std::ranges::uninitialized_default_construct_n
Defined in header <memory> |
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| Call signature | ||
template <no-throw-forward-iterator I> requires std::default_initializable<std::iter_value_t<I>> I uninitialized_default_construct_n( I first, std::iter_difference_t<I> n ); |
(since C++20) |
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 (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.
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 | - | 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.
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_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{};
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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 = 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';
}
Possible output:
1 █▓▒░ █▓▒░ 2 █▓▒░ █▓▒░ 3 █▓▒░ █▓▒░ 4 █▓▒░ █▓▒░ 1 2 3 4 5 6
See also
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(C++20)
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constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid) |
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(C++20)
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constructs objects by value-initialization in an uninitialized area of memory, defined by a range (niebloid) |
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(C++20)
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constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (niebloid) |
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(C++17)
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constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template) |
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