Return the n^th 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.

Parameters

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

#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';
}

See also