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 topolicy - 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: 305Eaxample 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
'Language > C++' 카테고리의 다른 글
| C++ array class definition (0) | 2022.06.22 |
|---|