std::ranges::fill_n - cppreference.com

From cppreference.com

Defined in header `<algorithm>`
Call signature
`template< class T, std::output_iterator<const T&> O > constexpr O fill_n( O first, std::iter_difference_t<O> n, const T& value );`		(since C++20) (until C++26)
`template< class O, class T = std::iter_value_t<O> > requires std::output_iterator<O, const T&> constexpr O fill_n( O first, std::iter_difference_t<O> n, const T& value );`		(since C++26)

Assigns the given value to all elements in the range [first, first + n).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup.
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first	-	the beginning of the range of elements to modify
n	-	number of elements to modify
value	-	the value to be assigned

Return value

An output iterator that compares equal to first + n.

Complexity

Exactly n assignments.

Possible implementation

struct fill_n_fn
{
    template<class O, class T = std::iter_value_t<O>>
    requires std::output_iterator<O, const T&>
    constexpr O operator()(O first, std::iter_difference_t<O> n, const T& value) const
    {
        for (std::iter_difference_t<O> i {}; i != n; ++first, ++i)
            *first = value;
        return first;
    }
};

inline constexpr fill_n_fn fill_n {};

Notes

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

Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <string>
#include <vector>

void println(const auto& v)
{
    for (const auto& elem : v)
        std::cout << ' ' << elem;
    std::cout << '\n';
}

int main()
{
    constexpr auto n{8};
    
    std::vector<std::string> v(n, "▓▓░░");
    println(v);
    
    std::ranges::fill_n(v.begin(), n, "░░▓▓");
    println(v);

    std::vector<std::complex<double>> nums{{1, 3}, {2, 2}, {4, 8}};
    println(nums);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::ranges::fill_n(nums.begin(), 2, {4, 2});
    #else
        std::ranges::fill_n(nums.begin(), 2, std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

 ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░
 ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓
 (1,3) (2,2) (4,8)
 (4,2) (4,2) (4,8)