Defined in header <algorithm>
template< class OutputIt, class Size, class T >
OutputIt fill_n( OutputIt first, Size count, const T& value );
(until C++20)
template< class OutputIt, class Size, class T >
constexpr OutputIt fill_n( OutputIt first, Size count, const T& value );
(since C++20)
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)
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 does not participate in overload resolution unless

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)


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. See execution policy for details.
Type requirements
-OutputIt must meet the requirements of LegacyOutputIterator.
-ForwardIt must meet the requirements of LegacyForwardIterator.
-value must be writable to first.
-Size must be convertible to integral type.

Return value

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


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


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

template<class OutputIt, class Size, class T>
OutputIt fill_n(OutputIt first, Size count, const T& value)
    for (Size i = 0; i < count; i++)
        *first++ = value;
    return first;


The following code uses fill_n() to assign -1 to the first half of a vector of integers:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
int main()
    std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::fill_n(v1.begin(), 5, -1);
    std::copy(begin(v1), end(v1), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';


-1 -1 -1 -1 -1 5 6 7 8 9

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

See also

copy-assigns the given value to every element in a range
(function template)
assigns a value to a number of elements

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.