< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations (on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library

Numeric operations
Operations on uninitialized memory
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)


[edit] 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. 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.

[edit] Return value

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

[edit] Complexity

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

[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

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;

[edit] Example

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

[edit] 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

[edit] See also

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