HP Integrity NonStop H-Series - Manual (Page 24)

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©Copyright 1996 Rogue Wave Software
Introduction to Iterators
Iterators
Fundamental to the use of the container classes and the associated algorithms provided by the
standard library is the concept of an iterator. Abstractly, an iterator is simply a pointer-like
object used to cycle through all the elements stored in a container. Because different algorithms
need to traverse containers in a variety of fashions, there are different forms of iterator. Each
container class in the standard library can generate an iterator with functionality appropriate to
the storage technique used in implementing the container. It is the category of iterators required
as arguments that chiefly distinguishes which algorithms in the standard library can be used
with which container classes.
Range
Just as pointers can be used in a variety of ways in traditional programming, iterators are also
used for a number of different purposes. An iterator can be used to denote a specific value, just
as a pointer can be used to reference a specific memory location. On the other hand, a pair of
iterators can be used to describe a range of values, just as two pointers can be used to describe a
contiguous region of memory. In the case of iterators, however, the values being described are
not necessarily physically in sequence, but are rather logically in sequence, because they are
derived from the same container, and the second follows the first in the order in which the
elements are maintained by the container.
Conventional pointers can sometimes be null, meaning they point at nothing. Iterators, as well,
can fail to denote any specific value. Just as it is a logical error to dereference a null pointer, it
is an error to dereference an iterator that is not denoting a value.
When two pointers that describe a region in memory are used in a C++ program, it is
conventional that the ending pointer is not considered to be part of the region. For example, an
array named x of length ten is sometimes described as extending from x to x+10, even though
the element at x+10 is not part of the array. Instead, the pointer value x+10 is the past-the-end
value - the element that is the next value after the end of the range being described. Iterators are
used similarly to describe a range. The second value is not considered to be part of the range
being denoted. Instead, the second value is a past-the-end element, describing the next value in
sequence after the final value of the range. Sometimes, as with pointers to memory, this will be
an actual value in the container. Other times it may be a special value, specifically constructed