[Back to POINTERS SWAG index] [Back to Main SWAG index] [Original]
{ TEST PROGRAM AT END !! }
{ Links Unit - Turbo Pascal 5.5
Patterned after the list processing facility in Simula class SIMSET.
Simula fans will note the same naming conventions as Simula-67.
Written by Bill Zech @CIS:[73547,1034]), May 16, 1989.
The Links unit defines objects and methods useful for implementing
list (set) membership in your own objects.
Any object which inherits object <Link> will acquire the attributes
needed to maintain that object in a doubly-linked list. Because the
Linkage object only has one set of forward and backward pointers, a
given object may belong to only one list at any given moment. This
is sufficient for many purposes. For example, a task control block
might belong in either a ready list, a suspended list, or a swapped
list, but all are mutually exclusive.
A list is defined as a head node and zero or more objects linked
to the head node. A head node with no other members is an empty
list. Procedures and functions are provided to add members to the
end of the list, insert new members in position relative to an
existing member, determine the first member, last member, size
(cardinality) of the list, and to remove members from the list.
Because your object inherits all these attributes, your program
need not concern itself with allocating or maintaining pointers
or other stuff. All the actual linkage mechanisms will be
transparent to your object.
*Note*
The following discussion assumes you have defined your objects
as static variables instead of pointers to objects. For most
programs, dynamic objects manipulated with pointers will be
more useful. Some methods require pointers as arguments.
Example program TLIST.PAS uses pointer type variables.
Define your object as required, inheriting object Link:
type
myObjType = object(Link)
xxx.....xxxx
end;
To establish a new list, declare a variable for the head node
as a type Head:
var
Queue1 :Head;
Queue2 :Head;
Define your object variables:
var
X : myObjType;
Y : myObjType;
Z : myObjType;
P :^myObjType;
Make sure the objects have been Init'ed as required for data
initialization, VMT setup, etc.
Queue1.Init;
Queue2.Init;
X.Init;
Y.Init;
Z.Init;
You can add your objects to a list with <Into>:
(Note the use of the @ operator to make QueueX a pointer to the
object.)
begin
X.Into(@Queue1);
Y.Into(@Queue2);
You can insert at a specific place with <Precede> or <Follow>:
Z.Precede(@Y);
Z.Follow(@Y);
Remove an object with <Out>:
Y.Out;
Then add it to another list:
Y.Into(@Queue1);
Note that <Into>, <Precede> and <Follow> all have a built-in
call to Out, so to move an object from one list to another can
be had with a single operation:
Z.Into(@Queue1);
You can determine the first and last elements with <First> and <Last>:
(Note the functions return pointers to objects.)
P := Queue1.First;
P := Queue1.Last;
The succcessor or predecessor of a given member can be found with
fucntions <Suc> and <Pred>:
P := X.Pred;
P := Y.Suc;
P := P^.Suc;
The number of elements in a list is found with <Cardinal>:
N := Queue1.Cardinal;
<Empty> returns TRUE is the list has no members:
if Queue1.Empty then ...
You can remove all members from a list with <Clear>:
Queue1.Clear;
GENERAL NOTES:
The TP 5.5 type compatibility rules allow a pointer to a
descendant be assigned to an ancestor pointer, but not vice-versa.
So although it is perfectly legal to assign a pointer to
type myObjType to a pointer to type Linkage, it won't let
us do it the opposite.
We would like to be able to assign returned values from
Suc, Pred, First, and Last to pointers of type myObjType,
and the least fussy way is to define these pointer types
internal to this unit as untyped pointers. This works fine
because all we are really doing is passing around pointers
to Self, anyway. The only down-side to this I have noticed
is you can't do: P^.Suc^.Pred because the returned pointer
type cannot be dereferenced without a type cast.
}
unit Links;
interface
type
pLinkage = ^Linkage;
pLink = ^Link;
pHead = ^Head;
Linkage = object
prede :pLinkage;
succ :pLinkage;
function Suc :pointer;
function Pred :pointer;
constructor Init;
end;
Link = object(Linkage)
procedure Out;
procedure Into(s :pHead);
procedure Follow (x :pLinkage);
procedure Precede(x :pLinkage);
end;
Head = object(Linkage)
function First :pointer;
function Last :pointer;
function Empty :boolean;
function Cardinal :integer;
procedure Clear;
constructor Init;
end;
implementation
constructor Linkage.Init;
begin
succ := NIL;
prede := NIL;
end;
function Linkage.Suc :pointer;
begin
if TypeOf(succ^) = TypeOf(Head) then
Suc := NIL
else Suc := succ;
end;
function Linkage.Pred :pointer;
begin
if TypeOf(prede^) = TypeOf(Head) then
Pred := NIL
else Pred := prede;
end;
procedure Link.Out;
begin
if succ <> NIL then
begin
succ^.prede := prede;
prede^.succ := succ;
succ := NIL;
prede := NIL;
end;
end;
procedure Link.Follow(x :pLinkage);
begin
Out;
if x <> NIL then
begin
if x^.succ <> NIL then
begin
prede := x;
succ := x^.succ;
x^.succ := @Self;
succ^.prede := @Self;
end;
end;
end;
procedure Link.Precede(x :pLinkage);
begin
Out;
if x <> NIL then
begin
if x^.succ <> NIL then
begin
succ := x;
prede := x^.prede;
x^.prede := @Self;
prede^.succ := @Self;
end;
end;
end;
procedure Link.Into(s :pHead);
begin
Out;
if s <> NIL then
begin
succ := s;
prede := s^.prede;
s^.prede := @Self;
prede^.succ := @Self;
end;
end;
function Head.First :pointer;
begin
First := suc;
end;
function Head.Last :pointer;
begin
Last := Pred;
end;
function Head.Empty :boolean;
begin
Empty := succ = prede;
end;
function Head.Cardinal :integer;
var
i :integer;
p :pLinkage;
begin
i := 0;
p := succ;
while p <> @Self do
begin
i := i + 1;
p := p^.succ;
end;
Cardinal := i;
end;
procedure Head.Clear;
var
x : pLink;
begin
x := First;
while x <> NIL do
begin
x^.Out;
x := First;
end;
end;
constructor Head.Init;
begin
succ := @Self;
prede := @Self;
end;
end.
{ ----------------------- TEST PROGRAM ----------- }
program tlist;
uses Links;
type
NameType = string[10];
person = object(link)
name :NameType;
constructor init(nameArg :NameType);
end;
Pperson = ^person;
constructor person.init(nameArg :NameType);
begin
name := nameArg;
link.init;
end;
var
queue : Phead;
man : Pperson;
man2 : Pperson;
n : integer;
tf : boolean;
begin
new(queue,Init);
tf := queue^.Empty;
new(man,Init('Bill'));
man^.Into(queue);
new(man,Init('Tom'));
man^.Into(queue);
new(man,Init('Jerry'));
man^.Into(queue);
man := queue^.First;
writeln('First man in queue is ',man^.name);
man := queue^.Last;
writeln('Last man in queue is ',man^.name);
n := queue^.Cardinal;
writeln('Length of queue is ',n);
if not queue^.Empty then writeln('EMPTY reports queue NOT empty');
new(man2,Init('Hugo'));
man2^.Precede(man);
new(man2,Init('Alfonso'));
man2^.Follow(man);
{ should now be: Bill Tom Hugo Jerry Alfonso }
writeln('After PRECEDE and FOLLOW calls, list should be:');
writeln(' {Bill, Tom, Hugo, Jerry, Alfonso}');
writeln('Actual list is:');
man := queue^.First;
while man <> NIL do
begin
write(man^.name,' ');
man := man^.Suc;
end;
writeln;
man := queue^.Last;
writeln('The same list backwards is:');
while man <> NIL do
begin
write(man^.name,' ');
man := man^.Pred;
end;
writeln;
n := queue^.Cardinal;
writeln('Queue size should be 5 now, is: ', n);
queue^.Clear;
writeln('After clear operation,');
n := queue^.Cardinal;
writeln(' Queue size is ',n);
tf := queue^.Empty;
if tf then writeln(' and EMTPY reports queue is empty.');
writeln;
writeln('Done with test.');
end.
[Back to POINTERS SWAG index] [Back to Main SWAG index] [Original]