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Swedish Research Institute
of National Defense
The CLASS and pointer
concepts in SIMULA
Mats Ohlin
75-07-31
The CLASS and pointer concepts in SIMULA Page 2
1. Introduction
Whenever SIMULA is taught to people unfamiliar with list
programming (as LISP, PL/1 etc) the same difficulties arise.
Since most of the participants in SIMULA courses have their basic
programming experience from languages like FORTRAN or ALGOL, the
introduction of the SIMULA concepts CLASS and REFerence variables
(pointers) almost invariably causes some headache.
This paper aims to introduce these concepts to the SIMULA beginner
in a stepwise and maybe somewhat tedious way. The reader is assumed to
have some experience of FORTRAN and a basic knowledge of elementary
ALGOL concepts as block- and program structure.
Once these things are fully understood, learning the rest of the
SIMULA language should present no special difficulties.
2. The SIMULA CLASS
Let us analyse what happens when you are calling a FORTRAN
subroutine (or an ALGOL/SIMULA procedure).
The statement
CALL SUB(A,B,C,...)
will have the effect that the parameters (arguments) of the subroutine
are evaluated, whereupon the code in the subroutine will be executed.
Usually in FORTRAN simple variables (undimensioned) are copied to
the sub- routine (and copied back at return), while the dimensioned
variables will be directly accessible to the subroutine by copying the
address of the first element or the array head.
A subsequent call of the same routine will execute the same code in
the routine (naturally), u s i n g t h e s a m e s t o r a g e
locations for parameters and variables local to the routine. This is
the reason why recursive subroutine calls are prohibited in FORTRAN.
The CLASS concept in SIMULA is in a way a generalization of the
procedure concept. A declaration of a CLASS looks quite the same as for
procedures :-
CLASS A(B,X); BOOLEAN B; PROCEDURE A(B,X); BOOLEAN B;
REAL X; REAL X;
<body>; <body>;
The CLASS and pointer concepts in SIMULA Page 3
The <body> contains the code for the respective modules.
The FORTRAN system uses the same storage locations for the
subroutine everytime. However, SIMULA uses a dynamic storage allocation
technique so that the used core may be expanded and the
d a t a w i t h i n t h e a l l o c a t e d
s p a c e m a y b e r e f e r r e d t o l a t e r o n.
The procedure call A(BX,XX);
will have its correspondence in NEW A(BX,XX);
This statement will have the following effect.
An unused part of the core will be used and the following
information is stored :-
<this part of core has the layout according to CLASS A>
<the formal boolean parameter B will store the value of BX>
<the formal real parameter X will store the value of XX>
or in more compact form (if BX was TRUE and XX was 2.5) :-
-------------------------
I - 'A' - I
-------------------------
I B = TRUE I
I X= 2.5 I
-------------------------
Of course existing code within the body of the CLASS will be
executed, just as in the procedure case.
Another call of A: NEW A(FALSE,3.5);
will add the information
-------------------------
I - 'A' - I
-------------------------
I B = FALSE I = a new object of class A
I X= 3.5 I
-------------------------
to the dynamic core.
We will call this data-description (including the specific data)
resulting from the "NEW A...." statement an object (or instance) of the
CLASS A.
All that hitherto has been discussed would be of little use if we
couldn't access the data in our objects after creating them.
The CLASS and pointer concepts in SIMULA Page 4
3. Reference variables - pointers.
In order to be able to access our generated data we must have the
possibility of accessing the objects. For this we need a special kind
of identifier - the REF-variable.
These identifiers can be regarded as holding two pieces of
information :-
REF-variable ------------------------------------
I - 'A' - I 001234 I
------------------------------------
qualification address
At first the information that the variable may be used in
connection with CLASS A is stored (this enables the SIMULA system to do
the qualification checking).
Then the address in core where the data of the referenced object
starts is stored.
For example :
CLASS A(B,X); BOOLEAN B; REAL X;
<body>;
REF (A) Y,Z;
Z:- NEW A(FALSE,1.5);
Y:- NEW A(TRUE,-1);
Note how the declaration is made. Y and Z are reference variables
which are allowed to point at objects of CLASS A (one at a time of
course).
After the declaration is made the address field contains a special
address which is called NONE, i.e. the variables do not point at any
object.
The compound symbol ':-' is pronounced "denotes", in contrast to
the ':=' meaning "becomes", the latter dealing with values
(corresponding to the FORTRAN use of '=').
So the first statement is interpreted as :
Z denotes a new object of class A having its B-data-value set
to FALSE and it's X-data-value set to 1.5.
The SIMULA language allows us to access the data of our created
objects.
Since the "Z's X-data-value = 1.5" is a bit lengthy to write we use
the notation :
The CLASS and pointer concepts in SIMULA Page 5
Z.X = 1.5
pronounced "Z dot X equals 1.5".
To the program above we might add the following statement :
Z.B:= Z.X > Y.X;
assigning Z.B the value TRUE.
The B and X are called ATTRIBUTES of class A.
Compare with a FORTRAN equivalent :
LOGICAL B(2)
REAL X(2),NAMES(2)
DATA NAMES/'Z','Y'/
B(1)= .FALSE.
X(1)= 1.5
B(2)= .TRUE.
X(2)= -1
Dimensioned variables are used much more in FORTRAN (and ALGOL)
than in SIMULA. In SIMULA classes and reference variables are often
preferred to dimensioned variables. This gives better protection
against programming errors, higher program readibility and the
possibility to vary the size of the data structure (the number of
objects) during execution.
Consider the following SIMULA program :
BEGIN
CLASS A(B,X); BOOLEAN B; REAL X;
BEGIN REAL C;
IF B THEN C:= X ELSE C:= 3;
END of a;
REF (A) Y,Z;
Z:- NEW A(FALSE,1.5);
Y:- NEW A(TRUE,-1);
END of program
After the execution the object at which Z is pointing to will look
like :
---------------------
I - 'A' - I
---------------------
I B = FALSE I
I X = 1.5 I
I C = 3 I
---------------------
* This shows that also variables declared within the class
* (at the outermost block level) are attributes as well as the
* formal parameters in the parameter list as explained before.
The CLASS and pointer concepts in SIMULA Page 6
So Z.C = 3 and Y.C = -1 (since Y.B is TRUE and thus the C:= X has
been executed for the Y-object).
Consider now an additional statement inserted before the last END
in the program above :
Z:- Y;
This will have the effect that t h e a d d r e s s
c o n t e n t in the Y-variable is copied to the address field of the
Z-variable. I.e. Z is made to point a t t h e s a m e t h i n g
t h a t Y p o i n t s t o (which may be an object or NONE).
* Since there are no other references the data once referenced
* by the Z-variable is now inaccessible and forever lost.
Now answer the following question :
If we add the statements :
Z:- Y; (as before)
Y:- NONE;
what does Z point at now?
-------------------------
Right - Z point at the A-object with .B=TRUE, .X= -1 and .C= -1,
i.e. the object which y pointed at earlier.
Multiple assignment is allowed :
Z:- Y:- NEW A(TRUE,4); is the same as
Y:- NEW A(TRUE,4);
Z:- Y;
Adding Y:- NONE; still means that Z points to our generated
object.
We have now introduced the simple use of a SIMULA CLASS and
reference variables. We have also experienced that the data - specified
in the parameter list (if present) as well as the class declarations (if
present) form the complete attribute list of an object.
Note that reference variables may also be attributes!
REF-variables are in this respect no different from other variables.
The CLASS and pointer concepts in SIMULA Page 7
An example :
BEGIN
CLASS A(X); REF (A) X;
BEGIN END; (the body is empty)
REF (A) Y,Z;
Z:- NEW A(NONE);
Y:- NEW A(Z);
END
Let us look at our objects :
Z Y
-------------------- --------------------
I - 'A' - I I - 'A' - I
+--> -------------------- --------------------
! I X:- NONE I I X :- -+ I
! -------------------- -----------!--------
! !
! !
+------------------------------------------+
So the X-data of the Y-object has a value which is referring to the
object currently referred by Z.
Thus Y.X == Z is now TRUE. The compound symbol '==' is the
operand for comparing references.
Adding the statement Z.X:- Y; creates a circular list, since Z.X
:- Y and Y.X:- Z.
We are allowed to use the attributes of the
attribute-reference-variable, just in the same way as an ordinary
reference variable.
So Z.X.X == Y.X since Z.X == Y !
Likewise :
Z.X.X == Z
Z.X.X.X == Y
Z.X.X.X.X == Z
....etc.
Compare the following program with the program above.
BEGIN
CLASS A;
BEGIN REF (A) X;
END;
REF (A) Y,Z;
Z:- NEW A;
Y:- NEW A; Y.X:- Z;
The CLASS and pointer concepts in SIMULA Page 8
END
The programs will generate identical data structures. The only
difference is at the program level where we have removed the formal
parameter list and declared X in the body instead.
Procedures may also be attributes. Consider
BEGIN
CLASS A(X,Y); REAL X,Y;
BEGIN
REAL PROCEDURE SUM; SUM:= X+Y;
PROCEDURE PRINT;
BEGIN OUTTEXT("A.X:"); OUTREAL(X,6,14);
OUTTEXT(" A.Y:");
OUTREAL(Y,6,16); OUTIMAGE
END;
END of A;
REF (A) Z;
Z:- NEW A(6,7);
OUTTEXT("ZSUM:"); OUTREAL(Z.SUM,6,14); OUTIMAGE;
Z.PRINT;
END
which will produce the following output :
ZSUM: 13.000000E+00
A.X: 6.000000E+00 A.Y: 7.000000E+00
Procedures returning REF-results are allowed.
A CLASS with a procedure returning a reference to a new object with
the x-attribute of the new object pointing at the original object would
read :
CLASS A(X); REF (A) X;
BEGIN
REF (A) PROCEDURE ADD; ADD:- NEW A(THIS A);
END of A;
REF (A) XA,XB;
XA:- NEW A(NONE); ! XA.X == NONE ;
XB:- XA.ADD; ! Note that XB.X == XA ;
XB:- XB.ADD; ! First is XB.ADD executed, then XB is switched
to point to the new object ;
XA.X:- XB; ! Creating a circular list ;
Use paper and pen a make a sketch of the situation after the execution
of the statements one by one.
The CLASS and pointer concepts in SIMULA Page 9
* Note the use of THIS which is the way to reference an
* object 'from inside'.
The CLASS and pointer concepts in SIMULA Page 10
4. CLASS hierarchy
Sometimes you want to add attributes to a class description, i.e.
define more detailed objects.
Assume that we have the class :
CLASS VEHICLE(WEIGHT); REAL WEIGHT; ;
and that we want to add an attribute for :
a) the maximum speed if it is a sportscar
b) number of seats if it is an ordinary car
c) maximum load if it is a lorry.
We then code :
VEHICLE CLASS SPORTSCAR(SPEED); REAL SPEED; ;
VEHICLE CLASS COMMONCAR(SEATS); INTEGER SEATS; ;
VEHICLE CLASS LORRY(MAXLOAD); REAL MAXLOAD; ;
These declarations will essentially have the same effect as if we
had declared :
CLASS SPORTSCAR(WEIGHT,SPEED); REAL WEIGHT,SPEED; ;
CLASS COMMONCAR(WEIGHT,SEATS); REAL WEIGTH; INTEGER SEATS;
;
CLASS LORRY(WEIGHT,MAXLOAD); REAL WEIGHT,MAXLOAD; ;
By using the prefix VEHICLE we saved some effort as we didn't need
to redeclare the attribute WEIGHT every time. We will also be able to
handle the different cars in a similar manner as shall be demonstrated
later on.
The class bodies of the respective classes will also be
concatenated (as well as the internal attributes).
New objects are thus created by adding the parameter(s) of the
subclass(es) to the parameter(s) of the basic class :
NEW SPORTSCAR(1600,150.0);
NEW COMMONCAR(1200,4);
The CLASS and pointer concepts in SIMULA Page 11
5. Qualification
We could now write a program :
BEGIN
CLASS VEHICLE(WEIGHT); REAL WEIGHT; ;
VEHICLE CLASS SPORTSCAR(SPEED); REAL SPEED; ;
VEHICLE CLASS COMMONCAR(SEATS); INTEGER SEATS; ;
VEHICLE CLASS LORRY(MAXLOAD); REAL MAXLOAD; ;
REF (VEHICLE) V;
REF (SPORTSCAR) S;
REF (COMMONCAR) U;
REF (LORRY) L;
V:- NEW VEHICLE(2000);
S:- NEW SPORTSCAR(1600,150.0);
U:- NEW COMMONCAR(1200,4);
L:- NEW LORRY(4000,2000);
.......
However, sometimes we need a variable which is allowed to point at
any kind of vehicle. Consider :
V:- NEW VEHICLE(2000);
Since the qualification match is exact (i.e. identical class
qualifications on both sides of the :- ) the compiler will not generate
any code for checking the match at run time.
However, it is allowed to write :
V:- NEW SPORTSCAR(1600,150.0);
and the V.WEIGHT gives the weight allright.
But the construction :
V.SPEED is illegal.
This is because the variable V is declared as a pointer which is
allowed to point at a vehicle-object which has no speed-attribute.
We might instead code :
X:= V QUA SPORTSCAR.SPEED;
in order to access the speed-attribute.
This will mean that the run time system will check that the V
actually points to a sportscar-object when this statement is executed.
The CLASS and pointer concepts in SIMULA Page 12
If not so a run time error will occur.
Note that the statement :
S:- NEW VEHICLE(1000);
is illegal since the S-variable definitely should give access to a
speed-attribute, which in this case is undefined.
The CLASS and pointer concepts in SIMULA Page 13
6. INNER
In a similar way as the parameters of nested classes are
concatenated, the code within the class body will be linked together.
Thus the following two examples will execute the same statements :
CLASS A; CLASS B;
BEGIN BEGIN
<statement 1> <statement 1>
END; <statement 2>
END;
A CLASS B;
BEGIN
<statement 2>
END;
The order of the execution may be altered by the use of INNER.
Consider :
CLASS A; CLASS B;
BEGIN BEGIN
<statement 1> <statement 1>
INNER; <statement 3>
<statement 2> <statement 2>
END; END;
A CLASS B;
BEGIN
<statement 3>
END;
The statement NEW B will execute the statements 1,3,2 in both
cases.
* INNER must be used at the outermost block level in the
* class body only.
It may be used at several levels in a class hierarchy.
The CLASS and pointer concepts in SIMULA Page 14
7. Some additional pieces of advice.
When you really think that you have comprehended the CLASS and
pointer concepts you are advised to analyze the COMMON BASE (1)
definition of the CLASS SIMSET. After that try SIMSET CLASS SIMULATION.
When writing SIMULA programs, try to build your classes as the most
natural carriers of the different information that is to be processed by
your program. Then add procedure attributes to manipulate the
information in the respective classes.
For example - if your are using SIMSET (a system defined class for
list- and queuing-problems) - it is often useful to define subclasses to
the standard class HEAD if operations will be performed on whole queues.
In such a case, define your own queue handling procedure attributes in
your subclass.
I.e.
SIMSET BEGIN
HEAD CLASS MYQUEUE;
BEGIN
PROCEDURE PRINT;
BEGIN REF (ITEM) X;
X:- FIRST;
WHILE X =/= NONE DO
BEGIN OUTINT(X.I,8); OUTIMAGE; X:- X.SUC END
END OF PRINT;
END OF MYQUEU;
LINK CLASS ITEM(I); INTEGER I;
BEGIN INTO(Q) END OF ITEM;
REF (MYQUEUE) Q;
Q:- NEW MYQUEUE;
NEW ITEM(1); COMMENT Note that ITEM puts itself into the Q;
NEW ITEM(2);
......
Q.PRINT; COMMENT The I-attributes will now be printed;
END OF PROGRAM
There is certainly a considerable threshold to pass before you will
be able to program. The process that constitues the initial structuring
of your SIMULA program in SIMULA classes and procedures will be
initially be slow. However, there is no other way to overcome these
difficulties than to START PROGRAMMING, how uncertain you ever may feel.
The CLASS and pointer concepts in SIMULA Page 15
You will later learn that the preliminary work done on the program
structuring will be payed back when you start to expand your program.
It could very well be said that the first program usually will be
re-written some times when you disover new structuring possibilities.
8. Acknowledgements
I want to thank Jacob Palme, Lars Enderin and Arne Fransen, all at
the Swedish Research Institute of National Defense, for helpfull
comments during the preparation of this memorandum.
References :
(1) COMMON BASE LANGUAGE by O. Dahl, B. Myhrhaug and
K. Nygaard, Norwegian Computing Center, October 1970,
Publication No S-22.