From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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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 = {} );
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(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 = {} );
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(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 = {} );
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(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 = {} );
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(4) | (since C++26) |
| Helper types |
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template< class I, class F >
using for_each_result = ranges::in_fun_result<I, F>;
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(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)
last4)
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;
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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.
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()
{
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
See also
| applies a unary function object to elements from a range (function template) | |
(C++20) |
applies a function object to the first N elements of a sequence (algorithm function object) |
(C++20) |
applies a function to a range of elements (algorithm function object) |
range-for loop(C++11)
|
executes loop over range |