std::ranges::prev(3)

NAME
std::ranges::prev - std::ranges::prev

Synopsis
   Defined in header <iterator>
   Call signature
   template< std::bidirectional_iterator I > (1) (since C++20)
   constexpr I prev( I i );
   template< std::bidirectional_iterator I > (2) (since C++20)
   constexpr I prev( I i, std::iter_difference_t<I> n );
   template< std::bidirectional_iterator I > (3) (since C++20)
   constexpr I prev( I i, std::iter_difference_t<I> n, I bound );
   Return the n^th predecessor of iterator i.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   i - an iterator
   n - number of elements i should be descended
   bound - iterator denoting the beginning of the range i points to

Return value
   1) The predecessor of i.
   2) The n^th predecessor of iterator i.
   3) The n^th predecessor of iterator i, or the first iterator that compares equal to
   bound, whichever is first.

Complexity
   1) Constant.
   2,3) Constant if I models std::random_access_iterator<I>; otherwise linear.

Possible implementation
   struct prev_fn
   {
       template<std::bidirectional_iterator I>
       constexpr I operator()(I i) const
       {
           --i;
           return i;
       }
       template<std::bidirectional_iterator I>
       constexpr I operator()(I i, std::iter_difference_t<I> n) const
       {
           ranges::advance(i, -n);
           return i;
       }
       template<std::bidirectional_iterator I>
       constexpr I operator()(I i, std::iter_difference_t<I> n, I bound) const
       {
           ranges::advance(i, -n, bound);
           return i;
       }
   };
   inline constexpr auto prev = prev_fn();

Notes
   Although the expression --r.end() often compiles for containers, it is not
   guaranteed to do so: r.end() is an rvalue expression, and there is no iterator
   requirement 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, --r.end() does not compile, while ranges::prev(r.end()) does.
   This is further exacerbated by ranges that do not model ranges::common_range. For
   example, for some underlying ranges, ranges::transform_view::end doesn't have the
   same return type as ranges::transform_view::begin, and so --r.end() won't compile.
   This isn't something that ranges::prev can aid with, but there are workarounds.

Example
// Run this code
 #include <iostream>
 #include <iterator>
 #include <vector>
 int main()
 {
     std::vector<int> v{3, 1, 4};
     auto pv = std::ranges::prev(v.end(), 2);
     std::cout << *pv << '\n';
     pv = std::ranges::prev(pv, 42, v.begin());
     std::cout << *pv << '\n';
 }

Output:
 1
 3

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