std::for_each(3)

NAME
std::for_each - std::for_each

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class UnaryFunc >
   UnaryFunc for_each( InputIt first, InputIt last, (1) (constexpr since C++20)
   UnaryFunc f );
   template< class ExecutionPolicy, class ForwardIt, class
   UnaryFunc >
   void for_each( ExecutionPolicy&& policy, (2) (since C++17)
                  ForwardIt first, ForwardIt last,
   UnaryFunc f );
   Applies the given 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.
   If UnaryFunc is not MoveConstructible, the behavior is undefined. (since C++11)
   2) f might not be applied in order. The algorithm is executed according to policy.
   This overload participates in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)
   If UnaryFunc is not CopyConstructible, the behavior is undefined.
   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.

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.

Return value
   1) f
   2) (none)

Complexity
   Exactly std::distance(first, last) 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
   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
   }

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.

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 (a.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
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       it was unclear whether f can modify
                       the elements
   LWG 475 C++98 of the sequence being iterated over made clear (allowed if the
                       (for_each is iterator type is mutable)
                       classified as “non-modifying sequence
                       operations”)
   LWG 2747 C++11 overload (1) returned std::move(f) returns f (which
                        implicitly moves)

See also
                        applies a function to a range of elements, storing results in
   transform a destination range
                        (function template)
   for_each_n applies a function object to the first N elements of a
   (C++17) sequence
                        (function template)
   ranges::for_each applies a function to a range of elements
   (C++20) (niebloid)
   ranges::for_each_n applies a function object to the first N elements of a
   (C++20) sequence
                        (niebloid)
   range-for loop(C++11) executes loop over range