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

std::ranges::uninitialized_fill_n - cppreference.com

Copies value to an uninitialized memory area first + [​0​, count) as if by return ranges::uninitialized_fill(std::counted_iterator(first, count),
                                  std::default_sentinel, value).base();

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

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.

As described above.

Linear in count.

Any exception thrown on construction of the elements in the destination range.

An implementation may improve the efficiency of the ranges::uninitialized_fill_n, e.g. by using ranges::fill_n, if the value type of the output range is TrivialType.

#include <iostream>
#include <memory>
#include <string>
 
int main()
{
    constexpr int n{3};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
 
    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last = std::ranges::uninitialized_fill_n(first, n, "cppreference");
 
        for (auto it{first}; it != last; ++it)
            std::cout << *it << '\n';
 
        std::ranges::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "Exception!\n";
    }
}

Output:

cppreference
cppreference
cppreference

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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