Pointers and references

In this task, the goal is to desmonstrate common operations on pointers and references.

Ada

Ada does not have pointers, only access types.

Create a pool specific access type for an integer

 type Int_Access is access Integer;
 Int_Acc : Int_Access := new Integer(5);

Create a general access type for an integer

 type Int_Ptr is access all Integer;
 Ref : Int_Ptr;
 Var : aliased Integer := 3;
 Ref := Var'access;

Ada does not provide pointer arithmetic, but does allow evaluation of the address

 Var : Integer;
 Var_Address : Address := Var'address;

Addresses support operations of comparison, addition, and subtraction Ada also supports conversion between address types and a pre-defined subtype of Integer named Integer_Address. This accomodates the conversion to a linear addressing of any hardware address scheme including address:offset used in the 8086 processor.

Ada allows the specification of a starting address for any object

 -- Demonstrate the overlay of one object on another
 A : Integer;
 B : Integer;
 for B'address use A'address;
 -- A and B start at the same address

C++

With pointers:

The following code creates a pointer to an int variable

 int var = 3;
 int* pointer = &var;
 // or alternatively:
 int* pointer2(&var);

Access the integer variable through the pointer:

 int v = *pointer; // sets v to the value of var (i.e. 3)
 *pointer = 42; // sets var to 42

Change the pointer to refer to another object

 int othervar;
 pointer = &othervar;

Change the pointer to not point to any object

 pointer = 0;
 // or alternatively:
 pointer = NULL;

Get a pointer to the first element of an array:

 int array[10];
 pointer = array;
 // or alternatively:
 pointer = &array[0];

Move the pointer to another object in the array

 pointer += 3; // pointer now points to array[3]
 pointer -= 2; // pointer now points to array[1]

Access another object in the same array through the pointer

 v = pointer[3]; // accesses third-next object, i.e. array[4]
 v = pointer[-1]; // accesses previous object, i.e. array[0]
 // or alternatively
 v = *(pointer + 3); // array[4]
 v = *(pointer - 1); // array[0]

With references:

The following code create a reference to an int variable:

 int var = 3;
 int& ref = var;
 // or alternatively:
 int& ref2(var);

Access the integer variable through the reference

 int v = ref; // sets v to the value of var, that is, 3
 ref = 42; // sets var to 42

References cannot be changed to refer to other objects, and cannot (legally) made to refer to no object.

Get a reference to the first element of an array:

 int array[10];
 int& ref3 = array[0];

Changing the reference to refer to another object of the array is not possible.

Accessing another object of the array through the reference:

 v = (&ref)[3]; // read value of array[3]; however doing this is bad style

Delphi

Delphi ( Object Pascal / Turbo Pascal ) fully supports both typed and untyped pointers. Since Delphi all pointers have been 32 bit. Previously you could have both long ( 32 bit ) and short ( 16 bit ) pointers.

Examples of pointers

Simple untyped pointer variable:

Variable Declaration

 pMyPointer : Pointer ;

Simple pointer to a pre-defined type:

Variable Declaration

 pIntPointer : ^Integer ;

A pointer to a Record. This is the equivilent to a Struct in C

Type Defintion

 MyRecord = Record
             FName : string[20];
             LName : string[20];
            end;

Variable Declaration

 pMyRecord : ^MyRecord ;

- Dereferencing a Pointer -

Pointers are dereferenced using the carrot '^' symbol. Dereferencing will cause the compiler or RTL to reveal the data that the pointer points to:

  IntVar := pIntPointer^ ;

Dereference with a type cast of an untyped pointer. It is not legal syntax to simply dereference an untyped pointer, since there is nothing to designate its type. It must therefor be "Type Cast" when the dereference takes place as below.

  IntVar := integer(MyPointer^);

- Pushing a Pointer -

Many programmers are familiar with C's ability to increment pointers of any type by using the ++ or -- operators. Delphi has a very limited implementation of this when using pointers to char. Delphi can use the Inc() & Dec() procedures to increment or decrement a pointer to char. This is the only special case. To increment all other pointers you must use a "Type Cast". The syntax for doing this is:

  p := Inc(integer(p)+1);

This will increment the pointer by one byte. Since all pointers are actualy 32 bit numbers. Additionaly, if you have a typed pointer you can increment the pointer by the size of the type it points to:

 pInt := Inc(integer(pInt)+sizeof(Integer)) ;

 pMyRecord := inc(integer(pMyRecord)+SizeOf(MyRecord));

E

E is a memory-safe object-graph language, and as such uses only object references; there are no non-reference values. Mutable storage always consists of an object; in particular, an assignable variable has, if necessary, an underlying "slot" object which can be retrieved.

var x := 0
def slot := &x # define "slot" to be x's slot

x := 1         # direct assignment; value is now 1
slot.put(2)    # via slot object; value is now 2

There are also mutable data structures:

def flexList := [].diverge()
flexList.push(1)
flexList.push(2)
flexList.snapshot() # returns [1, 2]

User-defined objects can have state by containing it:

var open := true
def door {
    to open()  { open := true }
    to close() { open := false }
    to walkThrough() { require(open) }
}

Perl

Interpeter: Perl v5.x

References are essentially how Perl handles pointers. Any scalar value, element in an array, key of a hash, or value of a hash may contain a reference to any other variable structurer.

# start with some var definitions
my $scalar = 'a string';
my @array = ('an', 'array');
my %hash = ( firstkey => 'a', secondkey => 'hash' );

# make pointers
my $scalarref = \$scalar;
my $arrayref = \@array;
my $hashref = \%hash;

Using a reference

# printing the value
print ${$scalar};
print $arrayref->[1];          # this would print "array"
print $hashref->{'secondkey'}; # this would print "hash"

# changing the value
${$scalar} = 'a new string';       # would change $scalar as well
$arrayref->[0] = 'an altered';     # would change the first value of @array as well
$hashref->{'firstkey'} = 'a good'; # would change the value of the firstkey name value pair in %hash

You may also create pointers or references without pointing to a previous variable.

my $scalarref;
${$scalarref} = 'a scalar';
my $arrayref = ['an', 'array'];
my $hashref = { firstkey => 'a', secondkey => 'hash' }

Tcl

Tcl does not have pointers, however if required a similar level of indirection can be had by storing a variable name in another variable, eg.

 set var 3
 set pointer var; # assign name "var" not value 3
 set pointer;     # returns "var"
 set $pointer;    # returns 3
 set $pointer 42; # variable var now has value 42

In practice it's safer and more convenient to use array keys, eg.

 set arr(var) 3
 set pointer var
 set arr($pointer);    # returns 3
 set arr($pointer) 42; # arr(var) now has value 42