std::uninitialized_default_construct_n
From cppreference.com
| Defined in header <memory>
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| template< class ForwardIt, class Size > ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n ); |
(1) | (since C++17) |
| template< class ExecutionPolicy, class ForwardIt, class Size > ForwardIt uninitialized_default_construct_n( ExecutionPolicy&& policy, ForwardIt first, Size n ); |
(2) | (since C++17) |
1) Constructs
n objects of type typename iterator_traits<ForwardIt>::value_type in the uninitialized storage starting at first by default-initialization, as if by
for (; n > 0; (void) ++first, --n) ::new (/*VOIDIFY*/(*first)) typename std::iterator_traits<ForwardIt>::value_type;
where /*VOIDIFY*/(e) is:
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(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>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.Parameters
| first | - | the beginning of the range of elements to initialize |
| n | - | the number of elements to construct |
| policy | - | the execution policy to use. See execution policy for details. |
| Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
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-No increment, assignment, comparison, or indirection through valid instances of ForwardIt may throw exceptions.
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Return value
The end of the range of objects (i.e., std::next(first, n)).
Complexity
Linear in n.
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.
Possible implementation
template<class ForwardIt, class Size> ForwardIt uninitialized_default_construct_n( ForwardIt first, Size n ) { using T = typename std::iterator_traits<ForwardIt>::value_type; ForwardIt current = first; try { for (; n > 0 ; (void) ++current, --n) { ::new (const_cast<void*>(static_cast<const volatile void*>( std::addressof(*current)))) T; } return current; } catch (...) { std::destroy(first, current); throw; } } |
Example
Run this code
#include <iostream> #include <memory> #include <string> 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 = std::uninitialized_default_construct_n(first, n); 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_n // generally does not zero-initialize the given uninitialized memory area. int v[] { 1, 2, 3, 4 }; const int original[] { 1, 2, 3, 4 }; std::uninitialized_default_construct_n(std::begin(v), std::size(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
See also
| constructs objects by default-initialization in an uninitialized area of memory, defined by a range (function template) | |
| constructs objects by value-initialization in an uninitialized area of memory, defined by a start and a count (function template) | |
| constructs objects by default-initialization in an uninitialized area of memory, defined by a start and count (niebloid) |