std::ranges::prev - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| 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 algorithm function objects (informally known as 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.
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
The predecessor of i.
The nth predecessor of iterator i.
The nth predecessor of iterator i, or the first iterator that compares equal to bound, whichever is first.
[edit] Complexity
- Constant.
[edit] Possible implementation
[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 #include #include int main() { std::vector 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:
[edit] See also
| | increment an iterator by a given distance or to a bound(algorithm function object)[edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | | advances an iterator by given distance or to a given bound(algorithm function object)[edit] | | | decrement an iterator (function template) [edit] |