std::uninitialized_default_construct
From cppreference.com
| Defined in header <memory>
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||
| template< class ForwardIt > void uninitialized_default_construct( ForwardIt first, ForwardIt last); |
(1) | (since C++17) |
| template< class ExecutionPolicy, class ForwardIt > void uninitialized_default_construct( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last ); |
(2) | (since C++17) |
1) Constructs objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage designated by the range
[first, last) by default-initialization, as if by
for (; first != last; ++first) ::new (/*VOIDIFY*/(*first)) typename std::iterator_traits<ForwardIt>::value_type;
where /*VOIDIFY*/(e) is:
|
(until C++20) |
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(since C++20) |
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to
policy. This overload does not participate in overload resolution unless |
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. |
(until C++20) |
|
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. |
(since C++20) |
Contents |
[edit] Parameters
| first, last | - | the range of the elements to initialize |
| policy | - | the execution policy to use. See execution policy for details. |
| Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
| ||
-No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.
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[edit] Return value
(none)
[edit] Complexity
Linear in the distance between first and last.
[edit] Exceptions
The overload with a template parameter named ExecutionPolicy reports errors as follows:
- If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicyis one of the standard policies, std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined. - If the algorithm fails to allocate memory, std::bad_alloc is thrown.
[edit] Possible implementation
template<class ForwardIt> void uninitialized_default_construct(ForwardIt first, ForwardIt last) { using Value = typename std::iterator_traits<ForwardIt>::value_type; ForwardIt current = first; try { for (; current != last; ++current) { ::new (const_cast<void*>(static_cast<const volatile void*>( std::addressof(*current)))) Value; } } catch (...) { std::destroy(first, current); throw; } } |
[edit] Example
Run this code
#include <iostream> #include <memory> #include <string> #include <cstring> int main() { struct S { std::string m{ "Default value" }; }; constexpr int n {3}; alignas(alignof(S)) unsigned char mem[n * sizeof(S)]; try { auto first {reinterpret_cast<S*>(mem)}; auto last {first + n}; std::uninitialized_default_construct(first, last); for (auto it {first}; it != last; ++it) { std::cout << it->m << '\n'; } std::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. int v[] { 1, 2, 3, 4 }; const int original[] { 1, 2, 3, 4 }; std::uninitialized_default_construct(std::begin(v), std::end(v)); // for (const int i : v) { std::cout << i << ' '; } // Maybe undefined behavior, pending CWG 1997. std::cout << (std::memcmp(v, original, sizeof(v)) == 0 ? "Unmodified\n" : "Modified\n"); // The result is unspecified. }
Possible output:
Default value Default value Default value Unmodified
[edit] See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and a count (function template) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a range (function template) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (niebloid) |
