The Standard Template Library defines five major categories of iterators. The following diagram illustrates these categories:
input iterators output iterators
random access iterators
Note that this illustration doesn't represent inheritance relationships; it merely describes the iterator categories and their interfaces. Each lower category is a superset of the category above it. For instance, a forward iterator provides the functionality of input and output iterators plus additional functionality. Here is a brief summary of the functionality and interfaces of these categories:
Input iterators allow the user to advance the iterator (by using the ++) and read the value to which the iterator points (using the * operator).
Output iterators allow the user to advance the iterator and assign a new value to the object to which the iterator is pointing.
Forward iterators support both read and write access, but traversal is permitted only in one direction.
Bidirectional iterators allow the user to traverse the sequence in both directions. In other words, you can use both ++ andwith a bidirectional iterator.
Random access iterators support random jumps and pointer arithmetic operations. For example:
string::iterator it = s.begin();
char c = *(it+5); // skip five elements in a sequence