NAME¶

std::fill_n - std::fill_n

Synopsis¶

Defined in header <algorithm>
template< class OutputIt, class Size, class T > (until C++11)
void fill_n( OutputIt first, Size count, const T& value );
template< class OutputIt, class Size, class T > (since C++11)
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 (1) (since C++20)
T& value );
template< class ExecutionPolicy, class ForwardIt, class Size,
class T > (2) (since C++17)
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 does not participate
in overload resolution unless
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) is true.

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¶

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

Complexity¶

Exactly count assignments, for count > 0.

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¶

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;
}

Example¶

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

// Run this code

#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';
}

Output:¶

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

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)