Object-oriented Software in Ada 95

Packages as classes 79
The body of the package contains the definition of the procedures and functions defined in the specification
part of the package. In accessing the Balance_Of contained in an instance of Account the . notation is used.
For example, in the function balance the result returned is obtained using the statement ‘return
The.Balance_Of;’. The . notation is used to access a component of an instance of a record type. In this case,
the instance of the record type is the object The and the component of the object is Balance_Of.
6.3.9 Terminology
The following terminology is used to describe the components of a class.
Terminology
Instance
attribute
Instance method
or just method
Note:
Example:
in class Account
Balance_Of
Deposit
Explanation
A data component of an object. In Ada this
will be a member of the type that is used to
declare the object.
A procedure or function used to access the
instance attributes in an object.
The terminology comes from the language Smalltalk.
6.4 The package as seen by a user
A user will normally only have access to the specification part of a package. This provides a specification of the
messages that can be sent to an object but does not show how the methods invoked by the messages have been
implemented. The implementation part will not normally be available, the implementor normally providing only a
compiled version of the package.
Unfortunately the details of the private type will normally be visible, though they cannot be accessed.
Note:
The details of the private type can be made invisible to a user, but this involves some complexity. One
approach to this is shown in Section 15.5.
6.5 The package as seen by an implementor
When building a package the implementor should ensure:
• That a user of the package can make effective use of its facilities.
• That the only visible components are:
(a) The messages that can be sent to an object.
(b) The private type declaration that is used to elaborate an object.
The visibility hierarchy for the package Class_Account is shown in Figure 6.5.
Visibility
Component is:
Visible to a client
of the package.
Invisible to a client
of the package
-
Deposit
Withdraw
Balance
Balance_Of
Implementation of
procedures and functions
In the public part of the
package specification.
-
In the private part of the
package specification.
-
In the body of the
package.
Figure 6.5 Visibility of methods and instance attributes of the package Class_Account.
© M A Smith - May not be reproduced without permission