std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transform_result

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.

struct transform_fn
{
 // First version
 template<std::input_iterator I, std::sentinel_for <I> 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 <R>, Proj>>>
 constexpr ranges::unary_transform_result<ranges::borrowed_iterator_t <R>, 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 <I1> S1,
 std::input_iterator I2, std::sentinel_for <I2> 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 <R1>, Proj1>,
 std::projected <ranges::iterator_t <R2>, Proj2>>>
 constexpr ranges::binary_transform_result<ranges::borrowed_iterator_t <R1>,
 ranges::borrowed_iterator_t <R2>, 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 .

Run this code

#include <algorithm>
#include <cctype>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
 
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 <Foo> 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

ranges::for_each

(C++20)

applies a unary function object to elements from a range
(algorithm function object)[edit]

ranges::transform_viewviews::transform

(C++20)

a view of a sequence that applies a transformation function to each element
(class template) (range adaptor object)[edit]

transform

applies a function to a range of elements, storing results in a destination range
(function template) [edit]

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std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transform_result

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[edit] Parameters

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[edit] Example

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