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std::fill_n

Defined in header `<algorithm>`
	(1)
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)

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.

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

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:

#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';
}

Output:

-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	returned

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