TOPS-20 PASCAL Language Manual - Bitsavers

PASCAL DATA TYPES
Pointer type definitions are the only place in a PASCAL program where
you can use an identifier before you define it. PASCAL allows you to
use the base type identifier in a pointer type definition before you
define the base type, for example:
TYPE Ptr_To_Movie ~
~Movie;
Nam(,~ :::: F',:':JCKED ARri:AY [:t •• 20:3 OF CHAFi:;
Mov :i. (.:~ :::: Fi:FCOI:;:D
Title, Director : Name;
Y f:'~ a Y' : I N T E C! E I~: ,
Stars : FILE OF Name;
Next : ptr_To_Movie
END;
The TYPE section specifies the type identifier Movie before defining
the type Movie.
The value of a pointer is the storage address of the variable to which
it points. Thus, in the example above, the value of the field Next is
a pointer to (or address of) a dynamic record variable of type Movie.
Pointers assume values at initialization, by assignment, and through
the NEW procedure. The value of a pointer can be any legal storage
address. The value NIL indicates that the pointer does not currently
specify an address. Thus, a NIL pointer does not point to a variable.
PASCAL allows you to define pointers to types containing files, for
example:
TYPE x::::
Y::~
VAR M:X;
"'Y;
RECORD
P : I NTEGEf;:;
Q : ARRAY[:ttt3] OF TEXT
END;
The pointer type X points to record type Y, which contains a file
component in field Q. The files denoted by Q are never closed until
execution of the program terminates, unless you use the CLOSE
procedure. For example, to close the files defined in the TYPE
section above, you must call CLOSE with the parameters MA.Q[l],
M A
.Q(2], M".Q[31.
You can assign the constant NIL to a pointer as follows:
As a result of the assignment, the pointer variable M does not point
to a variable. NIL is the only value you can specify to initialize a
pointer.
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