std::ranges::uninitialized_value_construct_n
Defined in header <memory> | ||
|---|---|---|
| Call signature | ||
template< /*nothrow-forward-iterator*/ I >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_value_construct_n( I first,
std::iter_difference_t<I> count );
| (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_value_construct_n( Ep&& policy, I first,
std::iter_difference_t<I> count );
| (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 value-initialization as if by return ranges::uninitialized_value_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_allocis 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::terminateis invoked).
Notes
An implementation may improve the efficiency of the ranges::uninitialized_value_construct_n (by using e.g. ranges::fill_n) if the value type of the range is a CopyAssignable 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_value_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_value_construct(iter, std::default_sentinel).base();
}
};
inline constexpr uninitialized_value_construct_n_fn uninitialized_value_construct_n{};
|
Example
#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_value_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_value_construct_n
// zero-initializes the given uninitialized memory area.
int v[]{1, 2, 3, 4, 5, 6, 7, 8};
std::cout << ' ';
for (const int i : v)
std::cout << i << ' ';
std::cout << "\n ";
std::ranges::uninitialized_value_construct_n(std::begin(v), std::size(v));
for (const int i : v)
std::cout << i << ' ';
std::cout << '\n';
}
Output:
1 █▓▒░ █▓▒░ █▓▒░ 2 █▓▒░ █▓▒░ █▓▒░ 3 █▓▒░ █▓▒░ █▓▒░ 4 █▓▒░ █▓▒░ █▓▒░ 1 2 3 4 5 6 7 8 0 0 0 0 0 0 0 0
See also
|
(C++20) | constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object) |
|
(C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a range (algorithm function object) |
|
(C++20) | constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object) |
|
(C++17) | constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template) |
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