cppdraft_translate/cppdraft/range/transform.md

[range.transform]

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]

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

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

Given subexpressions E and F, the expressionviews​::​transform(E, F) is expression-equivalent totransform_view(E, F).

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[Example 1: vector 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 && is_object_v &&regular_invocable<F&, range_reference_t> &&can-reference<invoke_result_t<F&, range_reference_t>>class transform_view : public view_interface<transform_view<V, F>> {private:// [range.transform.iterator], class template transform_view​::​iteratortemplate struct iterator; // exposition only// [range.transform.sentinel], class template transform_view​::​sentineltemplate struct sentinel; // exposition only V base_ = V(); // exposition only**movable-box fun_; // exposition onlypublic: transform_view() requires default_initializable && default_initializable = default; constexpr explicit transform_view(V base, F fun); constexpr V base() const & requires copy_constructible { return base_; }constexpr V base() && { return std::move(base_); }constexpr iterator begin(); constexpr iterator begin() constrequires range &&regular_invocable<const F&, range_reference_t>; constexpr sentinel end(); constexpr iterator end() requires common_range; constexpr sentinel end() constrequires range &&regular_invocable<const F&, range_reference_t>; constexpr iterator end() constrequires common_range &&regular_invocable<const F&, range_reference_t>; constexpr auto size() requires sized_range { return ranges::size(base_); }constexpr auto size() const requires sized_range{ return ranges::size(base_); }constexpr auto reserve_hint() requires approximately_sized_range{ return ranges::reserve_hint(base_); }constexpr auto reserve_hint() const requires approximately_sized_range{ return ranges::reserve_hint(base_); }}; template<class R, class F> transform_view(R&&, F) -> transform_view<views::all_t, F>;}

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constexpr explicit transform_view(V base, F fun);

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Effects: Initializes base_ with std​::​move(base) andfun_ with std​::​move(fun).

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constexpr iterator<false> begin();

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

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constexpr iterator<true> begin() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> && [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;

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

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constexpr sentinel<false> end();

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

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constexpr iterator<false> end() requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<V>;

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

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constexpr sentinel<true> end() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V> && [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;

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

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constexpr iterator<true> end() const requires [common_range](range.refinements#concept:common_range "25.4.6 Other range refinements [range.refinements]")<const V> && [regular_invocable](concept.regularinvocable#concept:regular_invocable "18.7.3 Concept regular_­invocable [concept.regularinvocable]")<const F&, range_reference_t<const V>>;

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

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

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namespace std::ranges {template<input_range V, move_constructible F>requires view && is_object_v &&regular_invocable<F&, range_reference_t> &&can-reference<invoke_result_t<F&, range_reference_t>>templateclass transform_view<V, F>::iterator {private:using Parent = maybe-const<Const, transform_view>; // exposition onlyusing Base = maybe-const<Const, V>; // exposition only iterator_t<Base> current_ = iterator_t<Base>(); // exposition onlyParent parent_ = nullptr; // exposition onlypublic:using iterator_concept = see below; using iterator_category = see below; // not always presentusing 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 i)requires Const && convertible_to<iterator_t, iterator_t<Base>>; constexpr const iterator_t<Base>& base() const & noexcept; constexpr iterator_t<Base> base() &&; constexpr decltype(auto) operator() constnoexcept(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) constrequires 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>>; };}

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iterator​::​iterator_concept is defined as follows:

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  If Base models random_access_range, theniterator_concept denotes random_access_iterator_tag.

• (1.2)

  Otherwise, if Base models bidirectional_range, theniterator_concept denotes bidirectional_iterator_tag.

• (1.3)

  Otherwise, if Base models forward_range, theniterator_concept denotes forward_iterator_tag.

• (1.4)

  Otherwise, iterator_concept denotes input_iterator_tag.

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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 typeiterator_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);

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Effects: Initializes current_ with std​::​move(current) andparent_ with addressof(parent).

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constexpr iterator(iterator<!Const> i) requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<iterator_t<V>, iterator_t<Base>>;

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

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

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Effects: Equivalent to: return current_;

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constexpr iterator_t<Base> base() &&;

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

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constexpr iterator& operator++();

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

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constexpr void operator++(int);

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Effects: Equivalent to ++current_.

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constexpr iterator operator++(int) requires [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator--() requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator operator--(int) requires [bidirectional_range](range.refinements#concept:bidirectional_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator+=(difference_type n) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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constexpr iterator& operator-=(difference_type n) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr bool operator==(const iterator& x, const iterator& y) requires [equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<iterator_t<Base>>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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Effects: Equivalent to: return y < x;

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friend constexpr bool operator<=(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr bool operator>=(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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

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friend constexpr auto operator<=>(const iterator& x, const iterator& y) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base> && [three_way_comparable](cmp.concept#concept:three_way_comparable "17.12.4 Concept three_­way_­comparable [cmp.concept]")<iterator_t<Base>>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>; friend constexpr iterator operator+(difference_type n, iterator i) requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<Base>;

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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.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<iterator_t<Base>, iterator_t<Base>>;

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

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

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namespace std::ranges {template<input_range V, move_constructible F>requires view && is_object_v &&regular_invocable<F&, range_reference_t> &&can-reference<invoke_result_t<F&, range_reference_t>>templateclass transform_view<V, F>::sentinel {private:using Parent = maybe-const<Const, transform_view>; // exposition onlyusing Base = maybe-const<Const, V>; // exposition only sentinel_t<Base> end_ = sentinel_t<Base>(); // exposition onlypublic:sentinel() = default; constexpr explicit sentinel(sentinel_t<Base> end); constexpr sentinel(sentinel i)requires Const && convertible_to<sentinel_t, sentinel_t<Base>>; constexpr sentinel_t<Base> base() const; templaterequires sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>friend constexpr bool operator==(const iterator& x, const sentinel& y); templaterequires sized_sentinel_for<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>>friend constexpr range_difference_t<maybe-const<OtherConst, V>>operator-(const iterator& x, const sentinel& y); templaterequires 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& x); };}

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constexpr explicit sentinel(sentinel_t<Base> end);

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Effects: Initializes end_ with end.

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constexpr sentinel(sentinel<!Const> i) requires Const && [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<sentinel_t<V>, sentinel_t<Base>>;

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

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constexpr sentinel_t<Base> base() const;

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Effects: Equivalent to: return end_;

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template<bool OtherConst> requires [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7 Concept sentinel_­for [iterator.concept.sentinel]")<sentinel_t<Base>, iterator_t<maybe-const<OtherConst, V>>> friend constexpr bool operator==(const iterator<OtherConst>& x, const sentinel& y);

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

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template<bool OtherConst> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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);

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

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template<bool OtherConst> requires [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<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);

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