26 Ranges library [ranges]

Mandates: C is a cv-unqualified class type.

Returns: An object of type C constructed from the elements of r in the following manner:

• (2.1)
  If C does not satisfy input_range or convertible_to<range_reference_t<R>, range_value_t<C>> is true:
  • (2.1.1)
    If constructible_from<C, R, Args...> is true: C(std::forward<R>(r), std::forward<Args>(args)...)
  • (2.1.2)
    Otherwise, if constructible_from<C, from_range_t, R, Args...> is true: C(from_range, std::forward<R>(r), std::forward<Args>(args)...)
  • (2.1.3)
    Otherwise, if
    • (2.1.3.1)
      common_range<R> is true,
    • (2.1.3.2)
      the qualified-id iterator_traits<iterator_t<R>>​::​iterator_category is valid and denotes a type that models derived_from<input_iterator_tag>, and
    • (2.1.3.3)
      constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...> is true:
    C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)
  • (2.1.4)
    Otherwise, if
    • (2.1.4.1)
      constructible_from<C, Args...> is true, and
    • (2.1.4.2)
      container-insertable<C, range_reference_t<R>> is true:
    C c(std::forward<Args>(args)...); if constexpr (sized_range<R> && reservable-container<C>) c.reserve(static_cast<range_size_t<C>>(ranges::size(r))); ranges::copy(r, container-inserter<range_reference_t<R>>(c));
• (2.2)
  Otherwise, if input_range<range_reference_t<R>> is true: to<C>(r | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);
• (2.3)
  Otherwise, the program is ill-formed.

Let input-iterator be an exposition-only type: struct input-iterator { // exposition only using iterator_category = input_iterator_tag; using value_type = range_value_t<R>; using difference_type = ptrdiff_t; using pointer = add_pointer_t<range_reference_t<R>>; using reference = range_reference_t<R>; reference operator*() const; pointer operator->() const; input-iterator& operator++(); input-iterator operator++(int); bool operator==(const input-iterator&) const; };

Let DEDUCE_EXPR be defined as follows:

• (4.1)
  C(declval<R>(), declval<Args>()...) if that is a valid expression,
• (4.2)
  otherwise, C(from_range, declval<R>(), declval<Args>()...) if that is a valid expression,
• (4.3)
  otherwise, C(declval<input-iterator>(), declval<input-iterator>(), declval<Args>()...) if that is a valid expression,
• (4.4)
  otherwise, the program is ill-formed.

Returns: to<decltype(DEDUCE_EXPR)>(std​::​forward<R>(r), std​::​forward<Args>(args)...).