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.
If Iterator has valid ([temp.deduct]) member types difference_type, value_type, pointer, reference, and iterator_category, iterator_traits<Iterator> shall have the following as publicly accessible members:
using difference_type = typename Iterator::difference_type; using value_type = typename Iterator::value_type; using pointer = typename Iterator::pointer; using reference = typename Iterator::reference; using iterator_category = typename Iterator::iterator_category;
Otherwise, iterator_traits<Iterator> shall have no members by any of the above names.
It is specialized for pointers as
namespace std {
template<class T> struct iterator_traits<T*> {
using difference_type = ptrdiff_t;
using value_type = T;
using pointer = T*;
using reference = T&;
using iterator_category = random_access_iterator_tag;
};
}
and for pointers to const as
namespace std {
template<class T> struct iterator_traits<const T*> {
using difference_type = ptrdiff_t;
using value_type = T;
using pointer = const T*;
using reference = const T&;
using iterator_category = random_access_iterator_tag;
};
}
[ 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 ]
27.4.3 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> constexpr 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> constexpr 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 InputIterator> constexpr InputIterator next(InputIterator x, typename iterator_traits<InputIterator>::difference_type n = 1);
Effects: Equivalent to: advance(x, n); return x;
template <class BidirectionalIterator> constexpr BidirectionalIterator prev(BidirectionalIterator x, typename iterator_traits<BidirectionalIterator>::difference_type n = 1);
Effects: Equivalent to: advance(x, -n); return x;
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