Object-oriented Software in Ada 95

40 Ada introduction: Part 2
Distance is a new type, instances of which may not be mixed with instances of other types. The following
table shows some examples of type declarations in Ada.
Type declaration
type T is range 0 .. 250_000;
type T is digits 8;
type T is digits 8 range 0.0 .. 10.0;
An instance of T will Declare
An object which can hold whole numbers in
the range 0 .. 250_000.
An object which can hold a floating point
number which has a precision of 8 digits.
An object which can hold a floating point
number which has a precision of 8 digits and
can store numbers in the range 0.0 .. 10.0.
4.3.1 Type conversions
To convert between compatible scalar types the type name of the required type is used to convert an object to the
required type. For example, the following program converts an object of type Apples into an object of type
French_Apples.
procedure Main is
type Apples is range 0 .. 100;
type French_Apples is range 0 .. 100;
Number : Apples;
Number_From_France : French_Apples;
begin
Number := 10;
Number_From_France := French_Apples( Number );
end Main;
It is, however, up to the programmer to determine whether the conversion is meaningful. Conversion,
however, can only take place between types that are compatible.
4.3.2 Universal integer
To avoid tedious type conversion when dealing with constants, Ada has the concept of a universal integer. The
compiler will automatically convert a universal integer to an appropriate type when used in an arithmetic
expression. In Ada all integer numeric constants are regarded as being of type universal integer. Likewise all
floating point constants are regarded as a universal float.
4.3.3 Constant declarations
To make a program more readable, all values other than 0 or 1 should normally be given a symbolic name. This
helps to improve the readability of a program and allows the programmer to change the value by means of a single
textual change. For example, the capacity of a car park could be described as:
Max_Parking_Spaces: constant := 100;
This describes Max_Parking_Spaces as a universal integer. However, if the declaration had been:
Max_Parking_Spaces: constant Parking_Spaces := 100;
then Max_Parking_Spaces would be a constant of type Parking_spaces.
Note:
The latter declaration will restrict the places where Max_Parking_Spaces can be used to only
those places where a value of type Parking_Spaces can occur.
© M A Smith - May not be reproduced without permission