std::ranges::for_each, std::ranges::for_each_result - cppreference.com

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)
template< /execution-policy/ Ep, std::random_access_iterator I, std::sized_sentinel_for<I> S, class Proj = std::identity, std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > I for_each( Ep&& policy, I first, S last, Fun f, Proj proj = {} );	(3)	(since C++26)
template< /execution-policy/ Ep, /sized-random-access-range/ R, class Proj = std::identity, std::indirectly_unary_invocable <std::projected<ranges::iterator_t<R>, Proj>> Fun > ranges::borrowed_iterator_t<R> for_each( Ep&& policy, R&& r, Fun f, Proj proj = {} );	(4)	(since C++26)
Helper types
template< class I, class F > using for_each_result = ranges::in_fun_result<I, F>;	(5)	(since C++20)

For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.

Applies the given invocable object f to each element (projected by proj) in the target range [first, last) or r. If f returns a result, the result is ignored.

1,2) f is applied in order from the beginning of the target range.

3,4) f might not be applied in order. The algorithm is executed according to policy.

Unlike other parallel algorithms, for_each is not allowed to make arbitrary copies of elements from the target range.

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.

Parameters

first, last	-	the pair of iterators defining the target range
r	-	the target range
f	-	the invocable object to be applied to the (projected) elements
proj	-	the projection to be applied to the elements
policy	-	the execution policy to use

Return value

1) {last, std::move(f)}

2) {ranges::next(ranges::begin(r), ranges::end(r)), std::move(f)} if R models forward_range, otherwise (since C++26){ranges::end(r), std::move(f)}

3) last

4) ranges::next(ranges::begin(r), ranges::end(r)) if R models forward_range, otherwise (since C++26)ranges::end(r)

Complexity

1,3) Exactly ranges::distance(first, last) applications of f and proj.

2,4) Exactly ranges::distance(r) applications of f and proj.

Exceptions

3,4) During the execution process:

If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

If the projection returns a mutable reference, f may modify the elements in the target range.

For overloads (1,2), f can be a stateful invocable object. The invocable object in the return value can be considered as the final state of the batch operation.

For overloads (3,4), multiple copies of f may be created to perform parallel invocation. The return value does not contain an invocable object because parallelization often does not permit efficient state accumulation.

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));
    }
    
    template<ranges::forward_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::next(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 an invocable object to compute their sum. Note that to compute the sum, it is recommended to use the dedicated algorithm std::accumulate.

#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()
{
    namespace ranges = std::ranges;
    
    std::vector<int> nums {3, 4, 2, 8, 15, 267};
    
    auto print = [](const auto& n) { std::cout << ' ' << n; };
    
    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