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 nth predecessor of iterator i

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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

// 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)