std::ranges::transform() algorithm

od C++20

Simplified

// (1)
constexpr unary_transform_result<I, O>
	transform( I first1, S last1, O result, F op, Proj proj = {} );

// (2)
constexpr unary_transform_result<ranges::borrowed_iterator_t<R>, O>
    transform( R&& r, O result, F op, Proj proj = {} );

// (3)
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 = {}
	);

// (4)
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 = {}
	);

The type of arguments are generic and have the following constraints:

• I, I1, I2 - std::input_iterator
• S, S1, S2 - std::sentinel_for<I>, std::sentinel_for<I1>, std::sentinel_for<I2>
• R, R1, R2 - std::ranges::input_range
• O - std::weakly_incrementable
• F - std::copy_constructible

The Proj template argument has a default type of std::identity for all overloads.

Additionally, each overload has the following constraints:

• (1) - indirectly_writable<O, indirect_result_t<F&, projected<I, Proj>> >
• (2) - indirectly_writable<O, indirect_result_t<F&, projected<ranges::iterator_t<R>, Proj>>
• (3) - indirect_result_t<F&, projected<I1, Proj1>, projected<I2, Proj2>>>
• (4) - indirectly_writable<O, indirect_result_t<F&, projected<ranges::iterator_t<R1>, Proj1>, projected<ranges::iterator_t<R2>, Proj2>>>

(The std:: namespace was ommitted here for readability)

Detailed

// (1)
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 = {} );

// (2)
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 = {} );

// (3)
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 = {}
	);

// (4)
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 = {}
	);

With the helper types defined as follows:

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

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

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 ).
  • 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.

Parameters

first1 last1	The first range of elements to transform (iterator, sentinel pair).
r r1	The first range of elements to transform (ranges).
first2 last2	The second range of elements to transform (iterator, sentinel pair).
r2	The second range of elements to transform (range).
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) Returns a unary_transform_result which contains an input iterator equal to last and an output iterator to the element past the last element transformed.
• (3 - 4) Returns 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.

Exceptions

(none)

Possible implementation

transform(1) and transform(2)

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.

Examples

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.

Main.cpp

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

int main()
{
    std::string s {"hello"};

    namespace ranges = std::ranges;

    ranges::transform(s.begin(), s.end(), s.begin(),
                      [](unsigned char c) -> unsigned char { return std::toupper(c); });

    std::vector<std::size_t> ordinals;
    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;

    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 bar;
    ranges::transform(f, std::back_inserter(bar), &Foo::bar);
    std::cout << bar << '\n';
}

Output

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