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std::prev(3)		      C++ Standard Libary		  std::prev(3)

NAME
       std::prev - std::prev

Synopsis
	  Defined in header <iterator>
	  template<		   class	       BidirIt		     >
       (since C++11)
	  BidirIt	  prev(		BidirIt		 it,	      typename
       (until C++17)
	  std::iterator_traits<BidirIt>::difference_type n = 1 );
	  template< class BidirIt >

	  constexpr
       (since C++17)

	  BidirIt prev(	BidirIt	it, typename
	  std::iterator_traits<BidirIt>::difference_type n = 1 );

	  Return the n^th predecessor (or -n^th	successor if n is negative) of
       iterator	it.

Parameters
	  it	    -	     an	iterator
	  n	    -	     number of elements	it should be descended

Type requirements
	  -
	  BidirIt must meet the	requirements of	LegacyBidirectionalIterator.

Return value
	  An  iterator	of  type  BidirIt  that	holds the n^th predecessor (or
       -n^th successor if n
	  is negative) of iterator it.

Complexity
	  Linear.

	  However, if BidirIt additionally meets the requirements of
	  LegacyRandomAccessIterator, complexity is constant.

Possible implementation
	  template<class BidirIt>
	  constexpr // since C++17
	  BidirIt      prev(BidirIt	  it,	    typename	   std::itera-
       tor_traits<BidirIt>::difference_type n =	1)
	  {
	      std::advance(it, -n);
	      return it;
	  }

Notes
	  Although  the	expression --c.end() often compiles, it	is not guaran-
       teed to do so:
	  c.end() is an	rvalue expression, and there is	no  iterator  require-
       ment that specifies
	  that	decrement  of  an rvalue is guaranteed to work.	In particular,
       when iterators are
	  implemented as pointers or its operator--  is	 lvalue-ref-qualified,
       --c.end() does
	  not compile, while std::prev(c.end())	does.

Example
       // Run this code

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

	int main()
	{
	    std::vector<int> v{3, 1, 4};

	    auto it = v.end();
	    auto pv = std::prev(it, 2);
	    std::cout << *pv <<	'\n';

	    it = v.begin();
	    pv = std::prev(it, -2);
	    std::cout << *pv <<	'\n';
	}

Output:
	1
	4

See also
	  next	       increment an iterator
	  (C++11)      (function template)
	  advance      advances	an iterator by given distance
		       (function template)
	  distance     returns the distance between two	iterators
		       (function template)
	  ranges::prev decrement an iterator by	a given	distance or to a bound
	  (C++20)      (niebloid)

http://cppreference.com		  2024.06.10			  std::prev(3)

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