Object-oriented Software in Ada 95

Ada introduction: Part 2 47
When performing arithmetic with an instance of a type's base type, no range checks take place. This allows an
implementor to implement the base type in the most efficient or effective way for a specific machine. However,
the exception Constraint_Error will be generated if the resultant arithmetic evaluation leads to a wrong
result. For example, the exception Constraint_Error is generated if an overflow is detected when
performing calculations with the base type.
4.11.3 Arithmetic with types and subtypes
In a program dealing with a student's exam marks, the following program is written to average the marks for a
student taking English, Maths and Computing:
with Ada.Text_Io;
use Ada.Text_Io;
procedure Main is
type Exam_Mark is new Integer range 0 .. 100;
English : Exam_Mark; --English exam mark
Maths : Exam_Mark; --Maths " "
Computing : Exam_Mark; --Computing " "
Average : Exam_Mark; --
begin
English := 72;
Maths := 68;
Computing := 76;
Put("Average exam mark is ");
Average := (English + Maths + Computing) / 3;
Put( Exam_Mark'Image(Average) ); New_Line;
end Main;
In executing the statement:
Average := (English+Maths+Computing) / 3;
the expression:
(English+Maths+Computing) / 3
will generate a result which is in the range 0 .. 100. However, the component of the statement
English+Maths+Computing will generate a temporary result which is outside the range of Exam_Mark.
In Ada the arithmetic operations are defined to process instances of the root types. In evaluating
English+Maths+Computing, English+Maths will deliver a temporary object of type Root_Enteger
(Exam_Mark'Base) which is then added to Computing. The result of the addition is divided by 3 at which
point a range check is performed on the temporary result before it is assigned to the object average.
Of course, for this to work the Root_Integer type must be sufficiently large to hold the sum of
English+Maths+Computing. Remember, this will be of type Root_Integer which has a range of -2 15 ..
2 15 -1.
4.11.4 Warning
If the declaration for Exam_mark where replaced by:
type Exam_Mark is range 0 .. 100;
then the above program would fail with a Constraint_Error if the base type of Exam_Mark were to be
implemented in a single byte.
© M A Smith - May not be reproduced without permission