Std::for_each (3) Linux Manual Page

std::for_each – std::for_each

Synopsis

Defined in header<algorithm>

    template <class InputIt, class UnaryFunction>
    (until C++ 20)

        UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f);

template <class InputIt, class UnaryFunction>
(1)(since C++ 20)

    constexpr UnaryFunction for_each(InputIt first, InputIt last, 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>> 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 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

  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

{
    Sum()
        : sum{0}
    {
    }
    void operator()(int n)
    {
        sum += n;
    }
    int sum;
};
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.begin(), nums.end(), 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.begin(), nums.end(), 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

std::for_each (3) Linux Manual Page