From cppreference.com
Defined in header
template<classNoThrowForwardIt>voiduninitialized_default_construct(NoThrowForwardItfirst,NoThrowForwardItlast); (1)(since C++17)
(constexpr since C++26)template<classExecutionPolicy,classNoThrowForwardIt>voiduninitialized_default_construct(ExecutionPolicy&&policy,NoThrowForwardItfirst,NoThrowForwardItlast); (2) (since C++17)1) Constructs elements in the destination range [first, last) by
as if by
for(;first!=last;++first)
::new(
(*first))
typenamestd::iterator_traits<NoThrowForwardIt>::value_type;
If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if the value of the following expression is true:
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20)Parameters
first, last - the pair of iterators defining the
of elements to initialize policy - the
to use Type requirements -NoThrowForwardIt must meet the requirements of
. -No increment, assignment, comparison, or indirection through valid instances of NoThrowForwardIt may throw exceptions. Exceptions
2) 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
macroValueStdFeature
__cpp_lib_raw_memory_algorithms
(C++26)constexpr for
specialized <memory> algorithms
, (
)Possible implementation
template<classNoThrowForwardIt>constexprvoiduninitialized_default_construct(NoThrowForwardItfirst,NoThrowForwardItlast){usingValue=typenamestd::iterator_traits<NoThrowForwardIt>::value_type;NoThrowForwardItcurrent=first;try{for(;current!=last;++current){::new(static_cast<void*>(std::addressof(*current)))Value;}}catch(...){std::destroy(first,current);throw;}}Example
Run this code
#include<cstring>#include<iostream>#include<memory>#include<string>structS{std::stringm{"Default value"};};intmain(){constexprintn{3};alignas(alignof(S))unsignedcharmem[n*sizeof(S)];try{autofirst{reinterpret_cast<S*>(mem)};autolast{first+n};std::uninitialized_default_construct(first,last);for(autoit{first};it!=last;++it)std::cout<<it->m<<'\n';std::destroy(first,last);}catch(...){std::cout<<"Exception!\n";}// For scalar types, uninitialized_default_construct// generally does not zero-fill the given uninitialized memory area.intv[]{1,2,3,4};constintoriginal[]{1,2,3,4};std::uninitialized_default_construct(std::begin(v),std::end(v));// Maybe undefined behavior, pending CWG 1997 to be resolved.// for (const int i : v)// std::cout << i << ' ';// The result is unspecified.std::cout<<(std::memcmp(v,original,sizeof(v))==0?"Unmodified\n":"Modified\n");}Possible output:
Default value Default value Default value Unmodified See also