std::for_each_n - cppreference.com (original) (raw)

Defined in header
template< class InputIt, class Size, class UnaryFunc >InputIt for_each_n( InputIt first, Size n, UnaryFunc f );	(1)	(since C++17) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class UnaryFunc >ForwardIt for_each_n( ExecutionPolicy&& policy, ForwardIt first, Size n, UnaryFunc f );	(2)	(since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n). If f returns a result, the result is ignored.

f is applied in order starting from first.
f might not be applied in order. The algorithm is executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

If n >= 0 is not true, 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_n is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

[edit] Parameters

first	-	the beginning of the range to apply the function to
n	-	the number of elements to apply the function to
policy	-	the execution policy to use
f	-	function object, to be applied to the result of dereferencing every iterator in the range [first, first + n) 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 LegacyInputIterator.
-ForwardIt must meet the requirements of LegacyForwardIterator.
-Size must be convertible to an integral type.

[edit] Return value

An iterator equal to first + n, or more formally, to std::advance(first, n).

[edit] Complexity

Exactly n applications of f.

[edit] 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.

[edit] Possible implementation

See also the implementation in libstdc++, libc++ and MSVC stdlib.

template<class InputIt, class Size, class UnaryFunc> InputIt for_each_n(InputIt first, Size n, UnaryFunc f) { for (Size i = 0; i < n; ++first, (void) ++i) f(*first); return first; }

[edit] Example

#include #include #include void println(auto const& v) { for (auto count{v.size()}; const auto& e : v) std::cout << e << (--count ? ", " : "\n"); } int main() { std::vector vi{1, 2, 3, 4, 5}; println(vi); std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; }); println(vi); }

Output:

1, 2, 3, 4, 5 2, 4, 6, 4, 5