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std::ranges::uninitialized_copy_n, std::ranges::uninitialized_copy_n_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_n
(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,

          no-throw-input-iterator O, no-throw-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)
Helper types
template< class I, class O >
using uninitialized_copy_n_result = ranges::in_out_result<I, O>;
(2) (since C++20)

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

Copies N elements from the range beginning at ifirst to an uninitialized memory area [ofirstolast) 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 [ofirstolast) overlaps with ifirst + [0count), the behavior is undefined.

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

Contents

[edit] 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

[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 the 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_raw_memory_algorithms 202411L (C++26) constexpr for specialized memory algorithms, (1)

[edit] Possible implementation

struct uninitialized_copy_n_fn
{
    template<std::input_iterator I, no-throw-input-iterator O, no-throw-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{};

[edit] 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"};

[edit] See also

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