27 Iterators library [iterators]

27.4 Iterator primitives [iterator.primitives]

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To simplify the task of defining iterators, the library provides several classes and functions:

27.4.1 Iterator traits [iterator.traits]

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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.
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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.
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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;
  };
}
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[ 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
 ]

27.4.2 Standard iterator tags [std.iterator.tags]

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It is often desirable for a function template specialization to find out what is the most specific category of its iterator argument, so that the function can select the most efficient algorithm at compile time.
To facilitate this, the library introduces category tag classes which are used as compile time tags for algorithm selection.
They are: input_­iterator_­tag, output_­iterator_­tag, forward_­iterator_­tag, bidirectional_­iterator_­tag and random_­access_­iterator_­tag.
For every iterator of type Iterator, iterator_­traits<Iterator>​::​iterator_­category shall be defined to be the most specific category tag that describes the iterator's behavior.
namespace std {
  struct input_iterator_tag { };
  struct output_iterator_tag { };
  struct forward_iterator_tag: public input_iterator_tag { };
  struct bidirectional_iterator_tag: public forward_iterator_tag { };
  struct random_access_iterator_tag: public bidirectional_iterator_tag { };
}
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[ Example
:
For a program-defined iterator BinaryTreeIterator, it could be included into the bidirectional iterator category by specializing the iterator_­traits template:
template<class T> struct iterator_traits<BinaryTreeIterator<T>> {
  using iterator_category = bidirectional_iterator_tag;
  using difference_type   = ptrdiff_t;
  using value_type        = T;
  using pointer           = T*;
  using reference         = T&;
};
— end example
 ]
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[ Example
:
If evolve() is well defined for bidirectional iterators, but can be implemented more efficiently for random access iterators, then the implementation is as follows:
template <class BidirectionalIterator>
inline void
evolve(BidirectionalIterator first, BidirectionalIterator last) {
  evolve(first, last,
    typename iterator_traits<BidirectionalIterator>::iterator_category());
}

template <class BidirectionalIterator>
void evolve(BidirectionalIterator first, BidirectionalIterator last,
  bidirectional_iterator_tag) {
  // more generic, but less efficient algorithm
}

template <class RandomAccessIterator>
void evolve(RandomAccessIterator first, RandomAccessIterator last,
  random_access_iterator_tag) {
  // more efficient, but less generic algorithm
}
— end example
 ]

27.4.3 Iterator operations [iterator.operations]

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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);
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Requires: n shall be negative only for bidirectional and random access iterators.
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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);
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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.
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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);
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Effects: Equivalent to: advance(x, n); return x;
template <class BidirectionalIterator> constexpr BidirectionalIterator prev(BidirectionalIterator x, typename iterator_traits<BidirectionalIterator>::difference_type n = 1);
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Effects: Equivalent to: advance(x, -n); return x;