Namespaces
Variants
Actions

std::for_each

From cppreference.com
< cpp‎ | algorithm
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
for_each
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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.
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.

Contents

[edit] 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.

[edit] Return value

1) f
2) (none)

[edit] Complexity

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

[edit] 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.

[edit] 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
}

[edit] 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.

[edit] 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()
{