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std::for_each

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

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

For both overloads, if the iterator type (InputIt/ForwardIt) 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 TriviallyCopyable.

Parameters

first, last	-	the range to apply the function to
policy	-	the execution policy to use. See execution policy for details.
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.
-`UnaryFunction` must meet the requirements of MoveConstructible. Does not have to be CopyConstructible.
-`UnaryFunction2` must meet the requirements of CopyConstructible.

Return value

`f`.	(until C++11)
`std::move(f)`.	(since C++11)

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 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-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.

#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)

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 function to a range of elements (niebloid)
ranges::for_each_n (C++20)	applies a function object to the first n elements of a sequence (niebloid)
range-`for` loop(C++11)	executes loop over range