< cpp‎ | algorithm
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
Defined in header <algorithm>
template< class OutputIt, class Size, class T >
void fill_n( OutputIt first, Size count, const T& value );
(until C++11)
template< class OutputIt, class Size, class T >
OutputIt fill_n( OutputIt first, Size count, const T& value );
(since C++11)
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 only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true


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 OutputIterator.
ForwardIt must meet the requirements of ForwardIterator.

Return value

(none) (until C++11)
Iterator one past the last element assigned if count > 0, first otherwise. (since C++11)


Exactly count assignments, for count > 0.


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 three 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 <vector>
#include <iostream>
#include <iterator>
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

See also

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