Iterator Categories and Iterator Category Hierarchy
Figure 15.5 shows the iterator categories. Each category provides a specific set of functionality. Figure 15.6 illustrates the hierarchy of iterator categories. As you follow the hierarchy from bottom to top, each iterator category supports all the functionality of the categories below it in the figure. Thus the “weakest” iterator types are at the bottom and the most powerful one is at the top. Note that this is not an inheritance hierarchy.