std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transform_result - cppreference.com (original) (raw)

Applies the given function to a range and stores the result in another range, beginning at result.

1. The unary operation op is applied to the range defined by [first1, last1) (after projecting with the projection proj).

2. Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

3. The binary operation binary_op is applied to pairs of elements from two ranges: one defined by [first1, last1) and the other defined by [first2, last2) (after respectively projecting with the projections proj1 and proj2).

4. Same as (3), but uses r1 as the first source range, as if using ranges::begin(r1) as first1 and ranges::end(r1) as last1, and similarly for r2.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

1,2) A unary_transform_result contains an input iterator equal to last and an output iterator to the element past the last element transformed.

3,4) A binary_transform_result contains input iterators to last transformed elements from ranges [first1, last1) and [first2, last2) as in1 and in2 respectively, and the output iterator to the element past the last element transformed as out.

[edit] Complexity

1,2) Exactly ranges::distance(first1, last1) applications of op and proj.

[edit] Possible implementation

struct transform_fn { // First version template<std::input_iterator I, std::sentinel_for S, std::weakly_incrementable O, std::copy_constructible F, class Proj = std::identity> requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<I, Proj>>> constexpr ranges::unary_transform_result<I, O> operator()(I first1, S last1, O result, F op, Proj proj = {}) const { for (; first1 != last1; ++first1, (void)++result) *result = std::invoke(op, std::invoke(proj, *first1));   return {std::move(first1), std::move(result)}; }   // Second version template<ranges::input_range R, std::weakly_incrementable O, std::copy_constructible F, class Proj = std::identity> requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<ranges::iterator_t, Proj>>> constexpr ranges::unary_transform_result<ranges::borrowed_iterator_t, O> operator()(R&& r, O result, F op, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(result), std::move(op), std::move(proj)); }   // Third version template<std::input_iterator I1, std::sentinel_for S1, std::input_iterator I2, std::sentinel_for S2, std::weakly_incrementable O, std::copy_constructible F, class Proj1 = std::identity, class Proj2 = std::identity> requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<I1, Proj1>, std::projected<I2, Proj2>>> constexpr ranges::binary_transform_result<I1, I2, O> operator()(I1 first1, S1 last1, I2 first2, S2 last2, O result, F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {}) const { for (; first1 != last1 && first2 != last2; ++first1, (void)++first2, (void)++result) *result = std::invoke(binary_op, std::invoke(proj1, *first1), std::invoke(proj2, *first2));   return {std::move(first1), std::move(first2), std::move(result)}; }   // Fourth version template<ranges::input_range R1, ranges::input_range R2, std::weakly_incrementable O, std::copy_constructible F, class Proj1 = std::identity, class Proj2 = std::identity> requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<ranges::iterator_t, Proj1>, std::projected<ranges::iterator_t, Proj2>>> constexpr ranges::binary_transform_result<ranges::borrowed_iterator_t, ranges::borrowed_iterator_t, O> operator()(R1&& r1, R2&& r2, O result, F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return (*this)(ranges::begin(r1), ranges::end(r1), ranges::begin(r2), ranges::end(r2), std::move(result), std::move(binary_op), std::move(proj1), std::move(proj2)); } };   inline constexpr transform_fn transform;

[edit] Notes

ranges::transform does not guarantee in-order application of op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use ranges::for_each.

[edit] Example

The following code uses ranges::transform to convert a string in place to uppercase using the std::toupper function and then transforms each char to its ordinal value. Then ranges::transform with a projection is used to transform elements of std::vector<Foo> into chars to fill a std::string.

#include #include #include #include #include #include   int main() { std::string s{"hello"}; auto op = [](unsigned char c) -> unsigned char { return std::toupper(c); };   namespace ranges = std::ranges;   // uppercase the string in-place ranges::transform(s.begin(), s.end(), s.begin(), op );   std::vector<std::size_t> ordinals; // convert each char to size_t ranges::transform(s, std::back_inserter(ordinals), [](unsigned char c) -> std::size_t { return c; });   std::cout << s << ':'; for (auto ord : ordinals) std::cout << ' ' << ord;   // double each ordinal ranges::transform(ordinals, ordinals, ordinals.begin(), std::plus{});   std::cout << '\n'; for (auto ord : ordinals) std::cout << ord << ' '; std::cout << '\n';   struct Foo { char bar; }; const std::vector f = {{'h'},{'e'},{'l'},{'l'},{'o'}}; std::string result; // project, then uppercase ranges::transform(f, std::back_inserter(result), op, &Foo::bar); std::cout << result << '\n'; }

Output:

HELLO: 72 69 76 76 79 144 138 152 152 158 HELLO

[edit] See also