std::ranges::uninitialized_value_construct
Defined in header <memory> |
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| Call signature | ||
template <no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requires std::default_initializable<std::iter_value_t<I>> I uninitialized_value_construct( I first, S last ); |
(1) | (since C++20) |
template <no-throw-forward-range R> requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R> uninitialized_value_construct( R&& 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 (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.
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 may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special compiler extensions.
Parameters
| first, last | - | iterator-sentinel pair denoting the range of elements to value-initialize |
| r | - | the range of the elements to value-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 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.
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{};
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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 {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
See also
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(C++20)
|
constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (niebloid) |
|
(C++20)
|
constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid) |
|
(C++20)
|
constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid) |
|
(C++17)
|
constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template) |
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