std::uninitialized_fill_n

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Memory management library

voidify

(exposition only*)

Allocators

allocator
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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)

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declare_reachable (C++11)(until C++23)
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Low level memory
management

operator newoperator new[]

operator deleteoperator delete[]

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(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,class Size,class T> NoThrowForwardIt uninitialized_fill_n( NoThrowForwardIt first, Size count,const T& value);	(1)	(constexpr since C++26)
template<class ExecutionPolicy, class NoThrowForwardIt,class Size,class T> NoThrowForwardIt uninitialized_fill_n( ExecutionPolicy&& policy, NoThrowForwardIt first, Size count,const T& value);	(2)	(since C++17)

1) Copiesvalue to an uninitialized memory areafirst + [0, count) as if by

for(; count--;++first)
::new(voidify(*first))
typenamestd::iterator_traits<NoThrowForwardIt>::value_type(value);
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 the elements to initialize
count	-	number of elements to construct
value	-	the value to construct the elements with
Type requirements
- `NoThrowForwardIt` must meet the requirements ofLegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of`NoThrowForwardIt` may throw exceptions. Applying&* to a`NoThrowForwardIt` value must yield a pointer to its value type.(until C++11)

[edit]Return value

As described above.

[edit]Complexity

Linear incount.

[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,class T>constexpr NoThrowForwardIt uninitialized_fill_n(NoThrowForwardIt first,                                                Size count,const T& value){using V=typenamestd::iterator_traits<NoThrowForwardIt>::value_type;    NoThrowForwardIt current= first;try{for(; count>0;++current,(void)--count)::new(static_cast<void*>(std::addressof(*current))) V(value);return current;}catch(...){for(; first!= current;++first)            first->~V();throw;}return current;}

[edit]Example

Run this code

#include <algorithm>#include <iostream>#include <memory>#include <string>#include <tuple> int main(){std::string* p;std::size_t sz;std::tie(p, sz)=std::get_temporary_buffer<std::string>(4);    std::uninitialized_fill_n(p, sz,"Example"); for(std::string* i= p; i!= p+ sz;++i){std::cout<<*i<<'\n';        i->~basic_string<char>();}std::return_temporary_buffer(p);}

Output:

ExampleExampleExampleExample

[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 866	C++98	given`T` as the value type of`NoThrowForwardIt`, if T::operator new exists, the program might be ill-formed	uses global placementnew instead
LWG 1339	C++98	the location of the first element following the filling range was not returned	returned
LWG 2433	C++11	this algorithm might be hijacked by overloadedoperator&	usesstd::addressof
LWG 3870	C++20	this algorithm might create objects on aconst storage	kept disallowed

[edit]See also

uninitialized_fill	copies an object to an uninitialized area of memory, defined by a range (function template)[edit]
ranges::uninitialized_fill_n (C++20)	copies an object to an uninitialized area of memory, defined by a start and a count (algorithm function object)[edit]