Object-oriented Software in Ada 95

74 Packages as classes
Likewise in the software world, there are objects that a user or programmer can make effective use of without
having to know how the object has been implemented. On a very simple level an Ada program may declare
objects to hold floating point numbers, which can then be used with arithmetic operations to sum, multiply, etc.
these values. Most programmers however, do not know the exact details of how these operations are performed;
they accept the interface provided by the programming language.
At one point it was fashionable for programming languages to provide a rich set of data types. The designers
of these languages hoped the data types provided would be adequate for all occasions. The problem was, and still
is, that no one language could ever hope to provide all the different types of item that a programmer may need or
wish to use.
Ada gives a programmer the ability to declare new data types, together with a range of operations that may be
performed on an instance of the type. Naturally, a programmer may also use types and operations on these types
that have been defined by other programmers.
6.2 Objects, messages and methods
A car can be thought of as an object. The car contains complex details and processes that are hidden from the
driver. For example, to make the car go faster the driver presses the accelerator pedal. The car receives the
message ‘go faster’ and evokes an internal method to speed up the engine.
In the above description of driving a car many object-oriented ideas have been used. These ideas are as
follows:
object
message
method
An item that has a hidden internal structure. The hidden structure
is manipulated or accessed by messages sent by a user.
A request sent to the object to obey one of its methods.
A set of actions that manipulates or accesses the internal state of
the object. The detail of these actions is hidden from a user of the
object.
6.3 Objects, messages and methods in Ada
In Ada an object is an instance of either a user-defined type or an instance of one of the in-built types.
An object for a user-defined type can be imagined diagrammatically as Figure 6.2.
Data
Method 1
Messages sent to an object
which evoke methods that
access the internal hidden
data
Method 2
Method 3
Figure 6.2 Diagrammatic representation of an object.
A message is implemented as either a procedure or function call, the body of which is the method that is
evoked when the message is sent to the object. The user of the object has no knowledge of the implementation
code contained in the body of the procedure or function.
Note:
The idea of binding code and data together in a unit that does not allow direct access to the data is
often referred to as encapsulation.
© M A Smith - May not be reproduced without permission