v_­).
  • Otherwise, v_­.emplace<i>(get<i>(x.v_­)).
where i is x.v_­.index().
Returns: *this

23.5.4.4 Accessors [common.iter.access]

decltype(auto) operator*(); decltype(auto) operator*() const requires dereferenceable<const I>;
Preconditions: holds_­alternative<I>(v_­) is true.
Effects: Equivalent to: return *get<I>(v_­);
decltype(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<I>> || constructible_­from<iter_value_t<I>, iter_reference_t<I>>)
Preconditions: holds_­alternative<I>(v_­) is true.
Effects:
  • If I is a pointer type or if the expression get<I>(v_­).operator->() is well-formed, equivalent to: return get<I>(v_­);
  • Otherwise, if iter_­reference_­t<I> is a reference type, equivalent to: auto&& tmp = *get<I>(v_); return addressof(tmp);
  • Otherwise, equivalent to: return proxy(*get<I>(v_­)); where proxy is the exposition-only class: class proxy { iter_value_t<I> keep_; proxy(iter_reference_t<I>&& x) : keep_(std::move(x)) {} public: const iter_value_t<I>* operator->() const { return addressof(keep_); } };

23.5.4.5 Navigation [common.iter.nav]

common_iterator& operator++();
Preconditions: holds_­alternative<I>(v_­) is true.
Effects: Equivalent to ++get<I>(v_­).
Returns: *this.
decltype(auto) operator++(int);
Preconditions: holds_­alternative<I>(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 constructible_­
from
<iter_­value_­t<I>, iter_­reference_­t<I>> && move_­constructible<iter_­value_­t<I>>
is
false, equivalent to: return get<I>(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<I> keep_; postfix-proxy(iter_reference_t<I>&& x) : keep_(std::forward<iter_reference_t<I>>(x)) {} public: const iter_value_t<I>& operator*() const { return keep_; } };

23.5.4.6 Comparisons [common.iter.cmp]

template<class I2, sentinel_­for<I> S2> requires sentinel_­for<S, I2> friend 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<i>(x.v_­) == get<j>(y.v_­), where i is x.v_­.index() and j is y.v_­.index().
template<class I2, sentinel_­for<I> S2> requires sentinel_­for<S, I2> && equality_­comparable_­with<I, I2> friend 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<i>(x.v_­) == get<j>(y.v_­), where i is x.v_­.index() and j is y.v_­.index().
template<sized_­sentinel_­for<I> I2, sized_­sentinel_­for<I> S2> requires sized_­sentinel_­for<S, I2> friend 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<i>(x.v_­) - get<j>(y.v_­), where i is x.v_­.index() and j is y.v_­.index().

23.5.4.7 Customizations [common.iter.cust]

friend 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>(i.v_­) is true.
Effects: Equivalent to: return ranges​::​iter_­move(get<I>(i.v_­));
template<indirectly_­swappable<I> I2, class S2> friend 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<I>(x.v_­) and holds_­alternative<I2>(y.v_­) are each true.
Effects: Equivalent to ranges​::​iter_­swap(get<I>(x.v_­), get<I2>(y.v_­)).