NAME¶

std::for_each - std::for_each

Synopsis¶

Defined in header <algorithm>
template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, (until C++20)
UnaryFunction f );
template< class InputIt, class UnaryFunction >
constexpr UnaryFunction for_each( InputIt first, InputIt last, (1) (since C++20)
UnaryFunction f );
template< class ExecutionPolicy, class ForwardIt, class
UnaryFunction2 > (2) (since C++17)
void for_each( ExecutionPolicy&& policy, ForwardIt first,
ForwardIt last, UnaryFunction2 f );

1) Applies the given function object f to the result of dereferencing every iterator
in the range [first, last), in order.
2) Applies the given function object f to the result of dereferencing every iterator
in the range [first, last) (not necessarily in order). The algorithm is executed
according to policy. This overload does not participate in overload resolution
unless
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) is true.

For both overloads, if the iterator type is mutable, f may modify the elements of
the range through the dereferenced iterator. If f returns a result, the result is
ignored.

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 trivially copyable.

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.
-
UnaryFunction must meet the requirements of MoveConstructible. Does not have to be
CopyConstructible
-
UnaryFunction2 must meet the requirements of CopyConstructible.

Return value¶

1)
f
(until C++11)
std::move(f)
(since C++11)
2) (none)

Complexity¶

Exactly last - first 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 UnaryFunction>
constexpr UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
for (; first != last; ++first) {
f(*first);
}
return f; // implicit move since C++11
}

Example¶

The following example uses a lambda function to increment all of the elements of a
vector and then uses an overloaded operator() in 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 <vector>
#include <algorithm>
#include <iostream>

struct Sum
{
void operator()(int n) { sum += n; }
int sum{0};
};

int main()
{
std::vector<int> nums{3, 4, 2, 8, 15, 267};

auto print = [](const int& n) { std::cout << " " << n; };

std::cout << "before:";
std::for_each(nums.cbegin(), nums.cend(), print);
std::cout << '\n';

std::for_each(nums.begin(), nums.end(), [](int &n){ n++; });

// calls Sum::operator() for each number
Sum s = std::for_each(nums.begin(), nums.end(), Sum());

std::cout << "after: ";
std::for_each(nums.cbegin(), nums.cend(), print);
std::cout << '\n';
std::cout << "sum: " << s.sum << '\n';
}

Output:¶

before: 3 4 2 8 15 267
after: 4 5 3 9 16 268
sum: 305

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