home | help 
std::uninitialized_copy_n(3)   C++ Standard Libary  std::uninitialized_copy_n(3)

NAME
     std::uninitialized_copy_n - std::uninitialized_copy_n

Synopsis
	Defined in header <memory>
	template< class InputIt, class Size, class NoThrowForwardIt >

	NoThrowForwardIt  uninitialized_copy_n(  InputIt first, Size count,  (1)
     (since C++11)

					       NoThrowForwardIt d_first );
	template< class ExecutionPolicy, class ForwardIt,

		  class Size, class NoThrowForwardIt >
	NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&&  policy,    (2)
     (since C++17)
					       ForwardIt first, Size
	count,

					       NoThrowForwardIt d_first );

	1)  Copies  count  elements from a range beginning at first to an unini-
     tialized memory
	area beginning at d_first as if by
	for (; n > 0; ++d_first, (void) ++first, --n)
	     ::new (static_cast<void*>(std::addressof(*d_first)))
		typename			std::iterator_traits<NoThrowFor-
     wardIt>::value_type(*first);
	If an exception is thrown during the initialization, the objects already
     constructed
	are destroyed in an unspecified order.

	If  d_first  +	[0,  n)  overlaps  with  first + [0, n), the behavior is
     (since C++20)
	undefined.

	2) Same as (1), but executed according to policy. This overload partici-
     pates in
	overload resolution only if

	std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>    is    true.
     (until
										  C++20)
	std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>      is
     true. (since
										  C++20)

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		   -	      the execution policy to use. See execution
     policy for
				      details.

Type requirements
	-
	InputIt must meet the requirements of LegacyInputIterator.
	-
	ForwardIt must meet the requirements of LegacyForwardIterator.
	-
	NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
	-
	No increment, assignment, comparison, or indirection through  valid  in-
     stances of
	NoThrowForwardIt may throw exceptions.

Return value
	Iterator to the element past the last element copied.

Complexity
	Linear in count.

Exceptions
	The overload with a template parameter named ExecutionPolicy reports er-
     rors as
	follows:

	  *  If  execution of a function invoked as part of the algorithm throws
     an exception
	    and ExecutionPolicy is one of the standard policies,  std::terminate
     is called.
	    For  any  other  ExecutionPolicy, the behavior is implementation-de-
     fined.
	  * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
     template<class InputIt, class  Size,  class  NoThrowForwardIt>  NoThrowFor-
     wardIt  uninitialized_copy_n(InputIt  first,  Size  count, NoThrowForwardIt
     d_first) {
	 using T = typename std::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; }

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

	Defect reports

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

	   DR	  Applied  to		 Behavior as published		    Cor-
     rect behavior
			    the effect description used a for loop with
			    the  iteration				    dis-
     cards the value
	LWG  2133 C++98      expression ++d_first, ++first, --n, which	  of one
     operand to
			    results					 disable
     those ADLs
			    in argument-dependent lookups of operator,
	LWG 2433 C++11	    this algorithm might be  hijacked  by	    uses
     std::addressof
			    overloaded operator&
	LWG  3870  C++20       this algorithm might create objects on a     kept
     disallowed
			    const storage

See also
				     copies a range of objects to an  uninitial-
     ized area of
	uninitialized_copy	     memory
				     (function template)
	ranges::uninitialized_copy_n copies a number of objects to an uninitial-
     ized area of
	(C++20) 		     memory
				     (niebloid)

http://cppreference.com 	   2024.06.10	    std::uninitialized_copy_n(3)
home | help