template<class OutputIterator, class Size, class T>
constexpr OutputIterator fill_n(OutputIterator first, Size n, const T& value);

template<class ExecutionPolicy, class ForwardIterator, class Size, class T>
ForwardIterator fill_n(ExecutionPolicy&& exec, ForwardIterator first, Size n, const T& value);

#include <iostream>
#include <algorithm>
#include <vector>

int main()
{
    std::vector<int> v(10);
    std::fill_n(std::begin(v), v.size(), 42);
    for (auto it : v) {
        std::cout << it << std::endl;
    }
    return 0;
}

42
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42
42
42
42
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42
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42