Object-oriented Software in Ada 95

Ada introduction: Part 2 49
4.12 Compile-time and run-time checks
The following program declares data types to represent: (a) the number of power points in a room, (b) the
capacity of a lecture room in seats, and (c) the capacity of a tutorial room, again in seats. In this program, various
assignments are made, some of which will fail to compile, some of which will fail in execution.
procedure Dec is
type Power_Points is range 0 .. 6;
type Room_Size is range 0 .. 120;
subtype Lecture_Room is Room_Size range 0 .. 75;
subtype Tutorial_Room is Room_Size range 0 .. 20;
Points_In_504 : Power_Points;
People_In_504 : Lecture_Room;
People_In_616 : Tutorial_Room;
begin
Points_In_504 := 3;
Points_In_504 := 80;
People_In_504 := 15;
People_In_616 := People_In_504;
People_In_504 := Points_In_504;
--Power outlets
--Size lecture room
--Size tutorial room
--OK
--Error / Warning
--OK
--OK
-- Type Mismatch
People_In_504 := Lecture_Room( Points_In_504 ); --Force
People_In_504 := 50;
People_In_616 := People_In_504;
end Dec;
--OK
--Constraint error
The compilation or execution of the following lines will fail for the following reasons:
Line
Points_In_504:= 80;
People_In_504 := Points_In_504;
People_In_616 := People_In_504;
Reason for failure
The range of values allowed for the object
Points_In_504 does not include 80.
This error will usually be detected at
compile-time.
The objects on the LHS and RHS of the
assignment statement are of different types
and will thus produce a compile-time error.
Will cause a constraint error when
executed, as the object People_In_504
contains 50.
In this example, the error could in theory be
detected at compile-time.
Note:
Depending on the quality of the compiler, some errors which in theory could be detected at compiletime,
will only be detected at run-time. Conversely possible run-time errors may be flagged through
warnings at compile time.
This shows the strength of Ada’s strong type checking: problems in a program can be identified at an early
stage of development. However, careful planning needs to be made when writing a program. Decisions about
which distinct data types to use and which data types should be a subtype of others are particularly important.
© M A Smith - May not be reproduced without permission