Object-oriented Software in Ada 95

Ada introduction: Part 2 45
For example, the speed of various forms of transport can be defined using the type and subtype mechanism as
follows:
type Speed_Mph is range 0 .. 25_000;
subtype Train_Speed is Speed_Mph range 0 .. 130;
subtype Bus_Speed is Speed_Mph range 0 .. 75;
subtype Cycling_Speed is Speed_Mph range 0 .. 30;
subtype Person_Speed is Speed_Mph range 0 .. 15;
A subtype is derived from an existing type and constrains the values that can be assigned to an instance of the
subtype. The compiler will enforce this constraint either by performing a compile-time check or by generating
code to check the constraint at run-time. Of course, the subtype inherits all the operations that can be performed
on an instance of the type.
Instances of a type and its subtypes may be freely mixed in arithmetic, comparison and assignment operations.
For example, using the above type and subtypes declarations for the speed of various forms of transport, the
following code can be written:
with Ada.Text_Io;
use Ada.Text_Io;
procedure main is
-- Type and subtype declarations for speeds
T0715 : Train_Speed; --07:15 Brighton - London
B0720 : Bus_Speed; --07:20 Brighton - London
begin
T0715 := 55; --Average speed Brighton - London (Train)
B0720 := 35; --Average speed Brighton - London (Bus)
if T0715 > B0720 then
Put("The train is faster then the bus");
else
Put("The bus is faster then the train");
end if;
New_Line;
end Main;
Note:
It is of course an error to mix instances of subtypes which are derived from different types.
4.10.1 Types vs. subtypes
Criteria Types Subtype
Instances may be mixed with only instances of the
same type
only instances of a type
and subtypes derived
from the type
May have a constraint Yes Yes
4.11 More on types and subtypes
In Ada only subtypes have names. The consequence of this is that the declaration:
type Speed_Mph is range 0 .. 25_000;
© M A Smith - May not be reproduced without permission