std::fill_n

Defined in header `<algorithm>`
template< class OutputIt, class Size, class T > OutputIt fill_n( OutputIt first, Size count, const T& value );	(1)	(until C++26) (constexpr since C++20)
template< class OutputIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > constexpr OutputIt fill_n( OutputIt first, Size count, const T& value );	(1)		(since C++26)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value );	(2)	(since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class Size, class T = typename std::iterator_traits <OutputIt>::value_type > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value );	(2)		(since C++26)

1) Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.

2) Same as (1), but executed according to policy.

This overload participates in overload resolution only if the value of the following expression is true:

`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)

If any of the following conditions is satisfied, the program is ill-formed:

value is not writable to first.
Size is not convertible to an integral type.

Parameters

first	-	the beginning of the range of elements to modify
count	-	number of elements to modify
value	-	the value to be assigned
policy	-	the execution policy to use
Type requirements
-`OutputIt` must meet the requirements of LegacyOutputIterator.
-`ForwardIt` must meet the requirements of LegacyForwardIterator.

Return value

Iterator one past the last element assigned if count > 0, first otherwise.

Complexity

Exactly std::max(0, count) assignments.

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.

Possible implementation

fill_n
template<class OutputIt, class Size, class T = typename std::iterator_traits<OutputIt>::value_type> OutputIt fill_n(OutputIt first, Size count, const T& value) { for (Size i = 0; i < count; i++) *first++ = value; return first; }

fill_n

template<class OutputIt, class Size,
         class T = typename std::iterator_traits<OutputIt>::value_type>
OutputIt fill_n(OutputIt first, Size count, const T& value)
{
    for (Size i = 0; i < count; i++)
        *first++ = value;
    return first;
}

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	List-initialization for algorithms (1,2)

Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

    // replace values of the first 5 elements with -1
    std::fill_n(v1.begin(), 5, -1);

    std::copy_n(v1.cbegin(), v1.size(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';

    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    #ifdef __cpp_lib_algorithm_default_value_type
        std::fill_n(nums.begin(), 2, {4, 2});
    #else
        std::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
    #endif
    std::copy_n(nums.cbegin(), nums.size(),
                std::ostream_iterator<std::complex<double>>(std::cout, " "));
    std::cout << '\n';
}

Output:

-1 -1 -1 -1 -1 5 6 7 8 9
(4,2) (4,2) (4,8)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	`T` was required to be CopyAssignable, but `T` is not always writable to `OutputIt`	required to be writable instead
LWG 426	C++98	the complexity requirement was “exactly `count` assignments”, which is broken if `count` is negative	no assignment if `count` is non-positive
LWG 865	C++98	the location of the first element following the filling range was not returned	returned

fill	copy-assigns the given value to every element in a range (function template & algorithm function object)
ranges::fill (C++20)
ranges::fill_n (C++20)	assigns a value to a number of elements (algorithm function object)