iterator/doc/quickbook/algorithms.qbk

[section:algorithms Algorithms]

[section:advance Function template `advance()`]

The `boost::iterators::advance` function template is an adapted version of `std::advance` for the Boost iterator [link iterator.concepts.traversal traversal concepts].

[heading Header]

    <boost/iterator/advance.hpp>

[heading Synopsis]

    template <typename Iterator, typename Distance>
    constexpr void advance(Iterator& it, Distance n);


[heading Description]

Moves `it` forward by `n` increments (or backward by `|n|` decrements if `n` is negative).

[heading Requirements]

`Iterator` should model Incrementable Iterator.

[heading Preconditions]

Let `it`[sub `i`] be the iterator obtained by incrementing (or decrementing if `n` is negative) `it` by `i`. All the iterators `it`[sub `i`] for `i` = 0, 1, 2, ..., `|n|` should be valid.

If `Iterator` does not model [link iterator.concepts.traversal.bidirectional Bidirectional Traversal Iterator], `n` should be non-negative.

[heading Complexity]

If `Iterator` models [link iterator.concepts.traversal.random_access Random Access Traversal Iterator], it takes constant time; otherwise it takes linear time.

[heading Notes]

* This function is not a customization point and is protected against being found by argument-dependent lookup (ADL).
* This function is `constexpr` only in C++14 or later.

[heading Acknowledgements]

Contributed by Michel Morin.

[endsect]

[section:distance Function template `distance()`]

The `boost::iterators::distance` function template is an adapted version of `std::distance` for the Boost iterator [link iterator.concepts.traversal traversal concepts].

[heading Header]

    <boost/iterator/distance.hpp>

[heading Synopsis]

    template <typename Iterator>
    constexpr typename iterator_difference<Iterator>::type
    distance(Iterator first, Iterator last);

[heading Description]

Computes the (signed) distance from `first` to `last`.

[heading Requirements]

`Iterator` should model [link iterator.concepts.traversal.single_pass Single Pass Iterator].

[heading Preconditions]

If `Iterator` models [link iterator.concepts.traversal.random_access Random Access Traversal Iterator], `[first, last)` or `[last, first)` should be valid; otherwise `[first, last)` should be valid.

[heading Complexity]

If `Iterator` models [link iterator.concepts.traversal.random_access Random Access Traversal Iterator], it takes constant time; otherwise it takes linear time.

[heading Notes]

* This function is not a customization point and is protected against being found by argument-dependent lookup (ADL).
* This function is `constexpr` only in C++14 or later.

[heading Acknowledgements]

Contributed by Michel Morin.

[endsect]

[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.

[heading 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.]

[heading Usage]

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.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.

[heading Acknowledgements]

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

[endsect]

[endsect]