std::ranges::uninitialized_move_n, std::ranges::uninitialized_move_n_result

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Defined in header <memory>
Call signature
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 );
 (1) (since C++20)
Helper types
template< class I, class O >
using uninitialized_move_n_result = ranges::in_out_result<I, O>;
 (2) (since C++20)

Moves N elements from the input range beginning at ifirst to the uninitialized storage designated by the range [ofirst, olast), where N is min(n, ranges::distance(ofirst, olast)).

The effect is equivalent 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));

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, that is:

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

Parameters

ifirst - the beginning of the input range of elements to move from
ofirst, olast - iterator-sentinel pair denoting the output range of elements 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.

Notes

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

Possible implementation

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(ifirst));
            return {std::move(ifirst), std::move(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{};

Example

Run this code
#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, in: ", std::begin(in), std::end(in));
            print("after move, out: ", first, last);
 
            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "Exception!\n";
        }
        std::free(out);
    }
}

Possible output: 

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

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 3870 C++20 this algorithm might create objects on a const storage kept disallowed

See also

(C++20)
 moves a range of objects to an uninitialized area of memory
(niebloid)
(C++17)
 moves a number of objects to an uninitialized area of memory
(function template)
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