Object-oriented Software in Ada 95

44 Ada introduction: Part 2
4.9 Type safety in a program
By using the type mechanism, errors in a program can be detected at compile-time. For example, a program which
processes distances in miles and kilometres can be made safer by defining separate types for miles and kilometres
as follows:
type Miles is digits 8 range 0.0 .. 25_000.0;
type Kilometres is digits 8 range 0.0 .. 50_000.0;
Note:
The range of values is adequate to accommodate any distance between two points on the earth.
A program which processes distances between cities could be defined as follows:
with Ada.Text_Io, Ada.Float_Text_Io;
use Ada.Text_Io, Ada.Float_Text_Io;
procedure Main is
type Miles is digits 8 range 0.0 .. 25_000.0;
type Kilometres is digits 8 range 0.0 .. 50_000.0;
London_Paris : Miles;
Paris_Geneva : Kilometres;
London_Paris_Geneva: Kilometres;
begin
London_Paris := 210.0; --Miles
Paris_Geneva := 420.0; --Kilometres
London_Paris_Geneva :=
Kilometres( London_Paris * 1.609_344 ) + Paris_Geneva;
Put("Distance london - paris - geneva (Kms) is " );
Put( Float( London_Paris_Geneva ), Aft=>2, Exp=>0 );
New_Line;
end Main;
Note: There is an explicit conversion of a distance in miles to kilometres using the type conversion
kilometres( london_paris * 1.609_344 ).
The contents of London_Paris_Geneva has been converted to a Float so that it can be printed
using the package Ada.Float_Text_Io. Chapter 17 describes how user defined types may be
output.
The parameters to put when outputting a floating point number control the number of decimal places
output and the format of the number. Section C.5, Appendix C lists the parameters used in outputting
numbers .
If by accident a programmer wrote:
London_Paris_Geneva := London_Paris + Paris_Geneva;
then the Ada compiler would detect a type mismatch at compile-time. London to Paris is in miles and Paris to
Geneva is in kilometres.
4.10 Subtypes
The type mechanism can on occasion, be restricting as a programmer wants the range checking provided by the
type mechanism but does not want to have to keep explicitly performing type conversions. A subtype of a type
provides the range checking associated with a type, but instances of a type and its subtypes may be freely mixed
in expressions.
© M A Smith - May not be reproduced without permission