Pointers

In a program, the value assigned to a variable is stored in a location in the computer’s volatile memory (RAM.)

The variable name is a reference to that memory location.

A memory location is identified by its memory address.

There are two types of variables: static and dynamic.

So far, we have used static variables.

Static variable

A static variable exists for the entire duration of a program’s execution.

A predetermined memory location is allocated to hold the value assigned to a static variable.

Example

var
   c : char;
{ . . .}
   c := 'A';

Dynamic variable

Dynamic memory allocation allows a memory location to be allocated and released as needed, thus ensuring optimization of memory usage.

A dynamic variable does not necessarily exist for the entire duration of a program’s execution. It is created as needed.

The memory location allocated to a dynamic variable is not determined in advance and can vary during program execution: memory allocation is dynamic.

Using the pointer data type allows you to manage dynamic memory allocation, and thus manage a dynamic variable.

The value assigned to a pointer type variable is a memory address.

This variable is a kind of reference to the content of a memory location identified by this address.

Pointer type variable

A variable of type pointer to a type T (the type of the pointer’s domain) holds the address of a memory location intended to store an underlying value of type T.

var
   p : ^char;
{ . . .}
   p^ := 'A';

Declaration

Syntax: declaring a pointer data type.

type
   identifier = ^type_of_domain;

Example

type
   pointer_char = ^char;
var
   p : pointer_char;

Using a pointer

var

{ declare a variable of type pointer to char }
p : ^char;

{ . . . }

{ allocate a memory location referenced by the pointer }
new(p);

{ dereference pointer }
p^ := 'A';
writeln(p^);

{ free the memory location referenced by the pointer }
dispose(p);

The pointer p is a kind of reference to an underlying value of type char.

The value of the pointer variable p is the address of a memory location intended to contain a value of type char.

Allocate memory

p : ^char;

This statement creates a variable of type pointer to a char.

This variable is intended to hold the address of a memory location allocated to hold a value of type char.

At this point, no memory location has yet been allocated to hold a value of type char.

The new() procedure is used to allocate a memory location.

The new() procedure takes a pointer variable as an argument and reserves enough memory space to hold a value of the pointer’s domain data type.

new(p);

This instruction allocates a memory location to hold a char value.

The pointer variable p contains the address of this allocated memory location.

The value contained in the allocated memory location is undefined (uninitialized).

Dereference a pointer

p^ := 'A';
writeln(p^);

To use the memory location allocated to this char value (variable), we must follow (dereference) the pointer.

We dereference a pointer variable by adding ^ to its name.

This char value (variable) has no name, but we use a pointer to access it.

On this dereferenced variable, we can apply all the operations allowed on the data type of the pointer’s domain, in our example the char type.

The expression p has the ^char data type (pointer to a char).

The expression p^ has the char data type.

Deallocate memory

The symmetric operation of memory allocation is memory deallocation.

The dispose() procedure is used to deallocate a memory location.

The dispose() procedure takes a pointer variable as an argument and deallocates the memory location previously allocated with the new() procedure.

dispose(p);

After using the dispose() procedure, the pointer can no longer be dereferenced.

nil pointer

The only value that can be assigned directly to a pointer is nil.

Otherwise, it is forbidden to assign a value directly to a pointer other than by using the new() procedure.

The nil value assigned to a pointer represents the notion of “pointing nowhere.”

Notes

The size of a variable declared in the var section is determined in advance.

In some situations, the size of the data to be stored and processed is not known in advance.

Pointers provide a way to overcome this limitation.

Pointers are used to implement the passing of parameters by variable to procedures and functions.

The pointer data type introduces an additional level of abstraction to the repertoire of data types.

Pointers are used to represent data structures such as lists, trees, and graphs, which allow data to be related.