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)