std::ranges::uninitialized_value_construct() algorithm
- od C++20
- Simplified
- Detailed
// (1)
I uninitialized_value_construct( I first, S last );
// (2)
ranges::borrowed_iterator_t<R>
uninitialized_value_construct( R&& r );
The type of arguments are generic and have following constraints:
I
-no-throw-forward-iterator
S
-no-throw-sentinel-for<I>
R
-no-throw-forward-range
Additionally, each overload has the following constraints:
- (1)
std::default_initializable<std::iter_value_t<I>>
- (2)
std::default_initializable<ranges::range_value_t<R>>
// (1)
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 );
// (2)
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 );
-
(1) Constructs objects of type
std::iter_value_t<I>
in the uninitialized storage designated by the range [first
;last
) byvalue-initialization
, as if by:for (; first != last; ++first)
::new (static_cast<void*>(std::addressof(*first)))
std::remove_reference_t<std::iter_reference_t<I>>();uwagaIf 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 usingranges::begin(r)
asfirst
, andranges::end(r)
aslast
.
The function-like entities described on this page are niebloids.
Parameters
first last | The range of elements to initialize. |
r | The range of 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.
Possible implementation
uninitialized_value_construct(1) and uninitialized_value_construct(2)
truct 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{};
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
.
Examples
#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';
}
1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
A B C D
A A A A
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