25 Ranges library [ranges]

25.7 Range adaptors [range.adaptors]

25.7.9 Transform view [range.transform]

25.7.9.1 Overview [range.transform.overview]

transform_view presents a view of an underlying sequence after applying a transformation function to each element.

The name views::transform denotes a range adaptor object ([range.adaptor.object]).

Given subexpressions E and F, the expression views::transform(E, F) is expression-equivalent to transform_view(E, F).

[Example 1: vector<int> is{ 0, 1, 2, 3, 4 }; auto squares = views::transform(is, [](int i) { return i * i; }); for (int i : squares) cout << i << ' '; // prints 0 1 4 9 16 — end example]

25.7.9.2 Class template transform_view [range.transform.view]

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namespace std::ranges { template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> class transform_view : public view_interface<transform_view<V, F>> { private: // [range.transform.iterator], class template transform_view::iterator template<bool> struct iterator; // exposition only // [range.transform.sentinel], class template transform_view::sentinel template<bool> struct sentinel; // exposition only V base_ = V(); // exposition only movable-box<F> fun_; // exposition only public: transform_view() requires default_initializable<V> && default_initializable<F> = default; constexpr explicit transform_view(V base, F fun); constexpr V base() const & requires copy_constructible<V> { return base_; } constexpr V base() && { return std::move(base_); } constexpr iterator<false> begin(); constexpr iterator<true> begin() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr sentinel<false> end(); constexpr iterator<false> end() requires common_range<V>; constexpr sentinel<true> end() const requires range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr iterator<true> end() const requires common_range<const V> && regular_invocable<const F&, range_reference_t<const V>>; constexpr auto size() requires sized_range<V> { return ranges::size(base_); } constexpr auto size() const requires sized_range<const V> { return ranges::size(base_); } }; template<class R, class F> transform_view(R&&, F) -> transform_view<views::all_t<R>, F>; }

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constexpr explicit transform_view(V base, F fun);

Effects: Initializes base_ with std::move(base) and fun_ with std::move(fun).

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constexpr iterator<false> begin();

Effects: Equivalent to: return iterator<false>{*this, ranges::begin(base_)};

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constexpr iterator<true> begin() const
  requires range<const V> &&
           regular_invocable<const F&, range_reference_t<const V>>;

Effects: Equivalent to: return iterator<true>{*this, ranges::begin(base_)};

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constexpr sentinel<false> end();

Effects: Equivalent to: return sentinel<false>{ranges::end(base_)};

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constexpr iterator<false> end() requires common_range<V>;

Effects: Equivalent to: return iterator<false>{*this, ranges::end(base_)};

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constexpr sentinel<true> end() const
  requires range<const V> &&
           regular_invocable<const F&, range_reference_t<const V>>;

Effects: Equivalent to: return sentinel<true>{ranges::end(base_)};

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constexpr iterator<true> end() const
  requires common_range<const V> &&
           regular_invocable<const F&, range_reference_t<const V>>;

Effects: Equivalent to: return iterator<true>{*this, ranges::end(base_)};

25.7.9.3 Class template transform_view::iterator [range.transform.iterator]

namespace std::ranges { template<input_range V, move_constructible F> requires view<V> && is_object_v<F> && regular_invocable<F&, range_reference_t<V>> && can-reference<invoke_result_t<F&, range_reference_t<V>>> template<bool Const> class transform_view<V, F>::iterator { private: using Parent = maybe-const<Const, transform_view>; // exposition only using Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition only Parent* parent_ = nullptr; // exposition only public: using iterator_concept = see below; using iterator_category = see below; // not always present using value_type = remove_cvref_t<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>>; using difference_type = range_difference_t<Base>; iterator() requires default_initializable<iterator_t<Base>> = default; constexpr iterator(Parent& parent, iterator_t<Base> current); constexpr iterator(iterator<!Const> i) requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>; constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) { return invoke(*parent_->fun_, *current_); } constexpr iterator& operator++(); constexpr void operator++(int); constexpr iterator operator++(int) requires forward_range<Base>; constexpr iterator& operator--() requires bidirectional_range<Base>; constexpr iterator operator--(int) requires bidirectional_range<Base>; constexpr iterator& operator+=(difference_type n) requires random_access_range<Base>; constexpr iterator& operator-=(difference_type n) requires random_access_range<Base>; constexpr decltype(auto) operator[](difference_type n) const requires random_access_range<Base> { return invoke(*parent_->fun_, current_[n]); } friend constexpr bool operator==(const iterator& x, const iterator& y) requires equality_comparable<iterator_t<Base>>; friend constexpr bool operator<(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator<=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr bool operator>=(const iterator& x, const iterator& y) requires random_access_range<Base>; friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>; friend constexpr iterator operator+(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr iterator operator+(difference_type n, iterator i) requires random_access_range<Base>; friend constexpr iterator operator-(iterator i, difference_type n) requires random_access_range<Base>; friend constexpr difference_type operator-(const iterator& x, const iterator& y) requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>; }; }

iterator::iterator_concept is defined as follows:

(1.1)
If Base models random_access_range, then iterator_concept denotes random_access_iterator_tag.
(1.2)
Otherwise, if Base models bidirectional_range, then iterator_concept denotes bidirectional_iterator_tag.
(1.3)
Otherwise, if Base models forward_range, then iterator_concept denotes forward_iterator_tag.
(1.4)
Otherwise, iterator_concept denotes input_iterator_tag.

The member typedef-name iterator_category is defined if and only if Base models forward_range .

In that case, iterator::iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<Base>>::iterator_category.

(2.1)
If is_reference_v<invoke_result_t<maybe-const<Const, F>&, range_reference_t<Base>>> is true, then
- (2.1.1)
  if C models derived_from<contiguous_iterator_tag>, iterator_category denotes random_access_iterator_tag;
- (2.1.2)
  otherwise, iterator_category denotes C.
(2.2)
Otherwise, iterator_category denotes input_iterator_tag.

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constexpr iterator(Parent& parent, iterator_t<Base> current);

Effects: Initializes current_ with std::move(current) and parent_ with addressof(parent).

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constexpr iterator(iterator<!Const> i)
  requires Const && convertible_to<iterator_t<V>, iterator_t<Base>>;

Effects: Initializes current_ with std::move(i.current_) and parent_ with i.parent_.

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constexpr const iterator_t<Base>& base() const & noexcept;

Effects: Equivalent to: return current_;

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constexpr iterator_t<Base> base() &&;

Effects: Equivalent to: return std::move(current_);

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constexpr iterator& operator++();

Effects: Equivalent to: ++current_; return *this;

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constexpr void operator++(int);

Effects: Equivalent to ++current_.

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constexpr iterator operator++(int) requires forward_range<Base>;

Effects: Equivalent to: auto tmp = *this; ++*this; return tmp;

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constexpr iterator& operator--() requires bidirectional_range<Base>;

Effects: Equivalent to: --current_; return *this;

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constexpr iterator operator--(int) requires bidirectional_range<Base>;

Effects: Equivalent to: auto tmp = *this; --*this; return tmp;

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constexpr iterator& operator+=(difference_type n)
  requires random_access_range<Base>;

Effects: Equivalent to: current_ += n; return *this;

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constexpr iterator& operator-=(difference_type n)
  requires random_access_range<Base>;

Effects: Equivalent to: current_ -= n; return *this;

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friend constexpr bool operator==(const iterator& x, const iterator& y)
  requires equality_comparable<iterator_t<Base>>;

Effects: Equivalent to: return x.current_ == y.current_;

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friend constexpr bool operator<(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return x.current_ < y.current_;

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friend constexpr bool operator>(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return y < x;

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friend constexpr bool operator<=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return !(y < x);

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friend constexpr bool operator>=(const iterator& x, const iterator& y)
  requires random_access_range<Base>;

Effects: Equivalent to: return !(x < y);

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friend constexpr auto operator<=>(const iterator& x, const iterator& y)
  requires random_access_range<Base> && three_way_comparable<iterator_t<Base>>;

Effects: Equivalent to: return x.current_ <=> y.current_;

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friend constexpr iterator operator+(iterator i, difference_type n)
  requires random_access_range<Base>;
friend constexpr iterator operator+(difference_type n, iterator i)
  requires random_access_range<Base>;

Effects: Equivalent to: return iterator{*i.parent_, i.current_ + n};

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friend constexpr iterator operator-(iterator i, difference_type n)
  requires random_access_range<Base>;

Effects: Equivalent to: return iterator{*i.parent_, i.current_ - n};

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friend constexpr difference_type operator-(const iterator& x, const iterator& y)
  requires sized_sentinel_for<iterator_t<Base>, iterator_t<Base>>;

Effects: Equivalent to: return x.current_ - y.current_;

25.7.9.4 Class template transform_view::sentinel [range.transform.sentinel]

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constexpr explicit sentinel(sentinel_t<Base> end);

Effects: Initializes end_ with end.

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constexpr sentinel(sentinel<!Const> i)
  requires Const && convertible_to<sentinel_t<V>, sentinel_t<Base>>;

Effects: Initializes end_ with std::move(i.end_).

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constexpr sentinel_t<Base> base() const;

Effects: Equivalent to: return end_;

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template<bool OtherConst>
  requires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

Effects: Equivalent to: return x.current_ == y.end_;

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template<bool OtherConst>
  requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr range_difference_t<maybe-const<OtherConst, V>>
  operator-(const iterator<OtherConst>& x, const sentinel& y);

Effects: Equivalent to: return x.current_ - y.end_;

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template<bool OtherConst>
  requires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>
friend constexpr range_difference_t<maybe-const<OtherConst, V>>
  operator-(const sentinel& y, const iterator<OtherConst>& x);

Effects: Equivalent to: return y.end_ - x.current_;