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C++ Iterators

Iterators are used to access members of the container classes, and can be used in a similar manner to pointers. For example, one might use an iterator to step through the elements of a vector. There are several different types of iterators:

Iterator Description
input_iterator Read values with forward movement. These can be incremented, compared, and dereferenced.
output_iterator Write values with forward movement. These can be incremented and dereferenced.
forward_iterator Read or write values with forward movement. These combine the functionality of input and output iterators with the ability to store the iterators value.
bidirectional_iterator Read and write values with forward and backward movement. These are like the forward iterators, but you can increment and decrement them.
random_iterator Read and write values with random access. These are the most powerful iterators, combining the functionality of bidirectional iterators with the ability to do pointer arithmetic and pointer comparisons.
reverse_iterator Either a random iterator or a bidirectional iterator that moves in reverse direction.

Each of the container classes is associated with a type of iterator, and each of the STL algorithms uses a certain type of iterator. For example, vectors are associated with random-access iterators, which means that they can use algorithms that require random access. Since random-access iterators encompass all of the characteristics of the other iterators, vectors can use algorithms designed for other iterators as well.

The following code creates and uses an iterator with a vector:

  vector<int> the_vector;
  vector<int>::iterator the_iterator;
  for( int i=0; i < 10; i++ )
    the_vector.push_back(i);
  int total = 0;
  the_iterator = the_vector.begin();
  while( the_iterator != the_vector.end() ) {
    total += *the_iterator;
    the_iterator++;
  }
  cout << "Total=" << total << endl;
Notice that you can access the elements of the container by dereferencing the iterator.