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[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]
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