24 Iterators library [iterators]

24.4 Iterator primitives [iterator.primitives]

24.4.3 Standard iterator tags [std.iterator.tags]

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 { };

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> > {
  typedef std::ptrdiff_t difference_type;
  typedef T value_type;
  typedef T* pointer;
  typedef T& reference;
  typedef bidirectional_iterator_tag iterator_category;

Typically, however, it would be easier to derive BinaryTreeIterator<T> from iterator<bidirectional_iterator_tag,T,ptrdiff_t,T*,T&>.  — end example ]

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 ]

Example: If a C++ program wants to define a bidirectional iterator for some data structure containing double and such that it works on a large memory model of the implementation, it can do so with:

class MyIterator :
  public iterator<bidirectional_iterator_tag, double, long, T*, T&> {
  // code implementing ++, etc.

Then there is no need to specialize the iterator_traits template.  — end example ]