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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) byvalue-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 value-initialize |
| r | - | therange of the elements to value-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 improve the efficiency of theranges::uninitialized_value_construct, e.g. by usingranges::fill, if the value type of the range is aCopyAssignableTrivialType.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_raw_memory_algorithms | 202411L | (C++26) | constexpr forspecialized memory algorithms,(1,2) |
struct uninitialized_value_construct_fn{template<no-throw-forward-iterator I, no-throw-sentinel-for<I> S> requires std::value_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::fill(first, last, ValueType()); 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_value_construct_fn uninitialized_value_construct{}; |
#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_value_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";} // For scalar types, uninitialized_value_construct// zero-fills the given uninitialized memory area.int v[]{0,1,2,3};std::cout<<' ';for(constint i: v)std::cout<<' '<<static_cast<char>(i+'A');std::cout<<"\n "; std::ranges::uninitialized_value_construct(std::begin(v),std::end(v));for(constint i: v)std::cout<<' '<<static_cast<char>(i+'A');std::cout<<'\n';}
Output:
1 ▄▀▄▀▄▀▄▀2 ▄▀▄▀▄▀▄▀3 ▄▀▄▀▄▀▄▀4 ▄▀▄▀▄▀▄▀ A B C D A A A A
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 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 (algorithm function object)[edit] | |
| 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 (function template)[edit] |
