Access variables

Access types

can declare variables of access types

    type int_pointer     is access INTEGER;
    type contact_pointer is access contact;
    PI : int_pointer;
    PC : contact_pointer;

Access Variables

use access variables to save pointers to allocated objects

Access variables are pointers

• do not contain data
• contain pointer to data

Access variables provide indirect access to data

Access variables are initialised by default to null

• null = does not point to any data

Accessing an object

to access the entire object pointed to by P

• P.all

(If the object is a record) to access a field:

• P.fieldname

(If the object is an array) to access an element:

• P(i)

CONSTRAINT_ERROR if P has value null

P is not affected when the object is accessed

Assignment with access variables

can assign access variables if they access the same type

    type contact is record
        initials  : string (1..3);
        extension : integer;
    end record;

    type contact_pointer is access contact;

    P1, P2, P3, P4 : contact_pointer;

Trace the effect of these statements:

    P1 := new contact'("CJL", 8817);
    P1.extension := 8060;
    P2 := new contact;
    P3 := P1;
    P2.all := P1.all;
    P3 := null;
    P4.extension := 8060;   -- constraint error
    P4 := new INTEGER;      -- syntax error

An example compares how the same problem can be solved with and without access types. Thus you can see how the access types are used.