std::ranges::uninitialized_value_construct
From cppreference.com
| Defined in header <memory>
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| Call signature |
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| template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S > requires std::default_initializable<std::iter_value_t<I>> |
(1) | (since C++20) |
| template< no-throw-forward-range R > requires std::default_initializable<ranges::range_value_t<R>> |
(2) | (since C++20) |
1) Constructs objects of type std::iter_value_t<I> in the uninitialized storage designated by the range
[first, last) by value-initialization, as if by
for (; first != last; ++first) ::new (static_cast<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.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as last.
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 |
[edit] Parameters
| first, last | - | iterator-sentinel pair denoting the range of elements to value-initialize |
| r | - | the range of the elements to value-initialize |
[edit] Return value
An iterator equal to last.
[edit] Complexity
Linear in the distance between first and last.
[edit] Exceptions
The exception thrown on construction of the elements in the destination range, if any.
[edit] Notes
An implementation may improve the efficiency of the ranges::uninitialized_value_construct, e.g. by using ranges::fill, if the value type of the range is TrivialType and CopyAssignable.
[edit] Possible implementation
struct uninitialized_value_construct_fn { template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requires std::default_initializable<std::iter_value_t<I>> I operator()(I first, S last) const { using T = std::remove_reference_t<std::iter_reference_t<I>>; if constexpr (std::is_trivial_v<T> && std::is_copy_assignable_v<T>) return ranges::fill(first, last, T()); I rollback{first}; try { for (; !(first == last); ++first) ::new (const_cast<void*>(static_cast<const volatile void*> (std::addressof(*first)))) T(); return first; } catch (...) // rollback: destroy constructed elements { for (; rollback != first; ++rollback) ranges::destroy_at(std::addressof(*rollback)); throw; } } template<no-throw-forward-range R> requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R> operator()(R&& r) const { return (*this)(ranges::begin(r), ranges::end(r)); } }; inline constexpr uninitialized_value_construct_fn uninitialized_value_construct{}; |
[edit] Example
Run this code
#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{first + n}; std::ranges::uninitialized_value_construct(first, last); 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_value_construct // zero-fills the given uninitialized memory area. int v[]{0, 1, 2, 3}; std::cout << ' '; for (const int i : v) std::cout << ' ' << static_cast<char>(i + 'A'); std::cout << "\n "; std::ranges::uninitialized_value_construct(std::begin(v), std::end(v)); for (const int i : v) std::cout << ' ' << static_cast<char>(i + 'A'); std::cout << '\n'; }
Output:
1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D A A A A
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed |
[edit] See also
| 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 range (niebloid) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template) |
