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std::ranges::prev

Defined in header `<iterator>`
Call signature
template< std::bidirectional_iterator I > constexpr I prev( I i );	(1)	(since C++20)
template< std::bidirectional_iterator I > constexpr I prev( I i, std::iter_difference_t<I> n );	(2)	(since C++20)
template< std::bidirectional_iterator I > constexpr I prev( I i, std::iter_difference_t<I> n, I bound );	(3)	(since C++20)

Return the nth 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 nth predecessor of iterator i

3) The nth 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

#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

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