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

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ranges::uninitialized_move_n
(C++20)
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C Library

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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> count,

                          O ofirst, S olast );
 (1) (since C++20)
(constexpr since C++26)
Helper types
template< class I, class O >
using uninitialized_move_n_result = ranges::in_out_result<I, O>;
 (2) (since C++20)

Let N be ranges::min(count, ranges::distance(ofirst, olast)).

Copies N elements from the range beginning at ifirst (using move semantics if supported) to an uninitialized memory area [ofirst, olast) 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.

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

Contents

[edit] 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

[edit] Return value

As described above.

[edit] Complexity

𝓞(N).

[edit] Exceptions

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

[edit] 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.

Feature-test macro Value Std Feature
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1)

[edit] 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>>
    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{};

[edit] 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"

[edit] 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

[edit] See also

(C++20)
 moves a range of objects to an uninitialized area of memory
(algorithm function object)[edit]
(C++17)
 moves a number of objects to an uninitialized area of memory
(function template) [edit]
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