[section:algorithms Algorithms]

[section:next_prior Function templates `next()` and `prior()`]

Certain data types, such as the C++ Standard Library's forward and bidirectional iterators, do not provide addition and subtraction via `operator+()` or `operator-()`. This means that non-modifying computation of the next or prior value requires a temporary, even though `operator++()` or `operator--()` is provided. It also means that writing code like `itr+1` inside a template restricts the iterator category to random access iterators.

The `next()` and `prior()` functions defined in `boost/next_prior.hpp` provide a simple way around these problems.

[section Synopsis]

    template <class T>
    T next(T x)
    {
        return ++x;
    }

    template <class T, class Distance>
    T next(T x, Distance n)
    {
        std::advance(x, n);
        return x;
    }

    template <class T>
    T prior(T x)
    {
        return --x;
    }

    template <class T, class Distance>
    T prior(T x, Distance n)
    {
        std::advance(x, -n);
        return x;
    }

[note Function implementations above are given for exposition only. The actual implementation has the same effect for iterators, but has different properties, as documented later.]

[endsect]

Usage is simple:

    const std::list<T>::iterator p = get_some_iterator();
    const std::list<T>::iterator prev = boost::prior(p);
    const std::list<T>::iterator next = boost::next(prev, 2);

The distance from the given iterator should be supplied as an absolute value. For example, the iterator four iterators prior to the given iterator `p` may be obtained by `prior(p, 4)`.

With C++11, the Standard Library provides `std::next()` and `std::prev()` function templates, which serve the same purpose. However, there are advantages to `boost::next()` and `boost::prior()`.

First, `boost::next()` and `boost::prior()` are compatible not only with iterators but with any type that provides arithmetic operators `operator++()`, `operator--()`, `operator+()`, `operator-()`, `operator+=()` or `operator-=()`. For example, this is possible:

    int x = 10;
    int y = boost::next(x, 5);
    assert(y == 15);

Second, `boost::next()` and `boost::prior()` use [link iterator.concepts.concepts_traversal traversal categories] to select the most efficient implementation. For some kinds of iterators, such as [link iterator.specialized.transform transform iterators], the standard iterator category does not reflect the traversal category correctly and therefore `std::next()` and `std::prev()` will fall back to linear complexity.

[section Acknowledgements]

Contributed by [@http://www.boost.org/people/dave_abrahams.htm Dave Abrahams]. Two-argument versions by Daniel Walker.

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