Object-oriented Software in Ada 95

Dynamic memory allocation 213
One way of managing dynamically allocated storage is to form a daisy chain of the allocated storage. The
usual approach when implementing this technique is to include with the record component a value which can hold
the access value of the next item in the daisy chain. The end of the chain is indicated by the value null. The
value null is special as it is considered the null value for any access type. The Ada system will guarantee that no
allocated access value can ever have the value null.
The code below forms a daisy chain of two items of storage which represent individual people.
declare
Max_Chs : constant := 7;
type Gender is ( Female, Male );
type Height_Cm is range 0 .. 300;
type Person;
--Incomplete declaration
type P_Person is access Person; --Access type
type Person is record
Name : String( 1 .. Max_Chs ); --Name as a String
Height : Height_Cm := 0; --Height in cm.
Sex : Gender; --Gender of person
Next : P_Person;
end record;
People : P_Person;
begin
People := new Person'("Mike ", 183, Male, null );
People.Next := new Person'("Corinna", 171, Female, null);
end;
Note: P_Person and Person are mutually dependent upon each other as both contain a reference to each
other. To fit in with the rule that all items must be declared before they can be used, Ada introduces the
concept of a tentative declaration. This is used when Person is defined as 'type Person;'. The
full declaration of person is defined a few lines further down.
The resultant data structure is illustrated in Figure 15.3.
People
Mike 183 Male
Corinna 171 Female null
Figure 15.3 Daisy chain of two people.
In the above code the daisy chain of people could have been created with the single assignment statement:
People := new Person'("Mike ", 183, Male,
new Person'("Corinna", 171, Female, null) );
© M A Smith - May not be reproduced without permission