NAME

std::ranges::for_each,std::ranges::for_each_result - std::ranges::for_each,std::ranges::for_each_result

Synopsis

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, class Proj =
std::identity,
(since
std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > (1) 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>, (since
Proj>> Fun > (2) 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
using for_each_result = ranges::in_fun_result<I, F>; 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 may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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