Iterators

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Iterator, a generic pointer. (Pointer to element in containers)

Basic Element of an Iterator

• begin()

  Point at the first element in the container

  vector<int> v = {1,2,3,4,5};
  auto it = v.begin(); // it -> 1
  

• end()

  Point at the element after the last element.

  \(element \in [begin(), end)\)

• operator ++

  Step the iterator to next element

  vector<int> v = {1,2,3,4,5};
  auto it = v.begin(); // it -> 1
  it++; // it -> 2
  

• **operator ***

  Get the element pointed to.

  vector<int> v = {1,2,3,4,5};
  auto it = v.begin(); // it -> 1
  *it == 1;
  

• operator ->

  Get the 'real' pointer to the element.

Iterator Traits

Defines info about the iterator

template <class Iterator>
struct iterator_traits {
 typedef typename Iterator::value_type value_type;
}

Partial specialization for pointer types

template <class T>
struct iterator_traits<T*> {
 typedef T value_type;
}

• Standard traits

  • iterator_category

  • value_type

    type of elements in container

  • difference_type

    type of it1 - it2

  • pointer

    type of pointer to elements on container

  • reference

    type of reference to elements on container

Iterator Category

• Inheritance tree

  

• Call corresponding functions regarding iterator category

  template<class Iterator, class Distance>
  inline void advance(Iterator &i, Distance n)
  {
    __advance(i, n, 
    iterator_traits<Iterator>::iterator_category());
  }
  

  Call functions with an instance of iterator_category.

  __advance have several overloaded versions, differed by the last argument.

  Compilers will then choose appropriate function signature !