std::ranges::uninitialized_fill_n

template< /*nothrow-forward-iterator*/ 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> count,
                        const T& value );

template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I,
          class T = std::iter_value_t<I> >
    requires std::constructible_from<std::iter_value_t<I>, const T&>
I uninitialized_fill_n( Ep&& exec, I first, std::iter_difference_t<I> count,
                        const T& value );

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

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.

Parameters

Return value

As described above.

Exceptions

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

• If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
• If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

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

Possible implementation

struct uninitialized_fill_n_fn
{
    template</*nothrow-forward-range*/ I, class T>
        requires std::constructible_from<std::iter_value_t<I>, const T&>
    constexpr I operator()(I first, std::iter_difference_t<I> count,
                           const T& value) const
    {
        I rollback{first};
        try
        {
            for (; count-- > 0; ++first)
                ranges::construct_at(std::addressof(*first), value);
            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