iterator_traits Struct


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A template helper struct used to specify all the critical type definitions that an iterator should have.

struct iterator_traits { typedef typename Iterator::iterator_category iterator_category; typedef typename Iterator::value_type value_type; typedef typename Iterator::difference_type difference_type; typedef difference_type distance_type; typedef typename Iterator::pointer pointer; typedef typename Iterator::reference reference; };

The template struct defines the member types

  • iterator_category: a synonym for Iterator::iterator_category.

  • value_type: a synonym for Iterator::value_type.

  • difference_type: a synonym for Iterator::difference_type.

  • distance_type: a synonym for Iterator::difference_type.

  • pointer: a synonym for Iterator::pointer.

  • reference: a synonym for Iterator::reference.

The partial specializations determine the critical types associated with an object pointer of type Type * or const Type *.

In this implementation you can also use several template functions that do not make use of partial specialization:

template <class Category, class Type, class Diff>
C _Iter_cat(const iterator<Category, Ty, Diff>&);

template <class Ty>
random_access_iterator_tag _Iter_cat(const Ty *);

template <class Category, class Ty, class Diff>
Ty *val_type(const iterator<Category, Ty, Diff>&);

template <class Ty>
Ty *val_type(const Ty *);

template <class Category, class Ty, class Diff>
Diff *_Dist_type(const iterator<Category, Ty, Diff>&);

template <class Ty>
ptrdiff_t *_Dist_type(const Ty *);

which determine several of the same types more indirectly. You use these functions as arguments on a function call. Their sole purpose is to supply a useful template class parameter to the called function.

// iterator_traits.cpp  
// compile with: /EHsc  
#include <iostream>  
#include <iterator>  
#include <vector>  
#include <list>  
using namespace std;  
template< class it >  
function( it i1, it i2 )  
   iterator_traits<it>::iterator_category cat;  
   cout << typeid( cat ).name( ) << endl;  
   while ( i1 != i2 )  
      iterator_traits<it>::value_type x;  
      x = *i1;  
      cout << x << " ";  
   cout << endl;  
int main( )   
   vector<char> vc( 10,'a' );  
   list<int> li( 10 );  
   function( vc.begin( ), vc.end( ) );  
   function( li.begin( ), li.end( ) );  
\* Output:   
struct std::random_access_iterator_tag  
a a a a a a a a a a   
struct std::bidirectional_iterator_tag  
0 0 0 0 0 0 0 0 0 0   

Header: <iterator>

Namespace: std

Thread Safety in the C++ Standard Library
Standard Template Library