std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transform_result - cppreference.com

Call signature

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 unary_transform_result<I, O>
    transform( I first1, S last1, O result, F op, Proj proj = {} );

(1) (since C++20)

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 unary_transform_result<ranges::borrowed_iterator_t<R>, O>
    transform( R&& r, O result, F op, Proj proj = {} );

(2) (since C++20)

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 binary_transform_result<I1, I2, O>
    transform( I1 first1, S1 last1, I2 first2, S2 last2, O result,
               F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} );

(3) (since C++20)

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 binary_transform_result<ranges::borrowed_iterator_t<R1>,
                                  ranges::borrowed_iterator_t<R2>, O>
    transform( R1&& r1, R2&& r2, O result, F binary_op,
               Proj1 proj1 = {}, Proj2 proj2 = {} );

(4) (since C++20)

Helper types

template< class I, class O >
using unary_transform_result = ranges::in_out_result<I, O>;

(5) (since C++20)

template< class I1, class I2, class O >
using binary_transform_result = ranges::in_in_out_result<I1, I2, O>;

(6) (since C++20)

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:

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.

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

3,4) Exactly ranges::min(ranges::distance(first1, last1), ranges::distance(first2, last2)) applications of binary_op and projections.

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;

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.

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 <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