std::ranges::uninitialized_default_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 default-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.
Parameters
| first, last | - | iterator-sentinel pair denoting the range of the elements to initialize |
| r | - | the range of the elements to initialize |
Return value
An iterator equal to last.
Complexity
Linear in the distance between first and last.
Exceptions
The exception thrown on construction of the elements in the destination range, if any.
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_v.
Possible implementation
struct uninitialized_default_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 ValueType = std::remove_reference_t<std::iter_reference_t<I>>; if constexpr (std::is_trivially_default_constructible_v<ValueType>) return ranges::next(first, last); // skip initialization I rollback{first}; try { for (; !(first == last); ++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; } } 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_default_construct_fn uninitialized_default_construct{}; |
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{first + n}; std::ranges::uninitialized_default_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_default_construct // generally does not zero-fill the given uninitialized memory area. constexpr char sample[]{'A', 'B', 'C', 'D', '\n'}; char v[]{'A', 'B', 'C', 'D', '\n'}; std::ranges::uninitialized_default_construct(std::begin(v), std::end(v)); if (std::memcmp(v, sample, sizeof(v)) == 0) { std::cout << " "; // Maybe undefined behavior, pending CWG 1997: // for (const char c : v) { std::cout << c << ' '; } for (const char c : sample) std::cout << c << ' '; } else std::cout << "Unspecified\n"; }
Possible output:
1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D
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 |
See also
| 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 (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 range (function template) |