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

From cppreference.com
< cpp‎ | memory
 
 
Memory management library
(exposition only*)
Uninitialized memory algorithms
(C++17)
(C++17)
(C++17)
Constrained uninitialized
memory algorithms
ranges::uninitialized_copy
(C++20)
C Library

Allocators
Memory resources
Garbage collection support
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
(C++11)(until C++23)
Uninitialized storage
(until C++20*)
(until C++20*)
Explicit lifetime management
 
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)
(constexpr since C++26)
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)
(constexpr since C++26)
Helper types
template< class I, class O >
using uninitialized_copy_result = ranges::in_out_result<I, O>;
(3) (since C++20)

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

1) Constructs N elements elements from the range [ifirstilast) to an uninitialized memory area [ofirstolast) as if by

for (; ifirst != ilast && ofirst != olast; ++ofirst, (void)++ifirst)
    ::new (voidify(*ofirst)) std::remove_reference_t<std::iter_reference_t<O>>(*ifirst);
return {std::move(ifirst), ofirst};

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
If [ofirstolast) overlaps with [ifirstilast), the behavior is undefined.
2) Equivalent to return ranges::uninitialized_copy(ranges::begin(in_range), ranges::end(in_range),
                                  ranges::begin(out_range), ranges::end(out_range));
.

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

Contents

[edit] Parameters

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

[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 ranges::uninitialized_copy 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, (1,2)

[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>>
    constexpr 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>,
    constexpr 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
(algorithm function object)[edit]
copies a range of objects to an uninitialized area of memory
(function template) [edit]