std::for_each

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.

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

[edit] Return value

[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

#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';
}

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.

[edit] See also