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Defined in header <memory> | ||
Call signature | ||
template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requiresstd::default_initializable<std::iter_value_t<I>> | (1) | (since C++20) (constexpr since C++26) |
template<no-throw-forward-range R> requiresstd::default_initializable<ranges::range_value_t<R>> | (2) | (since C++20) (constexpr since C++26) |
[first, last) bydefault-initialization, as if byfor(; first!= last;++first) ::new(voidify(*first)) std::remove_reference_t<std::iter_reference_t<I>>;
return first;
The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:
Contents |
| first, last | - | the iterator-sentinel pair defining therange of elements to initialize |
| r | - | therange of the elements to initialize |
As described above.
Linear in the distance betweenfirst andlast.
Any exception thrown on construction of the elements in the destination range.
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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_raw_memory_algorithms | 202411L | (C++26) | constexpr forspecialized memory algorithms,(1,2) |
struct uninitialized_default_construct_fn{template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requiresstd::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> requiresstd::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{}; |
#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
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 |
| constructs objects bydefault-initialization in an uninitialized area of memory, defined by a start and count (algorithm function object)[edit] | |
| constructs objects byvalue-initialization in an uninitialized area of memory, defined by a range (algorithm function object)[edit] | |
| constructs objects byvalue-initialization in an uninitialized area of memory, defined by a start and a count (algorithm function object)[edit] | |
| constructs objects bydefault-initialization in an uninitialized area of memory, defined by a range (function template)[edit] |
