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std::for_each - cppreference.com

From cppreference.com

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 invocable object f to each element in the target 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.

Unlike other parallel algorithms, for_each is not allowed to make arbitrary copies of elements from the target range.

This overload participates in overload resolution only if the value of the following expression is true:

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)

Parameters

first, last - the pair of iterators defining the target range
f - function object, to be applied to the elements

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.

​

policy - the execution policy to use
Type requirements
-InputIt must meet the requirements of LegacyInputIterator.
-ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

1) f

Complexity

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

Exceptions

2) During the execution process:

  • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
  • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements in the target range.

For overload (1), f can be a stateful invocable 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.

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
}

Example

The following example uses a lambda expression to increment all of the elements of a vector and then uses an overloaded operator() in an invocable object 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)
LWG 2747 C++11 overload (1) returned std::move(f) returns f (which implicitly moves)

See also

applies a unary function object to elements from a range
(algorithm function object)[edit]
applies a function object to the first N elements of a sequence
(function template & algorithm function object)[edit]
applies a function to a range of elements, storing results in a destination range
(function template & algorithm function object)[edit]
range-for loop(C++11) executes loop over range[edit]