Pascal - Pointer to Pointer

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Category: Chapter 3

Published: Sunday, 14 April 2013 17:10

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A pointer to a pointer is a form of multiple indirection, or a chain of pointers. Normally, a pointer contains the address of a variable. When we define a pointer to a pointer, the first pointer contains the address of the second pointer, which points to the location that contains the actual value as shown below.
 

 

A variable that is a pointer to a pointer must be declared as such. For example,

type
   iptr =^integer;
   pointerptr =^ iptr;
 

Following example would illustrate the concept as well as display the addresses:

program exPointertoPointers;
type
   iptr =^integer;
   pointerptr =^ iptr;
var
   num: integer;
   ptr: iptr;
   pptr: pointerptr;
   x, y :^word;
begin
   num :=3000;
   (* take the address of var*)
   ptr :=@num;
   (* take the address of ptr using address of operator@*)
   pptr :=@ptr;
  (* let us see the value and the adresses *)
   x:= addr(ptr);
   y := addr(pptr);
   writeln('Value of num = ', num );
   writeln('Value available at ptr^ = ', ptr^);
   writeln('Value available at pptr^^ = ', pptr^^);
   writeln('Address at ptr = ', x^); 
   writeln('Address at pptr = ', y^);
end.
 

When the above code is compiled and executed, it produces following result:

Value of num = 3000
Value available at ptr^ = 3000
Value available at pptr^^ = 3000
Address at ptr = 36864
Address at pptr = 36880