From cppreference.com
Defined in header
Call signature
template</*nothrow-forward-iterator*/I,/*nothrow-sentinel-for*/<I>S>requiresstd::default_initializable<std::iter_value_t<I>>Iuninitialized_value_construct(Ifirst,Slast); (1)(since C++20)
(constexpr since C++26)template</*nothrow-forward-range*/R>requiresstd::default_initializable<ranges::range_value_t<R>>ranges::borrowed_iterator_t<R>uninitialized_value_construct(R&&r); (2)(since C++20)
(constexpr since C++26)template</*execution-policy*/Ep,/*nothrow-random-access-iterator*/I,/*nothrow-sized-sentinel-for*/<I>S>requiresstd::default_initializable<std::iter_value_t<I>>Iuninitialized_value_construct(Ep&&policy,Ifirst,Slast); (3) (since C++26)template</*execution-policy*/Ep,/*nothrow-sized-random-access-range*/R>requiresstd::default_initializable<ranges::range_value_t<R>>ranges::borrowed_iterator_t<R>uninitialized_value_construct(Ep&&policy,R&&r); (4) (since C++26)For the definition of /*execution-policy*/, see
; for the definition of other exposition-only concepts, see
.
1) Constructs elements in the destination range [first, last) by
as if by
for(;first!=last;++first)
::new(
(*first))
std::remove_reference_t<std::iter_reference_t<I>>();
returnfirst;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but uses r as the destination range.
3,4) Same as (1,2), but executed according to policy.
The function-like entities described on this page are
(informally known as niebloids), that is:
Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to
.
When any of them are found by
as the name to the left of the function-call operator,
is inhibited.
Parameters
first, last - the iterator-sentinel pair defining the
of elements to value-initialize r - the
of the elements to value-initialize policy - the
to use Return value
As described above.
Exceptions
Any exception thrown on construction of the elements in the destination range.
3,4) During the execution process:
If the temporary memory resources required for parallelization are not available,
is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for
,
is invoked).
Notes
An implementation may improve the efficiency of the ranges::uninitialized_value_construct (by using e.g.
) if the value type of the range is a
.
macro ValueStdFeature
(C++26)Parallel range algorithms
__cpp_lib_raw_memory_algorithms
(C++26)constexpr for
specialized <memory> algorithms
, (
)Possible implementation
structuninitialized_value_construct_fn{template</*nothrow-forward-iterator*/I,/*nothrow-sentinel-for*/<I>S>requiresstd::value_initializable<std::iter_value_t<I>>constexprIoperator()(Ifirst,Slast)const{usingvalue_type=std::remove_reference_t<std::iter_reference_t<I>>;ifconstexpr(std::is_trivially_default_constructible_v<value_type>)returnranges::fill(first,last,ValueType());Irollback{first};try{for(;!(first==last);++first)::new(static_cast<void*>(std::addressof(*first)))value_type();returnfirst;}catch(...)// rollback: destroy constructed elements{for(;rollback!=first;++rollback)ranges::destroy_at(std::addressof(*rollback));throw;}}template</*nothrow-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::next(ranges::begin(r),ranges::end(r)));}};inlineconstexpruninitialized_value_construct_fnuninitialized_value_construct{};Example
Run this code
#include<iostream>#include<memory>#include<string>intmain(){structS{std::stringm{"▄▀▄▀▄▀▄▀"};};constexprintn{4};alignas(alignof(S))charout[n*sizeof(S)];try{autofirst{reinterpret_cast<S*>(out)};autolast{first+n};std::ranges::uninitialized_value_construct(first,last);autocount{1};for(autoit{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.intv[]{0,1,2,3};std::cout<<' ';for(constinti:v)std::cout<<' '<<static_cast<char>(i+'A');std::cout<<"\n ";std::ranges::uninitialized_value_construct(std::begin(v),std::end(v));for(constinti:v)std::cout<<' '<<static_cast<char>(i+'A');std::cout<<'\n';}Output:
1 ▄▀▄▀▄▀▄▀ 2 ▄▀▄▀▄▀▄▀ 3 ▄▀▄▀▄▀▄▀ 4 ▄▀▄▀▄▀▄▀ A B C D A A A A See also