random_access_iterator_tag Struct

Article
01/02/2018

The latest version of this topic can be found at random_access_iterator_tag Struct.

A class that provides a return type for iterator_category function that represents a random-access iterator.

Syntax

struct random_access_iterator_tag    : public bidirectional_iterator_tag {};

Remarks

The category tag classes are used as compile tags for algorithm selection. The template function needs to find the most specific category of its iterator argument so that it can use the most efficient algorithm at compile time. For every iterator of type Iterator, iterator_traits< Iterator> ::iterator_category must be defined to be the most specific category tag that describes the iterator's behavior.

The type is the same as iterator< Iter> ::iterator_category when Iter describes an object that can serve as a random-access iterator.

Example

// iterator_rait.cpp  
// compile with: /EHsc  
#include <iterator>  
#include <vector>  
#include <iostream>  
#include <list>  
  
using namespace std;  
  
int main( )  
{  
   vector<int> vi;  
   vector<char> vc;  
   list<char> lc;  
   iterator_traits<vector<int>:: iterator>::iterator_category cati;  
   iterator_traits<vector<char>:: iterator>::iterator_category catc;  
   iterator_traits<list<char>:: iterator>::iterator_category catlc;  
  
   // These are both random-access iterators  
   cout << "The type of iterator for vector<int> is "  
       << "identified by the tag:\n "   
       << typeid ( cati ).name( ) << endl;  
   cout << "The type of iterator for vector<char> is "  
       << "identified by the tag:\n "   
       << typeid ( catc ).name( ) << endl;  
   if ( typeid ( cati ) == typeid( catc ) )  
      cout << "The iterators are the same." << endl << endl;  
   else  
      cout << "The iterators are not the same." << endl << endl;  
  
   // But the list iterator is bidirectinal, not random access  
   cout << "The type of iterator for list<char> is "  
       << "identified by the tag:\n "   
       << typeid (catlc).name( ) << endl;  
  
   // cout << ( typeid ( vi.begin( ) ) == typeid( vc.begin( ) ) ) << endl;  
   if ( typeid ( vi.begin( ) ) == typeid( vc.begin( ) ) )  
      cout << "The iterators are the same." << endl;  
   else  
      cout << "The iterators are not the same." << endl;  
   // A random-access iterator is a bidirectional iterator.  
   cout << ( void* ) dynamic_cast< iterator_traits<list<char>:: iterator>  
          ::iterator_category* > ( &catc ) << endl;  
}

Sample Output

The following output is for x86.

The type of iterator for vector<int> is identified by the tag:
    struct std::random_access_iterator_tag
The type of iterator for vector<char> is identified by the tag:
    struct std::random_access_iterator_tag
The iterators are the same.

The type of iterator for list<char> is identified by the tag:
    struct std::bidirectional_iterator_tag
The iterators are not the same.
0012FF3B

Requirements

Header: <iterator>

Namespace: std

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