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      std::uninitialized_copy_n

      From cppreference.com
      <cpp‎ |memory
       
       
      Memory management library
      (exposition only*)
      Allocators
      Uninitialized memory algorithms
      Constrained uninitialized memory algorithms
      Memory resources
      Uninitialized storage(until C++20)
      (until C++20*)
      (until C++20*)
      Garbage collector support(until C++23)
      (C++11)(until C++23)
      (C++11)(until C++23)
      (C++11)(until C++23)
      (C++11)(until C++23)
      (C++11)(until C++23)
      (C++11)(until C++23)
       
      Defined in header<memory>
      template<class InputIt,class Size,class NoThrowForwardIt>

      NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count,

                                             NoThrowForwardIt d_first);
      		 (1)(since C++11)
      (constexpr since C++26)
      template<class ExecutionPolicy,class ForwardIt,

               class Size,class NoThrowForwardIt>
      NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy,
                                             ForwardIt first, Size count,

                                             NoThrowForwardIt d_first);
      		 (2)(since C++17)
      1) Copiescount elements from a range beginning atfirst to an uninitialized memory area beginning atd_first as if by

      for(; count>0;++d_first,(void)++first,--count)
          ::new(voidify(*d_first))
              typenamestd::iterator_traits<NoThrowForwardIt>::value_type(*first);

      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 topolicy.
      This overload participates in overload resolution only if all following conditions are satisfied:

      std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> istrue.

      		(until C++20)

      std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> istrue.

      		(since C++20)


      Ifd_first + [​0​, count) overlaps withfirst + [​0​, count), the behavior is undefined.

      		(since C++20)

      Contents

      [edit]Parameters

      first - the beginning of the range of the elements to copy
      count - the number of elements to copy
      d_first - the beginning of the destination range
      policy - theexecution policy to use
      Type requirements
      -
      InputIt must meet the requirements ofLegacyInputIterator.
      -
      ForwardIt must meet the requirements ofLegacyForwardIterator.
      -
      NoThrowForwardIt must meet the requirements ofLegacyForwardIterator.
      -
      No increment, assignment, comparison, or indirection through valid instances ofNoThrowForwardIt may throw exceptions.

      [edit]Return value

      Iterator to the element past the last element copied.

      [edit]Complexity

      Linear incount.

      [edit]Exceptions

      The overload with a template parameter namedExecutionPolicy reports errors as follows:

      • If execution of a function invoked as part of the algorithm throws an exception andExecutionPolicy is one of thestandard policies,std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined.
      • If the algorithm fails to allocate memory,std::bad_alloc is thrown.

      [edit]Notes

      Feature-test macroValueStdFeature
      __cpp_lib_raw_memory_algorithms202411L(C++26)constexpr forspecialized memory algorithms,(1)

      [edit]Possible implementation

      template<class InputIt,class Size,class NoThrowForwardIt>constexpr NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count,                                                NoThrowForwardIt d_first){using T=typenamestd::iterator_traits<NoThrowForwardIt>::value_type;    NoThrowForwardIt current= d_first;try{for(; count>0;++first,(void)++current,--count)::new(static_cast<void*>(std::addressof(*current))) T(*first);}catch(...){for(; d_first!= current;++d_first)            d_first->~T();throw;}return current;}

      [edit]Example

      Run this code
      #include <algorithm>#include <iostream>#include <memory>#include <string>#include <tuple>#include <vector> int main(){std::vector<std::string> v={"This","is","an","example"}; std::string* p;std::size_t sz;std::tie(p, sz)=std::get_temporary_buffer<std::string>(v.size());    sz=std::min(sz, v.size());     std::uninitialized_copy_n(v.begin(), sz, p); for(std::string* i= p; i!= p+ sz;++i){std::cout<<*i<<' ';        i->~basic_string<char>();}std::cout<<'\n'; std::return_temporary_buffer(p);}

      Possible output:

      This is an example

      [edit]Defect reports

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

      DRApplied toBehavior as publishedCorrect behavior
      LWG 2133C++98the effect description used afor loop with the iteration
      expression++d_first,++first,--count, which
      results in argument-dependent lookups ofoperator,      		discards the value
      of one operand to
      disable those ADLs
      LWG 2433C++11this algorithm might be hijacked by overloadedoperator&usesstd::addressof
      LWG 3870C++20this algorithm might create objects on aconst storagekept disallowed

      [edit]See also

      copies a range of objects to an uninitialized area of memory
      (function template)[edit]
      copies a number of objects to an uninitialized area of memory
      (algorithm function object)[edit]
      Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/uninitialized_copy_n&oldid=179703"
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