std::uninitialized_default_construct_n

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Memory management library

voidify

(exposition only*)

Allocators

allocator
allocator_traits (C++11)
allocation_result (C++23)
scoped_allocator_adaptor (C++11)
pmr::polymorphic_allocator (C++17)

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)
pmr::get_default_resource (C++17)
pmr::set_default_resource (C++17)
pmr::new_delete_resource (C++17)
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)

Explicit lifetime management
start_lifetime_as (C++23)
start_lifetime_as_array (C++23)

Types for composite class design
indirect (C++26)
polymorphic (C++26)

Uninitialized storage(until C++20)

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

return_temporary_buffer (until C++20*)

Garbage collector support(until C++23)

declare_reachable (C++11)(until C++23)
declare_no_pointers (C++11)(until C++23)
pointer_safety (C++11)(until C++23)

undeclare_reachable (C++11)(until C++23)
undeclare_no_pointers (C++11)(until C++23)
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)

[edit]

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

1) Constructs objects of typetypenamestd::iterator_traits<NoThrowForwardIt>::value_type in the uninitialized memory areafirst + [0, count) bydefault-initialization, as if by

for(; count>0;(void)++first,--count)
::new(voidify(*first))
typenamestd::iterator_traits<NoThrowForwardIt>::value_type;
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 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	-	the beginning of the range of elements to initialize
count	-	the number of elements to construct
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]Return value

As described above.

[edit]Complexity

Linear inn.

[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,class Size>constexpr ForwardIt uninitialized_default_construct_n(NoThrowForwardIt first, Size count){using T=typenamestd::iterator_traits<NoThrowForwardIt>::value_type;    NoThrowForwardIt current= first; try{for(; countn>0;(void)++current,--count)::new(static_cast<void*>(std::addressof(*current))) T;return current;}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= 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";} // For scalar types, uninitialized_default_construct_n// generally does not zero-initialize the given uninitialized memory area.int v[]{1,2,3,4};constint original[]{1,2,3,4};    std::uninitialized_default_construct_n(std::begin(v),std::size(v)); // An attempt to access v might be an 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?"un":"")<<"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 (C++17)	constructs objects bydefault-initialization in an uninitialized area of memory, defined by a range (function template)[edit]
uninitialized_value_construct_n (C++17)	constructs objects byvalue-initialization in an uninitialized area of memory, defined by a start and a count (function template)[edit]
ranges::uninitialized_default_construct_n (C++20)	constructs objects bydefault-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object)[edit]