Object-oriented Software in Ada 95

260 Containers
The procedure Next and Prev move the iterator on to the next / previous item in the list. If the iterator is not
currently pointing at an item, the iterator is unmodified. The end of the list is indicated by the iterator pointing to a
null value. By inspecting the list this case can be distinguished from the case of an empty list.
procedure Next( The:in out List_Iter ) is
begin
if The.Cur_Node /= null then --
The.Cur_Node := The.Cur_Node.Next;
end if;
end Next;
procedure Prev( The:in out List_Iter ) is
begin
if The.Cur_Node /= null then --
The.Cur_Node := The.Cur_Node.Prev;
end if;
end Prev;
end Class_List.Iterator;
--Next
--Previous
Note:
If you move the iterator to beyond the first element with prev then it is your responsibility to reset the
iterator’s position. The class list will consider the position at the end of the list.
17.3.2 Relationship between a list and its iterator
The list on which the iterator navigates must be writable. This is because the iterator may be used to insert or
delete an item in the list. Another solution would have been to have two distinct iterators for read and write
operations on the list.
17.4 Limitations of the list implementation
A limitation of this implementation is that a list object is physically duplicated when it is assigned. This is referred
to as a deep copy of an object. A deep copy of an object can involve the use of considerable time and storage
space.
There are two options for the implementation of assignment. These options are summarized in the table below:
Type of copy
Deep copy
Shallow copy
Commentary
The whole physical data structure is duplicated.
Only the pointer held directly in the object is duplicated.
For example, consider the data structure Original representing a list of colours held as a linked list. This is
illustrated in Figure 17.5 which shows the memory layout for the list container Original which holds the three
colours Red, Green and Blue.
Original
Red
Green
Blue
Figure 17.5 Illustration of the memory layout for a linked list of three colours.
© M A Smith - May not be reproduced without permission