std::ranges::uninitialized_fill_n() algorithm

since C++20

Simplified

// (1)
I uninitialized_fill_n( I first, std::iter_difference_t<I> n, const T& x );

The type of arguments are generic and have following constraints:

• I - no-throw-forward-range
• T - (none)

Additionally, each overload has the following constraints:

• (1) std::constructible_from<std::iter_value_t<I>, const T& >

Detailed

// (1)
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 (static_cast<void*>(std::addressof(*first)))
        std::remove_reference_t<std::iter_reference_t<I>>(x);
}

caution

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.

Parameters

first	The beginning of the range of the elements to initialize.
n	Number of elements to construct.
x	The value to construct the elements with.

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.

Possible implementation

uninitialized_fill_n(1)

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{};

Examples

Main.cpp

#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