std::for_each - cppreference.com [proxy]

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.

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:

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.

[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()
{
    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

[edit] 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)

std::for_each - cppreference.com

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