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