NAME¶

std::fill_n - std::fill_n

Synopsis¶

Defined in header <algorithm>
template< class OutputIt, class Size, class T (constexpr since
> C++20)
OutputIt fill_n( OutputIt first, Size count, (until C++26)
const T& value );
template< class OutputIt, class Size,

class T = typename
std::iterator_traits
(since C++26)
<OutputIt>::value_type >
constexpr OutputIt fill_n( OutputIt first,
Size count,

const T& value );
template< class ExecutionPolicy,

class ForwardIt, class Size, class (1)
T > (since C++17)
ForwardIt fill_n( ExecutionPolicy&& policy, (until C++26)

ForwardIt first, Size
count, const T& value );
template< class ExecutionPolicy,
(2)
class ForwardIt, class Size,
class T = typename
std::iterator_traits
(since C++26)
<OutputIt>::value_type >
ForwardIt fill_n( ExecutionPolicy&& policy,

ForwardIt first, Size
count, const T& value );

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

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)

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. See execution policy for details.

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;
}

Notes¶

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

Example¶

// Run this code

#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 required to be writable
T is not always writable to OutputIt instead
the complexity requirement was “exactly
LWG 426 C++98 count no assignment if
assignments”, which is broken if count is count is non-positive
negative
the location of the first element
LWG 865 C++98 following returned
the filling range was not returned

See also¶

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