std::ranges::uninitialized_default_construct
Defined in header <memory> |
||
|---|---|---|
| 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_default_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_default_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
default-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 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
#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 etalon[] { '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, etalon, sizeof(v)) == 0) {
std::cout << " ";
// Maybe undefined behavior, pending CWG 1997:
// for (const char c : v) { std::cout << c << ' '; }
for (const char c : etalon) { std::cout << c << ' '; }
} else {
std::cout << "Unspecified\n";
}
}
Possible output:
1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D
See also
|
(C++20)
|
constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid) |
|
(C++20)
|
constructs objects by value-initialization in an uninitialized area of memory, defined by a range (niebloid) |
|
(C++20)
|
constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (niebloid) |
|
(C++17)
|
constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/memory/ranges/uninitialized_default_construct
