std::for_each (3) - Linux Manuals

std::for_each: std::for_each

NAME

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