Object-oriented Software in Ada 95

Data structures 97
Then an assignment to an instance of Person is:
Mike := (4, Name=>"Mike" , Height=>183, Sex=>Male);
Corinna:= (7, Name=>"Corinna", Height=>171, Sex=>Female);
Note: The value of the discriminant must be specified in the record aggregate.
However, Mike, Corinna and Younger are not of the same type so the assignment:
Younger := Corinna; -- Fail at run-time
will fail at run-time as the discriminants of the record are not identical. The object Younger contains a String
of length of 10 whilst the object Corinna contains a String of length 7.
7.5 Default values to a discriminant
A discriminant to a type may have a default value. If a value is not specified with the declaration of a
discriminated type then it is an unconstrained discriminated type. An instance of an unconstrained discriminated
type may be assigned or compared with other unconstrained discriminants of the same type name.
For example, if the data structure Person is now defined as:
type Gender is ( Female, Male );
type Height_Cm is range 0 .. 300;
subtype Str_Range is Natural range 0 .. 20;
type Person( Chs:Str_Range := 0 ) is record --Length of name
Name : String( 1 .. Chs ); --Name as String
Height : Height_Cm := 0; --Height in cm.
Sex : Gender; --Gender
end record;
then the following code can be written:
declare
Mike : Person; --Unconstrained
Corinna: Person; --Unconstrained
Younger: Person; --Unconstrained
begin
Mike := (4, Name=>"Mike" , Height=>183, Sex=>Male);
Corinna:= (7, Name=>"Corinna", Height=>171, Sex=>Female);
Younger := Corinna;
if Corinna = Younger then
Put("Corinna is younger"); New_Line;
end if;
end;
Note:
It would still be an error to write:
Corinna:=(10,name=>"Corinna",height=>171,sex=>Female);
as the length of "Corinna" is not 10 characters.
© M A Smith - May not be reproduced without permission