std::ranges::fill_n

template< class T, std::output_iterator<const T&> O >
constexpr O
    fill_n( O first, std::iter_difference_t<O> n, const T& value );

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

The function-like entities described on this page are 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.

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

Parameters

Return value

An output iterator that compares equal to first + n.

Complexity

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 {};

Example

#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, "▓▓░░");
    println(v);
 
    std::ranges::fill_n(v.begin(), n, "░░▓▓");
    println(v);
}

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See also