std::ranges::uninitialized_fill_n

template <no-throw-forward-range I, class T>
requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x );

Constructs n copies of the given value x in an uninitialized memory area, designated by the range [first, first + n), as if by.

for (; n--; ++first) {
    ::new (
        const_cast<void*>(static_cast<const volatile void*>(std::addressof(*first)))
        ) std::remove_reference_t<std::iter_reference_t<I>>(x);
}

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 niebloids, that is:

• Explicit template argument lists may not be specified when calling any of them.
• None of them is visible to argument-dependent lookup.
• When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

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

Parameters

Return value

An iterator equal to first + n.

Complexity

Linear in n.

Exceptions

The exception thrown on construction of the elements in the destination range, if any.

Notes

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.

Possible implementation

struct uninitialized_fill_n_fn {
    template <no-throw-forward-range I, class T>
    requires std::constructible_from<std::iter_value_t<I>, const T&>
    I operator()( I first, std::iter_difference_t<I> n, const T& x ) const {
        I rollback {first};
        try {
            for (; n-- > 0; ++first)
                ranges::construct_at(std::addressof(*first), x);
            return first;
        } catch (...) { // rollback: destroy constructed elements
            for (; rollback != first; ++rollback)
                ranges::destroy_at(std::addressof(*rollback));
            throw;
        }
    }
};
 
inline constexpr uninitialized_fill_n_fn uninitialized_fill_n{};

Example

#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";
    }
}

cppreference
cppreference
cppreference

See also