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iterator_traits
class template
Iterator traits
Standard algorithms use some members of the iterator_traits class to determine the characteristics of the iterators passed to them.
For every iterator type, a corresponding specialization of iterator_traits class template shall exist, with at least the following member types defined:
| member | description |
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| difference_type | Type to express the result of subtracting one iterator from another |
| value_type | The type of the element the iterator can point to |
| pointer | The type of a pointer to an element the iterator can point to |
| reference | The type of a reference to an element the iterator can point to |
| iterator_category | The iterator category. It can be one of these:
- input_iterator_tag
- output_iterator_tag
- forward_iterator_tag
- bidirectional_iterator_tag
- random_access_iterator_tag
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The iterator_traits class template comes with a default definition for all iterator types that takes these member types from the iterator itself:
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template <class Iterator> struct iterator_traits {
typedef typename Iterator::difference_type difference_type;
typedef typename Iterator::value_type value_type;
typedef typename Iterator::pointer pointer;
typedef typename Iterator::reference reference;
typedef typename Iterator::iterator_category iterator_category;
}
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Therefore, if an iterator has these member types defined does not need to explicitly specialize the iterator_traits class.
The iterator_traits class template also comes with specialized versions for pointers and pointers to const:
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template <class T> struct iterator_traits<T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef T* pointer;
typedef T& reference;
typedef random_access_iterator_tag iterator_category;
}
template <class T> struct iterator_traits<const T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef const T* pointer;
typedef const T& reference;
typedef random_access_iterator_tag iterator_category;
}
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So there is no need to define specific iterator_traits specializations for pointer types.
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