std::for_each

 

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Algorithm library

Constrained algorithms and algorithms on ranges(C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
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) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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 objectf 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.

This overload participates in overload resolution only if all following conditions are satisfied:

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

If UnaryFunc is not CopyConstructible, the behavior is undefined.

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

Unlike the rest of the parallel algorithms, for_each is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

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

2) (none)

Complexity

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

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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}

Notes

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.

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 (i.k.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 <iostream>#include <vector>   int main(){std::vector<int> v{3, -4, 2, -8, 15, 267};   auto print =[](constint& 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)
for_each_n (C++17)	applies a function object to the first N elements of a sequence (function template)
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