std::ranges::uninitialized_default_construct() algorithm
- od C++20
- Simplified
- Detailed
// (1)
I uninitialized_default_construct( I first, S last );
// (2)
ranges::borrowed_iterator_t<R>
uninitialized_default_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_default_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_default_construct( R&& r );
Constructs n
objects of type std::iter_value_t<I>
in the uninitialized memory area starting at first
by default-initialization
,
as if by:
for (; n-- > 0; ++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.
The function-like entities described on this page are niebloids.
Parameters
first | The beginning of the range of elements to initialize. |
n | The number of elements to construct. |
Return value
The end of the range of objects (i.e., ranges::next(first, n)
).
Complexity
Linear in n
.
Exceptions
The exception thrown on construction of the elements in the destination range, if any.
Possible implementation
uninitialized_default_construct(1)
struct uninitialized_default_construct_n_fn
{
template<no-throw-forward-iterator I>
requires std::default_initializable<std::iter_value_t<I>>
I operator()(I first, std::iter_difference_t<I> n) 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, n); // skip initialization
I rollback{first};
try
{
for (; n-- > 0; ++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;
}
}
};
inline constexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};
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
.
Examples
#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 = std::ranges::uninitialized_default_construct_n(first, n);
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_n
// generally does not zero-fill the given uninitialized memory area.
constexpr int etalon[]{1, 2, 3, 4, 5, 6};
int v[]{1, 2, 3, 4, 5, 6};
std::ranges::uninitialized_default_construct_n(std::begin(v), std::size(v));
if (std::memcmp(v, etalon, sizeof(v)) == 0)
{
// Maybe undefined behavior, pending CWG 1997:
// for (const int i : v) { std::cout << i << ' '; }
for (const int i : etalon)
std::cout << i << ' ';
}
else
std::cout << "Unspecified!";
std::cout << '\n';
}
1 █▓▒░ █▓▒░
2 █▓▒░ █▓▒░
3 █▓▒░ █▓▒░
4 █▓▒░ █▓▒░
1 2 3 4 5 6
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