Pascal - Pointers

Pointers in Pascal are easy and fun to learn. Some Pascal programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Pascal programmer. Let s start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using the name of the pointer variable, which denotes an address in memory.

What Are Pointers?

A pointer is a dynamic variable, whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is −

type
   ptr-identifier = ^base-variable-type;

The pointer type is defined by prefixing the up-arrow of caret symbol (^) with the base type. The base-type defines the types of the data items. Once a pointer variable is defined to be of certain type, it can point data items of that type only. Once a pointer type has been defined, we can use the var declaration to declare pointer variables.

var
   p1, p2, ... : ptr-identifier;

Following are some vapd pointer declarations −

type
   Rptr = ^real;
   Cptr = ^char;
   Bptr = ^ Boolean;
   Aptr = ^array[1..5] of real;
   date-ptr = ^ date;
      Date = record
         Day: 1..31;
         Month: 1..12;
         Year: 1900..3000;
      End;
var
   a, b : Rptr;
   d: date-ptr;

The pointer variables are dereferenced by using the same caret symbol (^). For example, the associated variable referred by a pointer rptr, is rptr^. It can be accessed as −

rptr^ := 234.56;

The following example will illustrate this concept −

program exPointers;
var
   number: integer;
   iptr: ^integer;

begin
   number := 100;
   writeln( Number is:  , number);
   
   iptr := @number;
   writeln( iptr points to a value:  , iptr^);
   
   iptr^ := 200;
   writeln( Number is:  , number);
   writeln( iptr points to a value:  , iptr^);
end.

Number is: 100
iptr points to a value: 100
Number is: 200
iptr points to a value: 200

program exPointers;
var
   number: integer;
   iptr: ^integer;
   y: ^word;

begin
   number := 100;
   writeln( Number is:  , number);
   iptr := @number;
   writeln( iptr points to a value:  , iptr^);
   
   iptr^ := 200;
   writeln( Number is:  , number);
   writeln( iptr points to a value:  , iptr^);
   y := addr(iptr);
   writeln(y^); 
end.

Number is: 100
iptr points to a value: 100
Number is: 200
iptr points to a value: 200
45504

program exPointers;
var
   number: integer;
   iptr: ^integer;
   y: ^word;

begin
   iptr := nil;
   y := addr(iptr);
   
   writeln( the vaule of iptr is  , y^);
end.

The value of ptr is 0

if(ptr <> nill )then     (* succeeds if p is not null *)
if(ptr = nill)then    (* succeeds if p is null *)