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

NAME
     std::reverse - std::reverse

Synopsis
	Defined in header <algorithm>
	template<  class BidirIt >				  (1) (constexpr
     since C++20)
	void reverse( BidirIt first, BidirIt last );
	template< class ExecutionPolicy, class BidirIt >
	void reverse( ExecutionPolicy&&  policy,  BidirIt  first,    (2)  (since
     C++17)
	BidirIt last );

	1) Reverses the order of the elements in the range [first, last).
	Behaves as if applying std::iter_swap to every pair of iterators first +
     i and (last
	- i) - 1 for each integer i in [0, std::distance(first, last) / 2).
	2) Same as (1), but executed according to policy.
	This overload participates 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)

	If
	*first is not Swappable
	(until C++20)
	BidirIt is not ValueSwappable
	(since C++20), the behavior is undefined.

Parameters
	first, last -  the range of elements to reverse
	policy	    -  the execution policy to use. See execution policy for de-
     tails.

Type requirements
	-
	BidirIt must meet the requirements of LegacyBidirectionalIterator.

Complexity
	Exactly std::distance(first, last) / 2 swaps.

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
	See also the implementations in libstdc++, libc++, and MSVC STL.

	template<class BidirIt>
	constexpr // since C++20
	void reverse(BidirIt first, BidirIt last)
	{
	    using  iter_cat  =	typename   std::iterator_traits<BidirIt>::itera-
     tor_category;

	    // Tag dispatch, e.g. calling reverse_impl(first, last, iter_cat()),
	    // can be used in C++14 and earlier modes.
	    if	 constexpr   (std::is_base_of_v<std::random_access_iterator_tag,
     iter_cat>)
	    {
		if (first == last)
		    return;

		for (--last; first < last; (void)++first, --last)
		    std::iter_swap(first, last);
	    }
	    else
		while (first != last && first != --last)
		    std::iter_swap(first++, last);
	}

Notes
	Implementations (e.g. MSVC STL) may enable vectorization when the itera-
     tor type
	satisfies LegacyContiguousIterator and swapping  its  value  type  calls
     neither
	non-trivial special member function nor ADL-found swap.

Example
     // Run this code

      #include <algorithm>
      #include <iostream>
      #include <iterator>
      #include <vector>

      void println(auto rem, auto const& v)
      {
	  for (std::cout << rem; auto e : v)
	      std::cout << e << ' ';
	  std::cout << '\n';
      }

      int main()
      {
	  std::vector<int> v {1, 2, 3};
	  std::reverse(v.begin(), v.end());
	  println("after reverse, v = ", v);

	  int a[] = {4, 5, 6, 7};
	  std::reverse(std::begin(a), std::end(a));
	  println("after reverse, a = ", a);
      }

Output:
      after reverse, v = 3 2 1
      after reverse, a = 7 6 5 4

	Defect reports

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

	   DR	 Applied to	      Behavior as published		 Correct
     behavior
	LWG 223  C++98	    std::swap was applied to each  pair  of	 applies
     std::iter_swap
			    iterators				      instead
	LWG  2039  C++98       std::iter_swap was also applied when i	 not ap-
     plied
			    equals std::distance(first, last) / 2

See also
	reverse_copy	creates a copy of a range that is reversed
			(function template)
	ranges::reverse reverses the order of elements in a range
	(C++20) 	(niebloid)

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