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

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< cpp‎ | memory
 
 
Memory management library
(exposition only*)
Allocators
Uninitialized memory algorithms Constrained uninitialized memory algorithms Memory resources Uninitialized storage (until C++20)
(until C++20*)
(until C++20*)

Garbage collector support (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)
(C++11)(until C++23)
 
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 \(\scriptsize N\)N be ranges::min (ranges::distance (ifirst, ilast), ranges::distance (ofirst, olast)).

1) Constructs \(\scriptsize N\)N elements elements from the range [ifirst, ilast) to an uninitialized memory area [ofirst, olast) 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 [ofirst, olast) overlaps with [ifirst, ilast), 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

\(\scriptsize\mathcal{O}(N)\)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

Run this code
#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]

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