STL 之 iterator traits 備忘

標籤：iterator traits

//5種迭代器，為了啟用重載機制，定義的5個類型。每種迭代器就是一個類型。

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{};

為了定義自己的迭代器，可以繼承自下面的iterator類，避免迭代器型別的未定義。

//Base class for iterator classtemplate <class Category, class T, class Distance=ptrdiff_t, class Pointer = T*, class Reference =T&>struct iterator{    typedef Category iterator_category;    typedef T value_type;    typedef Distance difference_type;    typedef Pointer pointer;    typedef Reference reference;};

iterator_traits類，就是一些typedef，相當於就是將迭代器的最本質的類型忠實的呈現出來。這裡值得注意的是，要使得這個迭代器忠實的完成這項工作，那麼各個迭代器的定義就必須定義相應的5個性別。value_type difference_type  iterator_category pointer reference .

template<class Iterator>struct iterator_traits{    typedef typename Iterator::category iterator_category;    typedef typename Iterator::value_type value_type;    typedef typename Iterator::difference_type difference_type;    typedef typename Iterator::pointer pointer;    typedef typename Iterator::reference reference;};

然而這對於指標而言是不可行的，因為指標沒有結構體，我們無法在其中定義這5個型別，於是針對指標，我們定義了iterator_traits的特化版本。

template<class T>struct iterator_traits<T*>{    typedef random_access_iterator_tag iterator_category;    typedef T value_type;    typedef ptrdiff_t difference_type;    typedef T* pointer;    typedef T & reference;};template<class T>struct iterator_traits<const T*>{    typedef random_access_iterator_tag iterator_category;    typedef T value_type;    typedef ptrdiff_t difference_type;    typedef const T* pointer;    typedef const T & reference;    };

註：根據指標指向 的內容是否可以改變，定義了上述的兩種特化版本。

下面的函數則是直接返回各個型別的類型：

template<class Iterator>inline typename iterator_traits<Iterator>::iterator_categoryiterator_category(const Iterator&){  typedef typename iterator_traits<Iterator>::iterator_category category;  return category(); //random_access_iterator_tag or bidirectional_iterator_tag ...}//pointer to the difference_typetemplate< class Iterator>inline typename iterator_traits<Iterator>::difference_type*distance_type (const Iterator &){  return static_cast<typename iterator_traits<Iterator>::difference_type *>(0);}template< class Iterator>inline typename iterator_traits<Iterator>::value_type *distance_type (const Iterator &){  return static_cast<typename iterator_traits<Iterator>::value_type* > (0);}

distance函數在上述traits下的實現代碼：

//distance functiontemplate<class InputIterator>inline iterator_traits<InputIterator>::difference_type__distance(InputIterator first, InputIterator last, input_iterator_tag){  iterator_traits<InputIterator>::difference_type n=0;  while(first != last)  {      ++first;++n;  }  return n;}template<class RandomIterator>inline typename iterator_traits<RandomIterator>::difference_type__distance(RandomIterator first, RandomIterator last, random_access_iterator_tag){    return last-first;}//user template<class InputIterator>inline iterator_traits<InputIterator>::difference_typedistance(InputIterator first, InputIterator last){    typedef typename iterator_traits<InputIterator>::iterator_category category;    return __distance(first,last,category());//category()構造一個臨時對象，進行參數推導，決定重載函數。｝

參考：STL源碼剖析