std::ranges::uninitialized_copy_n, std::ranges::uninitialized_copy_n_result
From cppreference.com
| Defined in header <memory>
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| Call signature |
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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 );
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(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 );
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(2) | (since C++26) |
| Helper types |
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template< class I, class O >
using uninitialized_copy_n_result = ranges::in_out_result<I, O>;
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(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:
- 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.
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{};
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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
(C++20) |
copies a range of objects to an uninitialized area of memory (algorithm function object) |
(C++11) |
copies a number of objects to an uninitialized area of memory (function template) |