std::ranges...form_result,(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

🌸 std::ranges::transform + std::ranges::unary_transform_result 🌸

okay bestie, so this one is from the C++ Standard Library manual pages (part 1, more coming, don't go anywhere 💅). it's giving "apply a function to a range and dump the results somewhere else" and honestly it's iconic.

✨ NAME ✨

std::ranges::transform, std::ranges::unary_transform_result — the range-transform duo, together at last 🫶

✨ SYNOPSIS ✨

so first things first: it's Defined in header <algorithm> 📌

Call signature

this is overload (1), since C++20:

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 = {} );

overload (2), also since C++20 — the range-y one:

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 = {} );

overload (3), since C++20 — now with TWO ranges, look at her go:

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 = {} );

overload (4), since C++20 — two ranges, both as ranges, full main-character energy 👑:

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 = {} );

Helper types

(5) since C++20 — the unary one:

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

(6) since C++20 — the binary one:

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

✨ DESCRIPTION ✨

so what does she DO? she applies the given function to a range and stores the result in another range, beginning at result. that's it, that's the whole vibe ✨

okay but fr fr, here's the breakdown:

  1. the unary operation op is applied to the range defined by [first1, last1) — and yes, after projecting with the projection proj 💜
  2. same as (1), but it uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last. convenient, no notes.
  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. two ranges, one function, she ate 🔥
  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. symmetrical slay.
🫶 real talk: 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. that's just how it is, bestie 💅

✨ 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 🎀
  2. & 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. all accounted for 💖

✨ COMPLEXITY ✨

  1. & 2. exactly ranges::distance(first1, last1) applications of op and proj. exact. no guessing. ⭐
  2. & 4. exactly ranges::min(ranges::distance(first1, last1), ranges::distance(first2, last2)) applications of binary_op and projections. she stops at the shorter one, as a treat 🌸

✨ POSSIBLE IMPLEMENTATION ✨

ngl this is the fun part, look how clean she is:

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 ✨

⚠️ okay important one: ranges::transform does not guarantee in-order application of op or binary_op. so if you were counting on order, bestie, don't 😭

if you need to apply a function to a sequence in-order, or apply a function that modifies the elements of a sequence, use ranges::for_each instead. that's the one you want. periodt 💅

(part 1 done — part 2 is coming, don't touch that dial 🦋)

✨ EXAMPLE ✨

Okay bestie, this example is literally the moment you've been waiting for 🫶. The code below uses ranges::transform to convert a std::string in place to uppercase using the std::toupper function, and then it 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. No cap, it's giving range-algorithm excellence 💅.

// 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 💻

And the vibe check passes — here's what it prints:

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

Like, the string gets uppercased, the ordinals get printed, then doubled, then the projection magic turns those Foos back into HELLO. Obsessed 😭🔥.

👀 SEE ALSO 👀

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

That's the whole page, bestie 💖. Source is http://cppreference.com and the version/date on the original footer is 2024.06.10 — so yeah, go slay some ranges, periodt 🌈👑.

📦 where this page comes from