std::for_each_n
<algorithm>
InputIt for_each_n( InputIt first, Size n, UnaryFunc f );
(constexpr since C++20)
class ForwardIt, class Size, class UnaryFunc >
ForwardIt for_each_n( ExecutionPolicy&& policy,
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.
UnaryFunc
is not MoveConstructible, the behavior is undefined.std::is_execution_policy_v <std::decay_t <ExecutionPolicy>> is true.
(until C++20)std::is_execution_policy_v <std::remove_cvref_t <ExecutionPolicy>> is true.
(since C++20)UnaryFunc
is not CopyConstructible, the behavior is undefined.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.
Contents
[edit] Parameters
[
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.
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 otherExecutionPolicy
, 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 <algorithm> #include <iostream> #include <vector> void println(auto const& v) { for (auto count{v.size()}; const auto& e : v) std::cout << e << (--count ? ", " : "\n"); } int main() { std::vector <int> 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
[edit] See also
(function template) [edit]
(algorithm function object)[edit]