std::ranges::uninitialized_default_construct
From cppreference.com
| Defined in header <memory>
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| Call signature |
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template< /*nothrow-forward-iterator*/ I, /*nothrow-sentinel-for*/<I> S >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct( I first, S last );
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(1) | (since C++20) (constexpr since C++26) |
template< /*nothrow-forward-range*/ R >
requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
uninitialized_default_construct( R&& r );
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(2) | (since C++20) (constexpr since C++26) |
template< /*execution-policy*/ Ep, /*nothrow-random-access-iterator*/ I,
/*nothrow-sized-sentinel-for*/<I> S >
requires std::default_initializable<std::iter_value_t<I>>
I uninitialized_default_construct( Ep&& policy, I first, S last );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep, /*nothrow-sized-random-access-range*/ R >
requires std::default_initializable<ranges::range_value_t<R>>
ranges::borrowed_iterator_t<R>
uninitialized_default_construct( Ep&& policy, R&& r );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of other exposition-only concepts, see this page.
1) Constructs elements in the destination range
[first, last) by default-initialization as if by
for (; first != last; ++first)
::new (voidify(*first))
std::remove_reference_t<std::iter_reference_t<I>>;
return first;
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 destination range.3,4) Same as (1,2), but executed according to
policy.The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Parameters
| first, last | - | the iterator-sentinel pair defining the range of elements to initialize |
| r | - | the range of the elements to initialize |
| policy | - | the execution policy to use |
Return value
As described above.
Exceptions
Any exception thrown on construction of the elements in the destination range.
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_parallel_algorithm |
202506L |
(C++26) | Parallel range algorithms |
__cpp_lib_raw_memory_algorithms |
202411L |
(C++26) | constexpr for specialized <memory> algorithms, (1,2)
|
Possible implementation
struct uninitialized_default_construct_fn
{
template</*nothrow-forward-iterator*/ I, /*nothrow-sentinel-for*/<I> S>
requires std::default_initializable<std::iter_value_t<I>>
constexpr I operator()(I first, S last) const
{
using value_type = std::remove_reference_t<std::iter_reference_t<I>>;
if constexpr (std::is_trivially_default_constructible_v<value_type>)
return ranges::next(first, last); // skip initialization
I rollback{first};
try
{
for (; !(first == last); ++first)
::new (static_cast<void*>(std::addressof(*first))) value_type;
return first;
}
catch (...) // rollback: destroy constructed elements
{
for (; rollback != first; ++rollback)
ranges::destroy_at(std::addressof(*rollback));
throw;
}
}
template</*nothrow-forward-range*/ R>
requires std::default_initializable<ranges::range_value_t<R>>
constexpr ranges::borrowed_iterator_t<R> operator()(R&& r) const
{
return (*this)(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)));
}
};
inline constexpr uninitialized_default_construct_fn uninitialized_default_construct{};
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Example
Run this code
#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 sample[]{'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, sample, sizeof(v)) == 0)
{
std::cout << " ";
// Maybe undefined behavior, pending CWG 1997 to be resolved:
// for (const char c : v) { std::cout << c << ' '; }
for (const char c : sample)
std::cout << c << ' ';
}
else
std::cout << "Unspecified\n";
}
Possible output:
1 ▄▀▄▀▄▀▄▀
2 ▄▀▄▀▄▀▄▀
3 ▄▀▄▀▄▀▄▀
4 ▄▀▄▀▄▀▄▀
A B C D
See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (algorithm function object) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (algorithm function object) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template) |