6.13.1 C++ Standard Library Iterators

All Essential Tools Module standard library-based collection class templates provide standard iterators. These iterators are fully compliant with the C++ Standard Library requirements for iterators, making them a powerful tool for using the classes in conjunction with the C++ Standard Library—especially the algorithms. Although full treatment of iterators is beyond the scope of this guide, your C++ Standard Library reference and tutorials will provide ample information.

The standard library-based collection class templates provide three types of iterators: forward, bi-directional, and random-access. Forward iterators allow unidirectional traversal from beginning to end. As suggested by the name, bidirectional iterators allow traversal in both directions—front to back, and back to front. Random-access iterators are bidirectional as well, and further distinguished by their ability to advance over an arbitrary number of elements in constant time. All of these iterators allow access to the item at the current position via the dereference operator *.

Given iterator iter and an integral value n, the following basic operations are just some of those supported:

Again, your standard library documentation will describe all the operators and functions available for each type of iterator.

In addition to the iterators just described, the standard library-based collection class templates also provide two typedefs used to iterate over the items in a collection class: iterator, and const_iterator. You can use the iterator typedef to traverse a collection class and modify the elements within. You can use instances of const_iterator to traverse, but not modify, the collection class and access elements. For the associative container-based and sorted sequence-based collections, which do not allow modification of elements once they are in the collection class, the iterator and const_iterator types are the same.

Finally, the templates also provide two member functions that return actual iterators you can use for traversing their respective collection classes. These member functions are begin() and end(). Each of these member functions is overloaded by a const receiver so that the non-const version returns an instance of type iterator, and the const version returns an instance of type const_iterator.

Member function begin() always returns an iterator already positioned at the first item in the collection class. Member function end() returns an iterator which has a past-the-end value, the way a pointer to the NULL character of a null-terminated character string has a value that points "past the end." An iterator of past-the-end value can be used to compare with another iterator to see if you've finished visiting all the elements in the collection class. It can also be used as a starting point for moving backwards through collection classes that provide either bidirectional or random-access iterators. The one thing you cannot do with an end() iterator is deference it. Here's an example using iterators to move through a list and search for a match:

RWTValDlist<int> intCollection; // a list of integers // ... < put stuff in the list > // position iter at start: RWTValDlist<int>::iterator iter = intCollection.begin(); // set another iterator past the end: RWTValDlist<int>::iterator theEnd = intCollection.end(); // iterate through, looking for a 7: while (iter != theEnd) { // test for end of collection if (*iter == 7) // use '*' to access current element return true; // found a 7 ++iter; // not a 7, try next element } return false; // never found a 7

6.13.2 Map-Based Iteration and Pairs

6.13.3 Iterators as Generalized Pointers

It may not be obvious at first, but you can think of an iterator as a generalized pointer. Imagine a pointer to an array of int s. The array itself is a collection class, and a pointer to an element of that array is a random-access iterator. To advance to the next element, you simply use the unary operator ++. To move back to a previous element, you use --. To access the element at the current position of the iterator, you use the unary operator *. Finally, it is important to know when you have visited all the elements. C++ guarantees that you can always point to the first address past the end of an allocated array. For example:

int intCollection[10]; // an array of integers // ... < put stuff in the array > // position iter at start: int* iter = intCollection; // set another iterator past the end: int* theEnd = intCollection + 10; // iterate through, looking for a 7: while (iter != theEnd) { // test for end of array if (*iter == 7) // use '*' to access current element return true; // found a 7 ++iter; // not a 7, try next element } return false; // never found a 7