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:

Run this code

#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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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)