std::ranges::uninitialized_copy_n, std::ranges::uninitialized_copy_n_result

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

Copies \(\scriptsize N\)N elements from the range beginning at ifirst to an uninitialized memory area [ofirst, olast) 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.

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

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

[edit] Parameters

[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 the ranges::uninitialized_copy_n, by using e.g. ranges::copy_n , if the value type of the output range is TrivialType.

[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";
 }
}

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

[edit] See also

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