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std::ranges::prev

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Utilities
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
ranges::prev
(C++20)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

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

[edit] 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.

[edit] Complexity

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

[edit] Possible implementation

struct prev_fn {
  template<std::bidirectional_iterator I>
  constexpr I operator()(I i) const
  {
    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();

[edit] 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.

[edit] 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

[edit] See also

increment an iterator by a given distance or to a bound
(niebloid) [edit]
advances an iterator by given distance or to a given bound
(niebloid) [edit]
(C++11)
decrement an iterator
(function template) [edit]