# std::ranges::for_each, std::ranges::for_each_result

From cppreference.com
< cpp‎ | algorithm‎ | ranges

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 > constexpr for_each_result<I, Fun> for_each( I first, S last, Fun f, Proj proj = {} ); | (1) | (since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirectly_unary_invocable< std::projected<ranges::iterator_t<R>, Proj>> Fun > constexpr for_each_result<ranges::borrowed_iterator_t<R>, Fun> for_each( R&& r, Fun f, Proj proj = {} ); | (2) | (since C++20)
Helper types |  |
template< class I, class F > using for_each_result = ranges::in_fun_result<I, F>; | (3) | (since C++20)

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.

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

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

### Return value

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

### Complexity

Exactly last - first applications of f and proj.

### 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;

### 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');
    }

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

### See also

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/ranges/for_each&oldid=150656"