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std::ranges::uninitialized_copy, std::ranges::uninitialized_copy_result

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< cpp‎ | memory
 
 
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ranges::uninitialized_copy
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Defined in header <memory>
Call signature
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 );
(1) (since C++20)
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 );
(2) (since C++20)
Helper types
template< class I, class O >
using uninitialized_copy_result = ranges::in_out_result<I, O>;
(3) (since C++20)
1) Let N be ranges::min(ranges::distance(ifirst, ilast), ranges::distance(ofirst, olast)), constructs N elements in the output range [ofirstolast), which is an uninitialized memory area, from the elements in the input range [ifirstilast).
The input and output ranges must not overlap.
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, that is:

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

Contents

[edit] Parameters

ifirst, ilast - iterator-sentinel pair denoting the range of elements to copy from
in_range - the range of elements to copy from
ofirst, olast - iterator-sentinel pair denoting the destination range
out_range - the destination range

[edit] Return value

{ifirst + N, ofirst + N}

[edit] Complexity

𝓞(N).

[edit] Exceptions

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

[edit] Notes

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

[edit] Possible implementation

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{};

[edit] Example

#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 << (it == first ? "" : ", ") << 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"};

[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

copies a number of objects to an uninitialized area of memory
(niebloid)[edit]
copies a range of objects to an uninitialized area of memory
(function template) [edit]