std::ranges::uninitialized_move_n() algorithm

since C++20

Simplified

// (1)
uninitialized_move_n_result<I, O>
  uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O ofirst, S olast );

The type of arguments are generic and have following constraints:

• I - std::input_iterator
• O - no-throw-forward-iterator
• S - no-throw-sentinel-for<O>

Additionally, each overload has the following constraints:

• (1) std::constructible_from<std::iter_value_t<O>, std::iter_rvalue_reference_t<I>>

Detailed

// (1)
template<
  std::input_iterator I,
  no-throw-forward-iterator O,
  no-throw-sentinel-for<O> S
>
  requires std::constructible_from<std::iter_value_t<O>, std::iter_rvalue_reference_t<I>>
uninitialized_move_n_result<I, O>
  uninitialized_move_n( I ifirst, std::iter_difference_t<I> n, O ofirst, S olast );

With the helper types defined as follows:

template< class I, class O >
using uninitialized_move_n_result = ranges::in_out_result<I, O>;

Given N as min(n, ranges::distance(ofirst, olast)):

Moves N elements from the input range beginning at ifirst to the uninitialized storage designated by the range [ofirst; olast)

The function has the effect equal to:

for (; n-- > 0 && ofirst != olast; ++ifirst, ++ofirst)
  ::new (static_cast<void*>(std::addressof(*first)))
      std::remove_reference_t<std::iter_reference_t<O>>(ranges::iter_move(ifirst));

caution

If an exception is thrown during the initialization then the objects that already constructed in [ofirst; olast) are destroyed in an unspecified order. Also, the objects in the input range beginning at ifirst, that were already moved, are left in a valid but unspecified state.

The function-like entities described on this page are niebloids.

Parameters

ifirst	The beginning of the input range to move elements from.
ofirst olast	The destination range to initialize.
n	The number of elements to move.

Return value

{
  ifirst + N,
  ofirst + N
}

Complexity

Linear in N.

Exceptions

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

Possible implementation

uninitialized_move_n(1)

struct uninitialized_move_n_fn
{
    template<std::input_iterator I, no-throw-forward-iterator O,
             no-throw-sentinel-for<O> S>
    requires std::constructible_from<std::iter_value_t<O>,
             std::iter_rvalue_reference_t<I>>
    ranges::uninitialized_move_n_result<I, O>
    operator()(I ifirst, std::iter_difference_t<I> n, O ofirst, S olast) const
    {
        O current{ofirst};
        try
        {
            for (; n-- > 0 && current != olast; ++ifirst, ++current)
                ::new (const_cast<void*>(static_cast<const volatile void*>
                    (std::addressof(*current)))) std::remove_reference_t<
                        std::iter_reference_t<O>>(ranges::iter_move_n(ifirst));
            return {std::move_n(ifirst), std::move_n(current)};
        }
        catch (...) // rollback: destroy constructed elements
        {
            for (; ofirst != current; ++ofirst)
                ranges::destroy_at(std::addressof(*ofirst));
            throw;
        }
    }
};

inline constexpr uninitialized_move_n_fn uninitialized_move_n{};

Notes

An implementation may improve the efficiency of ranges::uninitialized_move_n if the value type of the output range is TrivialType.

Examples

Main.cpp

#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <memory>
#include <string>

void print(auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; ++first)
        std::cout << std::quoted(*first) << ' ';
    std::cout << '\n';
}

int main()
{
    std::string in[]{ "No", "Diagnostic", "Required", };
    print("initially, in: ", std::begin(in), std::end(in));

    if (
        constexpr auto sz = std::size(in);
        void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)
    )
    {
        try
        {
            auto first{static_cast<std::string*>(out)};
            auto last{first + sz};
            std::ranges::uninitialized_move_n(std::begin(in), sz, first, last);

            print("after move_n, in: ", std::begin(in), std::end(in));
            print("after move_n, out: ", first, last);

            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

Output

initially, in: "No" "Diagnostic" "Required"
after move_n, in: "" "" ""
after move_n, out: "No" "Diagnostic" "Required"