Object-oriented Software in Ada 95

48 Ada introduction: Part 2
4.11.5 Constrained and unconstrained types
In Ada there are no named types only subtypes. The Integer and Float types are derived from
Root_Integer and Root_Float respectively. Range checks only apply to constrained subtypes, but
overflow checks always apply. For example, using the declaration of Exam_Mark:
type Exam_Mark is new Integer range 0 .. 100;
the following properties hold.
Declaration Instance is Commentary
Exam_Mark Constrained Constrained to the range 0 .. 100.
Exam_Mark'Base Unconstrained
No range checks applied to assignment of
this variable. An implementor may allow
this to have a range greater than the base
range of the root type.
Declaration Instance is Commentary
Integer Constrained Constrained to the base range of Integer,
which is implementation dependent.
Integer'Base Unconstrained No range checks apply; may have a range greater
than Integer.
Note:
Regardless of whether an item is constrained or unconstrained, overflow checks will always apply.
Thus, the result obtained will always be mathematically correct.
Take note of the difference between Integer and Integer'Base. Instances of the type Integer
are constrained to the base range of the type, whilst instances of Integer'Base are not.
4.11.6 Implementation optimizations
An Ada compiler is allowed to represent an instance of a base type to a greater precision than is necessary. For
example, with the following declarations:
type Exam_Mark is new Integer range 0 .. 100;
type Temporary is Exam_mark'Base;
English : Exam_Mark;
Total : Temporary
the variable Total may be implemented to hold numbers of a greater range than is allowed by an Integer
declaration. This is to allow compiler writers the opportunity to perform optimizations such as holding a variable
or intermediate result in a CPU register which may have a greater precision than the range of normal Integer
values. Of course, overflow checks will be performed at all times, so the mathematical result is always correct.
The danger is that a program which compiles and runs successfully using a particular compiler on a machine
may fail to run successfully when compiled with a different compiler on the same machine, even though both
compilers have the same range for an Integer.
© M A Smith - May not be reproduced without permission