std::uninitialized_default_construct

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Memory management library

voidify

(exposition only*)

Allocators

allocator
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allocator_arg (C++11)
uses_allocator (C++11)
uses_allocator_construction_args (C++20)
make_obj_using_allocator (C++20)
uninitialized_construct_using_allocator (C++20)

Uninitialized memory algorithms

uninitialized_copy
uninitialized_fill
uninitialized_move (C++17)
uninitialized_copy_n (C++11)
uninitialized_fill_n
uninitialized_move_n (C++17)
construct_at (C++20)

uninitialized_default_construct (C++17)
uninitialized_value_construct (C++17)
destroy (C++17)
uninitialized_default_construct_n (C++17)
uninitialized_value_construct_n (C++17)
destroy_n (C++17)
destroy_at (C++17)

Constrained uninitialized memory algorithms

ranges::uninitialized_copy (C++20)
ranges::uninitialized_fill (C++20)
ranges::uninitialized_move (C++20)
ranges::uninitialized_copy_n (C++20)
ranges::uninitialized_fill_n (C++20)
ranges::uninitialized_move_n (C++20)
ranges::construct_at (C++20)

ranges::uninitialized_default_construct (C++20)
ranges::uninitialized_value_construct (C++20)
ranges::destroy (C++20)
ranges::uninitialized_default_construct_n (C++20)
ranges::uninitialized_value_construct_n (C++20)
ranges::destroy_n (C++20)
ranges::destroy_at (C++20)

Memory resources

pmr::memory_resource (C++17)
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pmr::set_default_resource (C++17)
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pmr::pool_options (C++17)

pmr::null_memory_resource (C++17)
pmr::synchronized_pool_resource (C++17)
pmr::unsynchronized_pool_resource (C++17)
pmr::monotonic_buffer_resource (C++17)

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Uninitialized storage(until C++20)

raw_storage_iterator (until C++20*)
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return_temporary_buffer (until C++20*)

Garbage collector support(until C++23)

declare_reachable (C++11)(until C++23)
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undeclare_reachable (C++11)(until C++23)
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get_pointer_safety (C++11)(until C++23)

Low level memory
management

operator newoperator new[]

operator deleteoperator delete[]

(C++11)

(C++11)

(C++17)

(C++20)

(C++17)

(C++11)

(C++11)

(C++11)

(until C++17*)

(C++11)

(C++17)

(C++26)

(C++26)

enable_shared_from_this

(C++11)

(C++11)

(C++11)

(C++23)

(C++23)

(C++11)

(C++20)

(C++11)

(C++11)

(C++20)

is_sufficiently_aligned

(C++26)

C Library

malloc
calloc
realloc

free
aligned_alloc (C++17)

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Defined in header`<memory>`
template<class NoThrowForwardIt> void uninitialized_default_construct( NoThrowForwardIt first, NoThrowForwardIt last);	(1)	(since C++17) (constexpr since C++26)
template<class ExecutionPolicy,class NoThrowForwardIt> void uninitialized_default_construct( ExecutionPolicy&& policy, NoThrowForwardIt first, NoThrowForwardIt last);	(2)	(since C++17)

1) Constructs objects of typetypenamestd::iterator_traits<NoThrowForwardIt>::value_type in the uninitialized memory area[first, last) bydefault-initialization, as if by

for(; first!= last;++first)
::new(voidify(*first))
typenamestd::iterator_traits<NoThrowForwardIt>::value_type;

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 topolicy.

This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> istrue.	(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> istrue.	(since C++20)

[edit]Parameters

first, last	-	the pair of iterators defining therange of elements to initialize
policy	-	theexecution policy to use
Type requirements
- `NoThrowForwardIt` must meet the requirements ofLegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of`NoThrowForwardIt` may throw exceptions.

[edit]Complexity

Linear in the distance betweenfirst andlast.

[edit]Exceptions

The overload with a template parameter namedExecutionPolicy reports errors as follows:

If execution of a function invoked as part of the algorithm throws an exception andExecutionPolicy is one of thestandard 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]Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_raw_memory_algorithms`	`202411L`	(C++26)	constexpr forspecialized memory algorithms,(1)

[edit]Possible implementation

template<class NoThrowForwardIt>constexprvoid uninitialized_default_construct(NoThrowForwardIt first,                                               NoThrowForwardIt last){using Value=typenamestd::iterator_traits<NoThrowForwardIt>::value_type;    NoThrowForwardIt current= first;try{for(; current!= last;++current){::new(static_cast<void*>(std::addressof(*current))) Value;}}catch(...){std::destroy(first, current);throw;}}

[edit]Example

Run this code

#include <cstring>#include <iostream>#include <memory>#include <string> struct S{std::string m{"Default value"};}; int main(){constexprint n{3};    alignas(alignof(S))unsignedchar 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";} // For scalar types, uninitialized_default_construct// generally does not zero-fill the given uninitialized memory area.int v[]{1,2,3,4};constint original[]{1,2,3,4};    std::uninitialized_default_construct(std::begin(v),std::end(v)); // Maybe undefined behavior, pending CWG 1997 to be resolved.// for (const int i : v)//     std::cout << i << ' '; // The result is unspecified.std::cout<<(std::memcmp(v, original, sizeof(v))==0?"Unmodified\n":"Modified\n");}

Possible output:

Default valueDefault valueDefault valueUnmodified

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3870	C++20	this algorithm might create objects on aconst storage	kept disallowed

[edit]See also

uninitialized_default_construct_n (C++17)	constructs objects bydefault-initialization in an uninitialized area of memory, defined by a start and a count (function template)[edit]
uninitialized_value_construct (C++17)	constructs objects byvalue-initialization in an uninitialized area of memory, defined by a range (function template)[edit]
ranges::uninitialized_default_construct (C++20)	constructs objects bydefault-initialization in an uninitialized area of memory, defined by a range (algorithm function object)[edit]