std::for_each(3)

NAME
std::for_each - std::for_each

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class UnaryFunction >
   UnaryFunction for_each( InputIt first, InputIt last, (until C++20)
   UnaryFunction f );
   template< class InputIt, class UnaryFunction >
   constexpr UnaryFunction for_each( InputIt first, InputIt last, (1) (since C++20)
   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>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) 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 parallel 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
   See also the implementations in libstdc++, libc++ and MSVC stdlib.
   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
 {
     void operator()(int n) { sum += n; }
     int sum{0};
 };
 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.cbegin(), nums.cend(), 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.cbegin(), nums.cend(), 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

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