NAME¶

std::for_each - std::for_each

Synopsis¶

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

void for_each( ExecutionPolicy&& policy, (2) (since C++17)

ForwardIt first, ForwardIt last,
UnaryFunc f );

Applies the given 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

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 range to apply the function to
policy - the execution policy to use. See execution policy for details.
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:

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

See also¶

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