std::for_each

 

C++

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

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Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Non-modifying sequence operations     Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)(C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)     stable_partition is_partitioned (C++11)     partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection          set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11)	Execution policies (C++17) is_execution_policy (C++17) execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)(C++17)(C++17)(C++20) execution-policy (exposition only*)(C++26) execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20) Numeric operations iota (C++11) inner_product      reduce (C++17) accumulate adjacent_difference transform_reduce (C++17) partial_sum inclusive_scan (C++17)     exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17) Specialized <memory> algorithms Specialized <random> algorithms ranges::generate_random (C++26) C library qsort bsearch

 

Defined in header <algorithm>
template< class InputIt, class UnaryFunc > UnaryFunc for_each( InputIt first, InputIt last, UnaryFunc f );	(1)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryFunc > void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunc f );	(2)	(since C++17)

Applies the given unary function object f to the result of dereferencing every iterator in the range [first, last). If f returns a result, the result is ignored.

1) f is applied in order starting from first.

If UnaryFunc is not MoveConstructible, the behavior is undefined.

(since C++11)

2) 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 ranges.

This overload participates in overload resolution only if the value of the following expression is true:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>	(since C++20)

If UnaryFunc is not CopyConstructible, the behavior is undefined.

Parameters

first, last	-	the pair of iterators defining the range of elements to which the function object will be applied
policy	-	the execution policy to use
f	-	function object, to be applied to the result of dereferencing every iterator in the range [first, last) 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. ​
Type requirements
- InputIt must meet the requirements of LegacyInputIterator.
- ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

1) f

Complexity

Exactly std::distance(first, last) applications of f.

Exceptions

2) 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 iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.

For overload (1), f can be a stateful function object. The return value can be considered as the final state of the batch operation.

For overload (2), multiple copies of f may be created to perform parallel invocation. No value is returned because parallelization often does not permit efficient state accumulation.

Possible implementation

See also the implementations in libstdc++, libc++ and MSVC stdlib.

template<class InputIt, class UnaryFunc> constexpr UnaryFunc for_each(InputIt first, InputIt last, UnaryFunc f) { for (; first != last; ++first) f(*first); return f; // implicit move since C++11 }

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 function 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 <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v{3, -4, 2, -8, 15, 267};
    
    auto print = [](const int& n) { std::cout << n << ' '; };
    
    std::cout << "before:\t";
    std::for_each(v.cbegin(), v.cend(), print);
    std::cout << '\n';
    
    // increment elements in-place
    std::for_each(v.begin(), v.end(), [](int &n) { n++; });
    
    std::cout << "after:\t";
    std::for_each(v.cbegin(), v.cend(), print);
    std::cout << '\n';
    
    struct Sum
    {
        void operator()(int n) { sum += n; }
        int sum {0};
    };
    
    // invoke Sum::operator() for each element
    Sum s = std::for_each(v.cbegin(), v.cend(), Sum());    
    std::cout << "sum:\t" << s.sum << '\n';
}

Output:

before:	3 -4 2 -8 15 267 
after:	4 -3 3 -7 16 268 
sum:	281

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 475	C++98	it was unclear whether f can modify the elements of the sequence being iterated over (for_each is classified as “non-modifying sequence operations”)	made clear (allowed if the iterator type is mutable)
LWG 2747	C++11	overload (1) returned std::move(f)	returns f (which implicitly moves)

See also

transform	applies a function to a range of elements, storing results in a destination range (function template & algorithm function object)
ranges::transform (C++20)
for_each_n (C++17)	applies a function object to the first N elements of a sequence (function template & algorithm function object)
ranges::for_each_n (C++20)
ranges::for_each (C++20)	applies a unary function object to elements from a range (algorithm function object)
ranges::for_each_n (C++20)	applies a function object to the first N elements of a sequence (algorithm function object)
range-for loop(C++11)	executes loop over range