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Showing content from https://en.cppreference.com/w/cpp/language/../memory/ranges/../../algorithm/ranges/fill.html below:

std::ranges::fill - cppreference.com

Call signature

1) Assigns the given value to the elements in the range [first, last).

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.

An output iterator that compares equal to last.

Exactly last - first assignments.

struct fill_fn
{
    template<class O, std::sentinel_for<O> S, class T = std::iter_value_t<O>>
    requires std::output_iterator<O, const T&>
    constexpr O operator()(O first, S last, const T& value) const
    {
        while (first != last)
            *first++ = value;
 
        return first;
    }
 
    template<class R, class T = ranges::range_value_t<R>>
    requires ranges::output_range<R, const T&>
    constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, const T& value) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value);
    }
};
 
inline constexpr fill_fn fill;

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>
 
void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<int> v{0, 1, 2, 3, 4, 5};
 
    // set all elements to -1 using overload (1)
    std::ranges::fill(v.begin(), v.end(), -1);
    println(v);
 
    // set all element to 10 using overload (2)
    std::ranges::fill(v, 10);
    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(nums, {4, 2}); // T gets deduced
    #else
        std::ranges::fill(nums, std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

-1 -1 -1 -1 -1 -1
10 10 10 10 10 10
(1,3) (2,2) (4,8)
(4,2) (4,2) (4,2)

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