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class template

<iterator>

std::iterator

template <class Category,              // iterator::iterator_category          class T,                     // iterator::value_type          class Distance = ptrdiff_t,  // iterator::difference_type          class Pointer = T*,          // iterator::pointer          class Reference = T&         // iterator::reference          > class iterator;

Iterator base class

[Note: This page describes the base class std::iterator. For general information about iterators, refer to header]

This is a base class template that can be used to derive iterator classes from it. It is not an iterator class and does not provide any of the functionality an iterator is expected to have.

This base class only provides some member types, which in fact are not required to be present in any iterator type (iterator types have no specific member requirements), but they might be useful, since they define the members needed for the default iterator_traits class template to generate the appropriate instantiation automatically (and such instantiation is required to be valid for all iterator types).

It is defined as:

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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;
  };

Template parameters

Category
Category to which the iterator belongs to. It must be one of the following iterator tags:
iterator tagCategory of iterators
input_iterator_tagInput Iterator
output_iterator_tagOutput Iterator
forward_iterator_tagForward Iterator
bidirectional_iterator_tagBidirectional Iterator
random_access_iterator_tagRandom-access Iterator
T
Type of elements pointed by the iterator.
Distance
Type to represent the difference between two iterators.
Pointer
Type to represent a pointer to an element pointed by the iterator.
Reference
Type to represent a reference to an element pointed by the iterator.

Member types

member typedefinitionnotes
iterator_categorythe first template parameter (Category)
value_typethe second template parameter (T)
difference_typethe third template parameter (Distance)defaults to: ptrdiff_t
pointerthe fourth template parameter (Pointer)defaults to: T*
referencethe fifth template parameter (Reference)defaults to: T&

Example

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// std::iterator example
#include <iostream>     // std::cout
#include <iterator>     // std::iterator, std::input_iterator_tag

class MyIterator : public std::iterator<std::input_iterator_tag, int>
{
  int* p;
public:
  MyIterator(int* x) :p(x) {}
  MyIterator(const MyIterator& mit) : p(mit.p) {}
  MyIterator& operator++() {++p;return *this;}
  MyIterator operator++(int) {MyIterator tmp(*this); operator++(); return tmp;}
  bool operator==(const MyIterator& rhs) const {return p==rhs.p;}
  bool operator!=(const MyIterator& rhs) const {return p!=rhs.p;}
  int& operator*() {return *p;}
};

int main () {
  int numbers[]={10,20,30,40,50};
  MyIterator from(numbers);
  MyIterator until(numbers+5);
  for (MyIterator it=from; it!=until; it++)
    std::cout << *it << ' ';
  std::cout << '\n';

  return 0;
}

Output:


See also

<iterator>
Iterator definitions (header)
iterator_traits
Iterator traits (class template)