std::for_each ( c++20 )

define

Defined in header

template< class InputIt, class UnaryFunction >
UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f ); //(until C++20)
template< class InputIt, class UnaryFunction >
constexpr UnaryFunction for_each( InputIt first, InputIt last, UnaryFunction f );//(since C++20)

template< class ExecutionPolicy, class ForwardIt, class UnaryFunction2 >
void for_each( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryFunction2 f );//(since C++17)

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.
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.

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 1

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.

#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

Eaxample 2

#include <iostream>
#include <array> //array
#include <algorithm> //for_each

using namespace std;
typedef array<int, 4> Myarray;

int main()
{
    Myarray mArr = { 1,2,3,4};
    for_each(mArr.begin(), mArr.end(), []<typename T>(T input){
       cout << input << endl; 
    });

    return 0;
}

reference

cppreference.com

array class

definition

• 길이가 N인 Ty 형식의 요소 시퀀스를 제어하는 개체를 설명합니다. 시퀀스는 array<Ty, N> 개체에 포함된 Ty의 배열로 저장됩니다.

template <class Ty, std::size_t N>
class array;

parameter

Ty 요소의 형식입니다.
N 요소의 수입니다.

explanation

형식에 기본 생성자 array()와 기본 대입 연산자 operator=가 있고 aggregate에 대한 요구 사항을 충족합니다. 따라서 집계 이니셜라이저를 사용하여 array<Ty, N> 형식의 개체를 초기화할 수 있습니다. 예를 들면 다음과 같습니다.

array<int, 4> ai = { 1, 2, 3 };

4개의 정수 값을 보유하는 ai 개체를 만들고, 처음 세 개의 요소를 값 1, 2, 3으로 각각 초기화한 다음 네 번째 요소를 0으로 초기화합니다.

example

#include <iostream>
#include <array> //array
#include <algorithm> //for_each

using namespace std;
typedef array<int, 4> Myarray;

int main()
{
    Myarray mArr = { 1,2,3,4};
    for_each(mArr.begin(), mArr.end(), []<typename T>(T input){
       cout << input << endl; 
    });

    return 0;
}

reference

Online compiler
modoocode