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std::ranges::for_each_n, std::ranges::for_each_n_result - cppreference.com

1) Applies the given function object f to the projected result by proj of dereferencing each iterator in the range [first, first + n), in order.

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. If n is less than zero, the behavior is undefined.

The function-like entities described on this page are algorithm function objects (informally known as 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.

An object {first + n, std::move(f)}, where first + n may be evaluated as std::ranges::next(std::move(first), n) depending on iterator category.

Exactly n applications of f and proj.

struct for_each_n_fn
{
    template<std::input_iterator I, class Proj = std::identity,
             std::indirectly_unary_invocable<std::projected<I, Proj>> Fun>
    constexpr for_each_n_result<I, Fun>
        operator()(I first, std::iter_difference_t<I> n, Fun fun, Proj proj = Proj{}) const
    {
        for (; n-- > 0; ++first)
            std::invoke(fun, std::invoke(proj, *first));
        return {std::move(first), std::move(fun)};
    }
};
 
inline constexpr for_each_n_fn for_each_n {};

#include <algorithm>
#include <array>
#include <iostream>
#include <ranges>
#include <string_view>
 
struct P
{
    int first;
    char second;
    friend std::ostream& operator<<(std::ostream& os, const P& p)
    {
        return os << '{' << p.first << ",'" << p.second << "'}";
    }
};
 
auto print = [](std::string_view name, auto const& v)
{
    std::cout << name << ": ";
    for (auto n = v.size(); const auto& e : v)
        std::cout << e << (--n ? ", " : "\n");
};
 
int main()
{
    std::array a {1, 2, 3, 4, 5};
    print("a", a);
    // Negate first three numbers:
    std::ranges::for_each_n(a.begin(), 3, [](auto& n) { n *= -1; });
    print("a", a);
 
    std::array s { P{1,'a'}, P{2, 'b'}, P{3, 'c'}, P{4, 'd'} };
    print("s", s);
    // Negate data members 'P::first' using projection:
    std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first);
    print("s", s);
    // Capitalize data members 'P::second' using projection:
    std::ranges::for_each_n(s.begin(), 3, [](auto& c) { c -= 'a'-'A'; }, &P::second);
    print("s", s);
}

Output:

a: 1, 2, 3, 4, 5
a: -1, -2, -3, 4, 5
s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}

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