To simplify the task of defining iterators, the library provides several classes and functions:
24.4.1 Iterator traits [iterator.traits]To implement algorithms only in terms of iterators, it is often necessary to determine the value and difference types that correspond to a particular iterator type. Accordingly, it is required that if Iterator is the type of an iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::iterator_category
be defined as the iterator's difference type, value type and iterator category, respectively. In addition, the types
iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointer
shall be defined as the iterator's reference and pointer types, that is, for an iterator object a, the same type as the type of *a and a->, respectively. In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type iterator_traits<Iterator>::reference iterator_traits<Iterator>::pointer
may be defined as void.
The template iterator_traits<Iterator> is defined as
namespace std {
template<class Iterator> struct iterator_traits {
typedef typename Iterator::difference_type difference_type;
typedef typename Iterator::value_type value_type;
typedef typename Iterator::pointer pointer;
typedef typename Iterator::reference reference;
typedef typename Iterator::iterator_category iterator_category;
};
}
It is specialized for pointers as
namespace std {
template<class T> struct iterator_traits<T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef T* pointer;
typedef T& reference;
typedef random_access_iterator_tag iterator_category;
};
}
and for pointers to const as
namespace std {
template<class T> struct iterator_traits<const T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef const T* pointer;
typedef const T& reference;
typedef random_access_iterator_tag iterator_category;
};
}
[ Example: To implement a generic reverse function, a C++ program can do the following:
template <class BidirectionalIterator>
void reverse(BidirectionalIterator first, BidirectionalIterator last) {
typename iterator_traits<BidirectionalIterator>::difference_type n =
distance(first, last);
--n;
while(n > 0) {
typename iterator_traits<BidirectionalIterator>::value_type
tmp = *first;
*first++ = *--last;
*last = tmp;
n -= 2;
}
}
— end example ]
24.4.2 Basic iterator [iterator.basic]The iterator template may be used as a base class to ease the definition of required types for new iterators.
namespace std {
template<class Category, class T, class Distance = ptrdiff_t,
class Pointer = T*, class Reference = T&>
struct iterator {
typedef T value_type;
typedef Distance difference_type;
typedef Pointer pointer;
typedef Reference reference;
typedef Category iterator_category;
};
}
24.4.4 Iterator operations [iterator.operations]
Since only random access iterators provide + and - operators, the library provides two function templates advance and distance. These function templates use + and - for random access iterators (and are, therefore, constant time for them); for input, forward and bidirectional iterators they use ++ to provide linear time implementations.
template <class InputIterator, class Distance> void advance(InputIterator& i, Distance n);
Requires: n shall be negative only for bidirectional and random access iterators.
Effects: Increments (or decrements for negative n) iterator reference i by n.
template<class InputIterator> typename iterator_traits<InputIterator>::difference_type distance(InputIterator first, InputIterator last);
Effects: If InputIterator meets the requirements of random access iterator, returns (last - first); otherwise, returns the number of increments needed to get from first to last.
Requires: If InputIterator meets the requirements of random access iterator, last shall be reachable from first or first shall be reachable from last; otherwise, last shall be reachable from first.
template <class ForwardIterator> ForwardIterator next(ForwardIterator x, typename std::iterator_traits<ForwardIterator>::difference_type n = 1);
Effects: Equivalent to advance(x, n); return x;
template <class BidirectionalIterator> BidirectionalIterator prev(BidirectionalIterator x, typename std::iterator_traits<BidirectionalIterator>::difference_type n = 1);
Effects: Equivalent to advance(x, -n); return x;
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