Constructs objects of typestd::iter_value_t<I> in the uninitialized memory areafirst + [​0​, count) bydefault-initialization, as if byreturnranges::uninitialized_default_construct(std::counted_iterator(first, count),
                                               std::default_sentinel).base();

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

The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:

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

[edit]Parameters

[edit]Return value

As described above.

[edit]Complexity

Linear incount.

[edit]Exceptions

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

[edit]Notes

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

[edit]Notes

[edit]Possible implementation

struct uninitialized_default_construct_n_fn{template<no-throw-forward-iterator I>        requiresstd::default_initializable<std::iter_value_t<I>>constexpr I operator()(I first,std::iter_difference_t<I> count)const{auto iter=std::counted_iterator(first, count);returnranges::uninitialized_default_construct(iter,std::default_sentinel).base();}}; inlineconstexpr uninitialized_default_construct_n_fn uninitialized_default_construct_n{};

[edit]Example

#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= std::ranges::uninitialized_default_construct_n(first, n); 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";} // For scalar types, uninitialized_default_construct_n// generally does not zero-fill the given uninitialized memory area.constexprint sample[]{1,2,3,4,5,6};int v[]{1,2,3,4,5,6};    std::ranges::uninitialized_default_construct_n(std::begin(v),std::size(v));if(std::memcmp(v, sample, sizeof(v))==0){// Maybe undefined behavior, pending CWG 1997:// for (const int i : v) { std::cout << i << ' '; }for(constint i: sample)std::cout<< i<<' ';}elsestd::cout<<"Unspecified!";std::cout<<'\n';}

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[edit]Defect reports

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

[edit]See also

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