std::ranges::uninitialized_default_construct
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ranges::uninitialized_default_construct
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Defined in header
<memory>
Call signature
template< no-throw-forward-iterator I, no-throw-sentinel-for<I> S >
(1)
(since C++20) requires std::default_initializable <std::iter_value_t <I>>
(constexpr since C++26)
template< no-throw-forward-range R >
(2)
(since C++20) requires std::default_initializable <ranges::range_value_t <R>>
ranges::borrowed_iterator_t <R>
(constexpr since C++26)
1) Constructs objects of type std::iter_value_t <I> in the uninitialized memory area
[first, last) by default-initialization, as if by
for (; first != last; ++first)
::new (voidify (*first))
std::remove_reference_t <std::iter_reference_t <I>>;
return first;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Equivalent to ranges::uninitialized_default_construct(ranges::begin (r), ranges::end (r)).
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.
Contents
[edit] Parameters
first, last
-
the iterator-sentinel pair defining the range of elements to initialize
r
-
the
range of the elements to initialize
[edit] Return value
As described above.
[edit] Complexity
Linear in the distance between first and last.
[edit] Exceptions
Any exception thrown on construction of the elements in the destination range.
[edit] 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_raw_memory_algorithms |
202411L |
(C++26) | constexpr for specialized memory algorithms, (1,2) |
[edit] 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>> constexpr 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 (static_cast<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>> constexpr 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{};
[edit] 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 to be resolved: // 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
[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 default-initialization in an uninitialized area of memory, defined by a start and count
(algorithm function object)[edit]
(algorithm function object)[edit]
constructs objects by value-initialization in an uninitialized area of memory, defined by a range
(algorithm function object)[edit]
(algorithm function object)[edit]
constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count
(algorithm function object)[edit]
(algorithm function object)[edit]
(C++17)
(function template) [edit]