std::ranges::uninitialized_default_construct_n
| Defined in header <memory>
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| Call signature |
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| template <__NoThrowForwardIterator I> requires std::default_initializable<std::iter_value_t<I>> |
(1) | (since C++20) |
Constructs n objects of type std::iter_value_t<I> in the uninitialized memory area starting at first by default-initialization, as if by
for (; n-- > 0; ++first) ::new ( const_cast<void*>(static_cast<const volatile void*>(std::addressof(*first))) ) std::remove_reference_t<std::iter_reference_t<I>>;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
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.
Contents |
Parameters
| first | - | the beginning of the range of elements to initialize |
| n | - | the number of elements to construct. |
Return value
An iterator equal to ranges::next(first, n) that points to one past the last element of the range of constructed objects.
Complexity
Linear in n.
Notes
An implementation may skip the objects construction (without changing the observable effect) if the expression std::is_trivially_default_constructible_v<T> evaluates to true, where T is the value type of given range.
Possible implementation
struct uninitialized_default_construct_n_fn { template <__NoThrowForwardIterator I> requires std::default_initializable<std::iter_value_t<I>> I operator()( I first, std::iter_difference_t<I> n ) const { using ValueType = std::remove_reference_t<std::iter_reference_t<I>>; if constexpr (std::is_trivially_default_constructible_v<ValueType>) return ranges::next(first, n); // skip initialization I rollback {first}; try { for (; n-- > 0; ++first) ::new (const_cast<void*>(static_cast<const volatile void*> (std::addressof(*first)))) ValueType; return first; } catch (...) { // rollback: destroy constructed elements for (; rollback != first; ++rollback) ranges::destroy_at(std::addressof(*rollback)); throw; } } }; inline constexpr uninitialized_default_construct_n_fn uninitialized_default_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_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"; } // Notice that for "trivial types" the uninitialized_default_construct_n // does not zero-initialize the given uninitialized memory area. int v[] { 1, 2, 3, 4, 5, 6 }; std::ranges::uninitialized_default_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
See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (niebloid) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (niebloid) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template) |
