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)

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

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


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.


Exactly n assignments.

Possible implementation

struct fill_n_fn
    template<class T, std::output_iterator<const T&> O>
    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 {};


#include <algorithm>
#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, "▓▓░░");
    std::ranges::fill_n(v.begin(), n, "░░▓▓");


 ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░ ▓▓░░
 ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓ ░░▓▓

See also

assigns a range of elements a certain value
copies a number of elements to a new location
saves the result of a function in a range
applies a function to a range of elements
copy-assigns the given value to N elements in a range
(function template)

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