std::ranges...each_result(3)

💖 std::ranges::for_each, std::ranges::for_each_result 💖

Okay bestie, this is the std::ranges...each_result(3) page from the C++ Standard Libary manual — yes, the header spells it like that, and we're keeping the whole vibe intact. This is the main-character page for std::ranges::for_each and std::ranges::for_each_result, and every detail is staying right here. 🫶

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🌸 NAME 🌸

std::ranges::for_each,std::ranges::for_each_result — std::ranges::for_each,std::ranges::for_each_result

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🎀 Synopsis 🎀

Defined in header <algorithm>

✨ Call signature ✨

template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
          std::indirectly_unary_invocable<std::projected<I, Proj>> Fun >
    (1) (since C++20)
constexpr for_each_result<I, Fun>
    for_each( I first, S last, Fun f, Proj proj = {} );

template< ranges::input_range R, class Proj = std::identity,
          std::indirectly_unary_invocable<
              std::projected<ranges::iterator_t<R>, Proj>> Fun >
    (2) (since C++20)
constexpr for_each_result<ranges::borrowed_iterator_t<R>, Fun>
    for_each( R&& r, Fun f, Proj proj = {} );

🧚 Helper types 🧚

template< class I, class F >
    (3) (since C++20)
using for_each_result = ranges::in_fun_result<I, F>;

1. Applies the given function object f to the result of the value projected by each iterator in the range [first, last), in order. ✨
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. 💅

For both overloads, if the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored. Noted, bestie.

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.

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💫 Parameters 💫

• first, last — iterator-sentinel pair denoting the range to apply the function to.
• r — the range of elements to apply the function to.
• f — the function to apply to the projected range.
• proj — projection to apply to the elements.

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🔥 Return value 🔥

{std::ranges::next(std::move(first), last), std::move(f)}

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⭐ Complexity ⭐

Exactly last - first applications of f and proj.

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🧚 Possible implementation 🧚

struct for_each_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
    constexpr ranges::for_each_result<I, Fun>
        operator()(I first, S last, Fun f, Proj proj = {}) const
    {
        for (; first != last; ++first)
            std::invoke(f, std::invoke(proj, *first));
        return {std::move(first), std::move(f)};
    }

    template<ranges::input_range R, class Proj = std::identity,
             std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>,
             Proj>> Fun>
    constexpr ranges::for_each_result<ranges::borrowed_iterator_t<R>,
    Fun>
        operator()(R&& r, Fun f, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
        std::move(f), std::ref(proj));
    }
};

inline constexpr for_each_fn for_each;

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🦋 Example 🦋

The following example uses a lambda expression to increment all of the elements of a vector and then uses an overloaded operator() in a functor to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate. 💖

// Run this code

#include <algorithm>
#include <cassert>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

struct Sum
{
    void operator()(int n) { sum += n; }
    int sum {0};
};

int main()
{
    std::vector<int> nums {3, 4, 2, 8, 15, 267};

    auto print = [](const auto& n) { std::cout << ' ' << n; };

    namespace ranges = std::ranges;
    std::cout << "before:";
    ranges::for_each(std::as_const(nums), print);
    print('\n');

    ranges::for_each(nums, [](int& n) { ++n; });

    // calls Sum::operator() for each number
    auto [i, s] = ranges::for_each(nums.begin(), nums.end(), Sum());
    assert(i == nums.end());

    std::cout << "after: ";
    ranges::for_each(nums.cbegin(), nums.cend(), print);

    std::cout << "\n" "sum: " << s.sum << '\n';

    using pair = std::pair<int, std::string>;
    std::vector<pair> pairs {{1,"one"}, {2,"two"}, {3,"tree"}};

    std::cout << "project the pair::first: ";
    ranges::for_each(pairs, print, [](const pair& p) { return p.first; });

    std::cout << "\n" "project the pair::second:";
    ranges::for_each(pairs, print, &pair::second);
    print('\n');
}

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🌈 Output 🌈

before: 3 4 2 8 15 267
after:  4 5 3 9 16 268
sum: 305
project the pair::first:  1 2 3
project the pair::second: one two tree

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🍓 See also 🍓

• range-for loop(C++11) — executes loop over range
• ranges::transform — applies a function to a range of elements (C++20) (niebloid)
• ranges::for_each_n — applies a function object to the first N elements of a sequence (C++20) (niebloid)
• for_each — applies a function to a range of elements (function template)

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source/version/date check, bestie: http://cppreference.com · 2024.06.10 · page std::ranges...each_result(3) — that's the footer, and we are out. Byeee 💅🎀