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

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, 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 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.

#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

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

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