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std::ranges::uninitialized_move_n, std::ranges::uninitialized_move_n_result - cppreference.com

From cppreference.com

Defined in header <memory>

Call signature

 
template< std::input_iterator I,
          /*nothrow-forward-iterator*/ O, /*nothrow-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> count,
                          O ofirst, S olast );
(1) (since C++20)
(constexpr since C++26)		 
template< /*execution-policy*/ Ep, std::random_access_iterator I,
          /*nothrow-random-access-iterator*/ O,
          /*nothrow-sized-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( Ep&& policy, I ifirst,
                          std::iter_difference_t<I> count,
                          O ofirst, S olast );
(2) (since C++26)

Helper types

 
template< class I, class O >
using uninitialized_move_n_result = ranges::in_out_result<I, O>;
(3) (since C++20)

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

1) Constructs elements in the destination range [ofirst, olast) from elements in the source range ifirst + [0, count) as if by

auto ret = ranges::uninitialized_move(std::counted_iterator(std::move(ifirst), count),
                                      std::default_sentinel, ofirst, olast);
return {std::move(ret.in).base(), ret.out};

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.

If [ofirst, olast) overlaps with ifirst + [0, count), the behavior is undefined.

2) Same as (1), but executed according to policy.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Parameters

ifirst - the beginning of the input range of elements to move from
ofirst, olast - the iterator-sentinel pair defining the output range of elements to initialize
count - the number of elements to move
policy - the execution policy to use

Return value

As described above.

Exceptions

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

2) During the execution process:

  • 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_move_n (by using e.g. ranges::copy_n) if the value type of the output range is TrivialType.

Feature-test macro Value Std Feature
__cpp_lib_parallel_algorithm 202506L (C++26) Parallel range algorithms
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized <memory> algorithms, (1)

Possible implementation

struct uninitialized_move_n_fn
{
    template<std::input_iterator I,
             /*nothrow-forward-iterator*/ O, /*nothrow-sentinel-for*/<O> S>
        requires std::constructible_from<std::iter_value_t<O>,
                                         std::iter_rvalue_reference_t<I>>
    constexpr ranges::uninitialized_move_n_result<I, O>
        operator()(I ifirst, std::iter_difference_t<I> count, O ofirst, S olast) const
    {
        auto iter = std::counted_iterator(std::move(ifirst), count);
        auto ret = ranges::uninitialized_move(iter, std::default_sentinel, ofirst, olast);
        return {std::move(ret.in).base(), ret.out};
    }
};

inline constexpr uninitialized_move_n_fn uninitialized_move_n{};

Example

#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"

See also