std::fill_n

Since C++26

Header: <algorithm>

Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.

# Declarations

template< class OutputIt, class Size, class T >
OutputIt fill_n( OutputIt first, Size count, const T& value );

(constexpr since C++20) (until C++26)

template< class OutputIt, class Size,
class T = typename std::iterator_traits
<OutputIt>::value_type >
constexpr OutputIt fill_n( OutputIt first, Size count,
const T& value );

(since C++26)

template< class ExecutionPolicy,
class ForwardIt, class Size, class T >
ForwardIt fill_n( ExecutionPolicy&& policy,
ForwardIt first, Size count, const T& value );

(since C++17) (until C++26)

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

(since C++26)

# 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

# Return value

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

# Notes

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

# Example

#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';
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be CopyAssignable, butT is not always writable to OutputIt	required to be writable instead
LWG 426	C++98	the complexity requirement was “exactly countassignments”, which is broken if count is negative	no assignment ifcount is non-positive
LWG 865	C++98	the location of the first element followingthe filling range was not returned	returned