std::ranges::transform,std::ranges::unary_transform_result,(3)

NAME
std::ranges::transform,std::ranges::unary_transform_result, - std::ranges::transform,std::ranges::unary_transform_result,

Synopsis
   Defined in header <algorithm>
   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, (1) (since
                        std::indirect_result_t<F&, C++20)
   std::projected<I, Proj>>>
   constexpr unary_transform_result<I, O>
       transform( I first1, S last1, O result, F op, Proj proj = {} );
   template< ranges::input_range R, std::weakly_incrementable O,
             std::copy_constructible F, class Proj = std::identity >
   requires std::indirectly_writable<O, (since
                std::indirect_result_t<F&, (2) C++20)
   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 = {} );
   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, (3) (since
                std::indirect_result_t<F&, C++20)
                        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 = {} );
   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, (since
                std::indirect_result_t<F&, (4) C++20)
                    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 = {} );

Helper types
   template< class I, class O > (5) (since
   using unary_transform_result = ranges::in_out_result<I, O>; C++20)
   template< class I1, class I2, class O > (6) (since
   using binary_transform_result = ranges::in_in_out_result<I1, I2, O>; 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 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.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first1, last1 - the first range of elements to transform
   r, r1 - the first range of elements to transform
   first2, last2 - the second range of elements to transform
   r2 - the second range of elements to transform
   result - the beginning of the destination range, may be equal to first1 or
                   first2
   op, binary_op - operation to apply to the projected element(s)
   proj1 - projection to apply to the elements in the first range
   proj2 - projection to apply to the elements in the second range

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.

Complexity
   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.

Possible implementation
 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 {first1, 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), result,
                        std::ref(op), std::ref(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 {first1, first2, 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),
                        result, std::ref(binary_op),
                        std::ref(proj1), std::ref(proj2));
     }
 };
 inline constexpr transform_fn transform;

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.

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

See also
   ranges::for_each applies a function to a range of elements
   (C++20) (niebloid)
   ranges::transform_view a view of a sequence that applies a transformation function
   views::transform to each element
   (C++20) (class template) (range adaptor object)
                        applies a function to a range of elements, storing results in
   transform a destination range
                        (function template)