std::ranges::uninitialized_default_construct

 

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Defined in header <memory>
Call signature
template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S >     requires std::default_initializable<std::iter_value_t<I>> I uninitialized_default_construct( I first, S last );	(1)	(since C++20) (constexpr since C++26)
template<no-throw-forward-range R >     requires std::default_initializable<ranges::range_value_t<R>> ranges::borrowed_iterator_t<R>     uninitialized_default_construct( R&& r );	(2)	(since C++20) (constexpr since C++26)

1) Constructs objects of type std::iter_value_t<I> in the uninitialized memory area [first, last) by default-initialization, as if by

for(; first != last;++first)
    ::new(voidify(*first))
        std::remove_reference_t<std::iter_reference_t<I>>;
return first;

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.

2) Equivalent to ranges::uninitialized_default_construct(ranges::begin(r), ranges::end(r)).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	the iterator-sentinel pair defining the range of elements to initialize
r	-	the range of the elements to initialize

Return value

As described above.

Complexity

Linear in the distance between first and last.

Exceptions

Any exception thrown on construction of the elements in the destination range.

Notes

An implementation may skip the objects construction (without changing the observable effect) if no non-trivial default constructor is called while default-initializing a std::iter_value_t<I> object, which can be detected by std::is_trivially_default_constructible.

Feature-test macro	Value	Std	Feature
__cpp_lib_raw_memory_algorithms	202411L	(C++26)	constexpr for specialized memory algorithms, (1,2)

Possible implementation

struct uninitialized_default_construct_fn {template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requires std::default_initializable<std::iter_value_t<I>>constexpr I operator()(I first, S last)const{using ValueType =std::remove_reference_t<std::iter_reference_t<I>>;ifconstexpr(std::is_trivially_default_constructible_v<ValueType>)returnranges::next(first, last);// skip initialization I rollback{first};try{for(;!(first == last);++first)::new(static_cast<void*>(std::addressof(*first))) ValueType;return first;}catch(...)// rollback: destroy constructed elements{for(; rollback != first;++rollback)ranges::destroy_at(std::addressof(*rollback));throw;}}   template<no-throw-forward-range R> requires std::default_initializable<ranges::range_value_t<R>>constexprranges::borrowed_iterator_t<R> operator()(R&& r)const{return(*this)(ranges::begin(r), ranges::end(r));}};   inlineconstexpr uninitialized_default_construct_fn uninitialized_default_construct{};

Example

Run this code

#include <cstring>#include <iostream>#include <memory>#include <string>   int main(){struct S {std::string m{"▄▀▄▀▄▀▄▀"};};   constexprint n{4}; alignas(alignof(S))char out[n * sizeof(S)];   try{auto first{reinterpret_cast<S*>(out)};auto last{first + n};   std::ranges::uninitialized_default_construct(first, last);   auto count{1};for(auto it{first}; it != last;++it)std::cout<< count++<<' '<< it->m <<'\n';   std::ranges::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.constexprchar sample[]{'A', 'B', 'C', 'D', '\n'};char v[]{'A', 'B', 'C', 'D', '\n'}; std::ranges::uninitialized_default_construct(std::begin(v), std::end(v));if(std::memcmp(v, sample, sizeof(v))==0){std::cout<<" ";// Maybe undefined behavior, pending CWG 1997 to be resolved:// for (const char c : v) { std::cout << c << ' '; }for(constchar c : sample)std::cout<< c <<' ';}elsestd::cout<<"Unspecified\n";}

Possible output:

1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D

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 a const storage	kept disallowed

See also

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