iterator_traits Struct

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

Syntax

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;
   };

Remarks

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 class template parameter to the called function.

Example

// iterator_traits.cpp
// compile with: /EHsc
#include <iostream>
#include <iterator>
#include <vector>
#include <list>

using namespace std;

template< class it >
void
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 << " ";
      i1++;
   }
   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
*/

Requirements

Header: <iterator>

Namespace: std

See also

<iterator>
Thread Safety in the C++ Standard Library
C++ Standard Library Reference