std::ranges::for_each() algorithm

since C++20

Simplified

// (1)
constexpr for_each_result<I, Fun>
    for_each( I first, S last, Fun f, Proj proj = {} );


// (2)
constexpr for_each_result<ranges::borrowed_iterator_t<R>, Fun>
    for_each( R&& r, Fun f, Proj proj = {} );

The type of arguments are generic and have following constraints:

• I - std::input_iterator
• S - std::sentinel_for<I>
• R - std::ranges::input_range
• Fun:
  • (1) - std::indirectly_unary_invocable<std::projected<I, Proj>>
  • (2) - std::indirectly_unary_invocable<std::projected<ranges::iterator_t<R>, Proj>>
• Proj - (none)

The Proj template argument has a default type of std::identity for all overloads.

Detailed

// (1)
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 = {} );


// (2)
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 = {} );

With the helper types defined as follows:

template< class I, class F >
using for_each_result = ranges::in_fun_result<I, F>;

Applies the given function to all elements in range.

• (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. If f returns a result, the result is ignored.

The function-like entities described on this page are niebloids.

Parameters

first last	The range of elements to apply the function to.
r	The range of elements to apply the function to.
proj	Projection to apply to the elements.
f	Function object, to be applied to every element of the range. The signature of the function should be equivalent to the following: void fun(const Type& a); The signature does not need to have const&. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.

Return value

Value of type for_each_result initialized as follows:

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

Complexity

Exactly last - first applications of f and proj.

Exceptions

(none)

Possible implementation

for_each(1) and for_each(2)

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;

Examples

Main.cpp

#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