std::for_each

std::for_each

 

C++

 

Algorithm library

 

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 InputIt is a mutable iterator, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored.

Parameters

first, last	-	the range to apply the function to
policy	-	the execution policy to use. See execution policy for details.
f	-	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:  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 InputIterator.
- 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) (nothing)

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,

• if policy is std::parallel_vector_execution_policy, std::terminate is called
• if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
• if policy is some other type, the behavior is implementation-defined

• If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

Possible implementation

template<class InputIt, class UnaryFunction> UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f) { for (; first != last; ++first) { f(*first); } return f; }

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:

Run this code

#include 
#include 
#include 
 
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};
 
    std::cout << "before:";
    for (auto const &n : nums)
    {
        std::cout << ' ' << n;
    }
    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: ";
    for (auto const &n : nums)
    {
        std::cout << ' ' << n;
    }
    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