std::ranges::uninitialized_copy() algorithm

od C++20

Simplified

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

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

The type of arguments are generic and have following constraints:

• I - std::input_iterator
• S1, S2 - 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_reference_t<I>>
• (1) std::constructible_from<ranges::range_value_t<OR>, ranges::range_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_reference_t<I>>
uninitialized_copy_result<I, O>
  uninitialized_copy( 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_reference_t<IR>>
uninitialized_copy_result< ranges::borrowed_iterator_t<IR>,
                           ranges::borrowed_iterator_t<OR> >
  uninitialized_copy( IR&& in_range, OR&& out_range );

With the helper types defined as follows:

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

• (1) Let N be ranges::min(ranges::distance(ifirst, ilast), ranges::distance(ofirst, olast)): Constructs N elements in the output range [ofirst; olast), which is an uninitialized memory area, from the elements in the input range [ifirst; ilast).

  important

  The input and output ranges must not overlap.

  uwaga

  If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

  The function has the effect equal to:

  for (; !(ifirst == ilast || ofirst == olast); ++ofirst, ++ifirst)
  {
      ::new (static_cast<void*>(std::addressof(*ofirst)))
          std::remove_reference_t<std::iter_reference_t<O>>(*ifirst);
  }

• (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 range of elements to copy from.
in_range	The range of elements to copy from.
ofirst olast	The destination range.
out_range	The destination range.

Return value

{
  ifirst + N,
  ofirst + N
}

Complexity

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

O(N)

Exceptions

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

Possible implementation

uninitialized_copy(1) and uninitialized_copy(2)

struct uninitialized_copy_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_reference_t<I>>
    ranges::uninitialized_copy_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)
                ranges::construct_at(std::addressof(*current), *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_reference_t<IR>>
    ranges::uninitialized_copy_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_copy_fn uninitialized_copy{};

Notes

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

Examples

Main.cpp

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

int main()
{
    const char* v[]{ "This", "is", "an", "example", };

    if (const auto sz{std::size(v)};
        void* pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
    {
        try
        {
            auto first {static_cast<std::string*>(pbuf)};
            auto last {first + sz};
            std::ranges::uninitialized_copy(std::begin(v), std::end(v), first, last);

            std::cout << "{ ";
            for (auto it{first}; it != last; ++it)
                std::cout << std::quoted(*it) << ", ";
            std::cout << "};\n";

            std::ranges::destroy(first, last);
        }
        catch (...)
        {
            std::cout << "uninitialized_copy exception\n";
        }
        std::free(pbuf);
    }
}

Output

{ "This", "is", "an", "example", };