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std::ranges::uninitialized_copy_n, std::ranges::uninitialized_copy_n_result - cppreference.com

From cppreference.com

Defined in header <memory>

Call signature

 
template< std::input_iterator I,
          /*nothrow-input-iterator*/ O, /*nothrow-sentinel-for*/<O> S >
    requires std::constructible_from<std::iter_value_t<O>,
                                     std::iter_reference_t<I>>
uninitialized_copy_n_result<I, O>
    uninitialized_copy_n( I ifirst, std::iter_difference_t<I> count,
                          O ofirst, S olast );
(1) (since C++20)
(constexpr since C++26)		 
template< /*execution-policy*/ Ep, std::random_access_iterator I,
          /*nothrow-random-access-iterator*/ O,
          /*nothrow-sized-sentinel-for*/<O> S>
    requires std::constructible_from<std::iter_value_t<O>,
                                     std::iter_reference_t<I>>
uninitialized_copy_n_result<I, O>
    uninitialized_copy_n( Ep&& policy,
                          I ifirst, std::iter_difference_t<I> count,
                          O ofirst, S olast );
(2) (since C++26)

Helper types

 
template< class I, class O >
using uninitialized_copy_n_result = ranges::in_out_result<I, O>;
(3) (since C++20)

For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.

1) Constructs elements in the destination range [ofirst, olast) from elements in the source range ifirst + [0, count) as if by

auto ret = ranges::uninitialized_copy(std::counted_iterator(std::move(ifirst), count),
                                      std::default_sentinel, ofirst, olast);
return {std::move(ret.in).base(), ret.out};

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

If [ofirst, olast) overlaps with ifirst + [0, count), the behavior is undefined.

2) Same as (1), but executed according to policy.

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

Parameters

ifirst - the beginning of the range of elements to copy from
count - the number of elements to copy
ofirst, olast - the iterator-sentinel pair defining the destination range of elements
policy - the execution policy to use

Return value

As described above.

Exceptions

Any exception thrown on construction of the elements in the destination range.

2) During the execution process:

  • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
  • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Notes

An implementation may improve the efficiency of ranges::uninitialized_copy_n (by using e.g. ranges::copy_n) if the value type of the output range is TrivialType.

Feature-test macro Value Std Feature
__cpp_lib_parallel_algorithm 202506L (C++26) Parallel range algorithms
__cpp_lib_raw_memory_algorithms 202411L (C++26) constexpr for specialized <memory> algorithms, (1)

Possible implementation

struct uninitialized_copy_n_fn
{
    template<std::input_iterator I,
             /*nothrow-input-iterator*/ O, /*nothrow-sentinel-for*/<O> S>
        requires std::constructible_from<std::iter_value_t<O>, std::iter_reference_t<I>>
    constexpr ranges::uninitialized_copy_n_result<I, O>
        operator()(I ifirst, std::iter_difference_t<I> count, O ofirst, S olast) const
    {
        auto iter = std::counted_iterator(std::move(ifirst), count);
        auto ret = ranges::uninitialized_copy(iter, std::default_sentinel, ofirst, olast);
        return {std::move(ret.in).base(), ret.out};
    }
};

inline constexpr uninitialized_copy_n_fn uninitialized_copy_n{};

Example

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

int main()
{
    const char* stars[]{"Procyon", "Spica", "Pollux", "Deneb", "Polaris"};
    
    constexpr int n{4};
    alignas(alignof(std::string)) char out[n * sizeof(std::string)];
    
    try
    {
        auto first{reinterpret_cast<std::string*>(out)};
        auto last{first + n};
        auto ret{std::ranges::uninitialized_copy_n(std::begin(stars), n, first, last)};
        
        std::cout << '{';
        for (auto it{first}; it != ret.out; ++it)
            std::cout << (it == first ? "" : ", ") << std::quoted(*it);
        std::cout << "};\n";
        
        std::ranges::destroy(first, last);
    }
    catch (...)
    {
        std::cout << "uninitialized_copy_n exception\n";
    }
}

Output: 

{"Procyon", "Spica", "Pollux", "Deneb"};

See also