std::ranges::uninitialized_default_construct_n
From cppreference.com
| Defined in header <memory>
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| Call signature |
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template< /*nothrow-forward-iterator*/ I >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct_n( I first,
std::iter_difference_t<I> count );
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(1) | (since C++20) (constexpr since C++26) |
template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct_n( Ep&& policy, I first,
std::iter_difference_t<I> count );
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(2) | (since C++26) |
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
first + [0, count) by default-initialization as if by
return ranges::uninitialized_default_construct(std::counted_iterator(first, count),
std::default_sentinel).base();
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
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
| first | - | the beginning of the range of elements to initialize |
| count | - | the number of elements to construct |
| 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 skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible.
| 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)
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Possible implementation
struct uninitialized_default_construct_n_fn
{
template</*nothrow-forward-iterator*/ I>
requires std::default_initializable<std::iter_value_t<I>>
constexpr I operator()(I first, std::iter_difference_t<I> count) const
{
auto iter = std::counted_iterator(first, count);
return ranges::uninitialized_default_construct(iter, std::default_sentinel).base();
}
};
inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};
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Example
Run this code
#include <cstring>
#include <iostream>
#include <memory>
#include <string>
int main()
{
struct S { std::string m{"█▓▒░ █▓▒░ "}; };
constexpr int n{4};
alignas(alignof(S)) char out[n * sizeof(S)];
try
{
auto first{reinterpret_cast<S*>(out)};
auto last = std::ranges::uninitialized_default_construct_n(first, n);
auto count{1};
for (auto it{first}; it != last; ++it)
std::cout << count++ << ' ' << it->m << '\n';
std::ranges::destroy(first, last);
}
catch (...)
{
std::cout << "Exception!\n";
}
// For scalar types, uninitialized_default_construct_n
// generally does not zero-fill the given uninitialized memory area.
constexpr int sample[]{1, 2, 3, 4, 5, 6};
int v[]{1, 2, 3, 4, 5, 6};
std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
if (std::memcmp(v, sample, sizeof(v)) == 0)
{
// Maybe undefined behavior, pending CWG 1997:
// for (const int i : v) { std::cout << i << ' '; }
for (const int i : sample)
std::cout << i << ' ';
}
else
std::cout << "Unspecified!";
std::cout << '\n';
}
Possible output:
1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6
See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (algorithm function object) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (algorithm function object) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template) |