std::ranges::uninitialized_move() algorithm

od C++20

Simplified

// (1)
uninitialized_move_result<I, O>
  uninitialized_move( I ifirst, S1 ilast, O ofirst, S2 olast );

// (2)
uninitialized_move_result< ranges::borrowed_iterator_t<IR>,
                           ranges::borrowed_iterator_t<OR> >
  uninitialized_move( IR&& in_range, OR&& out_range );

The type of arguments are generic and have following constraints:

• I - std::input_iterator
• S1 - std::sentinel_for<I>, no-throw-sentinel-for<O>
• O - no-throw-forward-iterator
• IR - ranges::input_range
• OR - no-throw-forward-range

Additionally, each overload has the following constraints:

• (1) std::constructible_from< std::iter_value_t<O>, std::iter_rvalue_reference_t<I> >
• (1) std::constructible_from< ranges::range_value_t<OR>, ranges::range_rvalue_reference_t<IR> >

Detailed

// (1)
template<
  std::input_iterator I,
  std::sentinel_for<I> S1,
  no-throw-forward-iterator O,
  no-throw-sentinel-for<O> S2
>
  requires std::constructible_from< std::iter_value_t<O>,
                                    std::iter_rvalue_reference_t<I> >
uninitialized_move_result<I, O>
  uninitialized_move( I ifirst, S1 ilast, O ofirst, S2 olast );

// (2)
template<
  ranges::input_range IR,
  no-throw-forward-range OR
>
  requires std::constructible_from< ranges::range_value_t<OR>,
                                    ranges::range_rvalue_reference_t<IR> >
uninitialized_move_result< ranges::borrowed_iterator_t<IR>,
                           ranges::borrowed_iterator_t<OR> >
  uninitialized_move( IR&& in_range, OR&& out_range );

With the helper types defined as follows:

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

• (1) Moves N elements from the input range [ifirst; ilast) to the output range [ofirst; olast), where N is min(ranges::distance(ifirst, ilast), ranges::distance(ofirst, olast)).

  The function has the effect equal to:

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

  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 [ifirst; ilast) that were already moved, are left in a valid but unspecified state.

• (2) Same as (1), but uses in_range as the first range and out_range as the second range, as if using ranges::begin(in_range) as ifirst, ranges::end(in_range) as ilast, ranges::begin(out_range) as ofirst, and ranges::end(out_range) as olast.

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

Parameters

ifirst ilast	The input range to move elements from.
in_range	The input range to move elements from.
ofirst olast	The destination range to initialize.
out_range	The destination range to initialize.

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(1) and uninitialized_move(2)

struct uninitialized_move_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S1,
             no-throw-forward-iterator O, no-throw-sentinel-for<O> S2>
    requires std::constructible_from<std::iter_value_t<O>,
             std::iter_rvalue_reference_t<I>>
    ranges::uninitialized_move_result<I, O>
    operator()(I ifirst, S1 ilast, O ofirst, S2 olast) const
    {
        O current{ofirst};
        try
        {
            for (; !(ifirst == ilast or 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;
        }
    }

    template<ranges::input_range IR, no-throw-forward-range OR>
    requires std::constructible_from<ranges::range_value_t<OR>,
             ranges::range_rvalue_reference_t<IR>>
    ranges::uninitialized_move_result<ranges::borrowed_iterator_t<IR>,
             ranges::borrowed_iterator_t<OR>>
    operator()(IR&& in_range, OR&& out_range) const
    {
        return (*this)(ranges::begin(in_range), ranges::end(in_range),
                       ranges::begin(out_range), ranges::end(out_range));
    }
};

inline constexpr uninitialized_move_fn uninitialized_move{};

Notes

An implementation may improve the efficiency of ranges::uninitialized_move 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[]{"Home", "World"};
    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(std::begin(in), std::end(in), 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);
    }
}

Output

initially, in: "Home" "World"
after move, in: "" ""
after move, out: "Home" "World"