NAME¶

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

Synopsis¶

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>> (1) (since C++20)
uninitialized_copy_n_result<I, O>
uninitialized_copy_n( I ifirst, std::iter_difference_t<I>
count,

O ofirst, S olast );

Helper types¶

template< class I, class O > (2) (since C++20)
using uninitialized_copy_n_result = ranges::in_out_result<I, O>;

Let \(\scriptsize N\)N be ranges::min(count, ranges::distance(ofirst, olast)),
constructs \(\scriptsize N\)N elements in the output range [ofirst, olast), which is
an uninitialized memory area, from the elements in the input range beginning at
ifirst.

The input range [ifirst, ifirst + count) must not overlap with the output range
[ofirst, olast).

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

The function has the effect equivalent to:

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

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

ifirst - the beginning of the range of elements to copy from
count - the number of elements to copy
ofirst, olast - iterator-sentinel pair denoting the destination range

Return value¶

{ifirst + N, ofirst + N}

Complexity¶

\(\scriptsize\mathcal{O}(N)\)𝓞(N).

Exceptions¶

The exception thrown on construction of the elements in the destination range, if
any.

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.

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>>
ranges::uninitialized_copy_n_result<I, O>
operator()(I ifirst, std::iter_difference_t<I> count, O ofirst, S olast) const
{
O current{ofirst};
try
{
for (; count > 0 && current != olast; ++ifirst, ++current, --count)
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;
}
} };

inline constexpr uninitialized_copy_n_fn uninitialized_copy_n{};

Example¶

// Run this code

#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¶

ranges::uninitialized_copy copies a range of objects to an uninitialized area of
(C++20) memory
(niebloid)
uninitialized_copy_n copies a number of objects to an uninitialized area of
(C++11) memory
(function template)